Hagfish predatory behaviour and slime defence mechanism

Evolutionary insights into gut acidification: invertebrate-like mechanisms in the basal vertebrate hagfish

Acidification is a key component of digestion throughout metazoans. The gut digestive fluid of many invertebrates is acidified by the vesicular-type H+-ATPase (VHA). In contrast, vertebrates generate acidic gut fluids using the gastric H+/K+-ATPase (HKA), an evolutionary innovation linked with the appearance of a true stomach that greatly improves digestion, absorption and immune function. Hagfishes are the most basal extant vertebrates, and their mechanism of digestive acidification remains unclear. Herein, we report that the stomachless Pacific hagfish (Eptatretus stoutii) acidify their gut using the VHA, and searches of E. stoutii gut transcriptomes and the genome of a closely related hagfish species (E. burgerii) indicate they lack HKA, consistent with its emergence following the 2R whole-genome duplication. Immunostaining revealed prominent VHA presence in the apical membrane of enterocytes and sub-apical expression of both VHA and soluble adenylyl cyclase. Interestingly, akin to vertebrates, VHA was also observed in immature pancreatic-like zymogen granules and was noticeably absent from the mature granules. Furthermore, isolated gut sacs from fed hagfish demonstrate increased VHA-dependent luminal H+ secretion that is stimulated by the cAMP pathway. Overall, these results suggest that the hagfish gut shares the trait of VHA-dependent acidification with invertebrates, while simultaneously performing some roles of the pancreas and intestine of gnathostomes.

Fast 3D printing of fine, continuous, and soft fibers via embedded solvent exchange

Nature uses fibrous structures for sensing and structural functions as observed in hairs, whiskers, stereocilia, spider silks, and hagfish slime thread skeins. Here, we demonstrate multi-nozzle printing of 3D hair arrays having freeform trajectories at a very high rate, with fiber diameters as fine as 1.5 µm, continuous lengths reaching tens of centimeters, and a wide range of materials with elastic moduli from 5 MPa to 3500 MPa. This is achieved via 3D printing by rapid solvent exchange in high yield stress micro granular gel, leading to radial solidification of the extruded polymer filament at a rate of 2.33 μm/s. This process extrudes filaments at 5 mm/s, which is 500,000 times faster than meniscus printing owing to the rapid solidification which prevents capillarity-induced fiber breakage. This study demonstrates the potential of 3D printing by rapid solvent exchange as a fast and scalable process for replicating natural fibrous structures for use in biomimetic functions.

Odorant transport in a hagfish

Odorant transport is of fundamental and applied importance. Using computational simulations, we studied odorant transport in an anatomically accurate model of the nasal passage of a hagfish (probably Eptatretus stoutii). We found that ambient water is sampled widely, with a significant ventral element. Additionally, there is a bilateral element to olfactory flow, which enters the single nostril in two narrow, laminar streams that are then split prior to the nasal chamber by the anterior edge of the central olfactory lamella. An appendage on this lamella directs a small portion (10-14%) of the overall nasal flow to the olfactory sensory channels. Much of the remaining flow is diverted away from the sensory channels by two peripheral channels. The anterior edge of the central olfactory lamella, together with a jet-impingement mechanism, disperses flow over the olfactory surfaces. Diffusion of odorant from bulk water to the olfactory surfaces is facilitated by the large surface area:volume ratio of the sensory channels, and by a resistance-based hydrodynamic mechanism that leads to long residence times (up to 4.5 s) in the sensory channels. With increasing volumetric flow rate, the rate of odorant transfer to the olfactory surfaces increases, but the efficiency of odorant uptake decreases, falling in the range 2-6%. Odorant flux decreases caudally across the olfactory surfaces, suggesting in vivo a preponderance of olfactory sensory neurons on the anterior part of each olfactory surface. We conclude that the hagfish has a subtle anatomy for locating and capturing odorant molecules.

Hagfish-inspired hydrogel for root caries: A multifunctional approach including immediate protection, antimicrobial phototherapy, and remineralization

Root caries is the main cause of oral pain and tooth loss in the elderly. Protecting root lesions from environmental disturbances, resisting pathogens, and facilitating remineralization over time are essential for addressing root caries, but are challenging due to the irregular root surface and the complex oral environment. Hagfish secretes slime when facing danger, which converts into gels upon contact with seawater, suffocating the predators. Inspired by hagfish's defense mechanism, a fluid-hydrogel conversion strategy is proposed to establish a mechanical self-regulating multifunctional platform for root caries treatment. The fluid system (silk fibroin-tannic acid-black phosphorene-urea, ST-BP-U), in which urea disrupts the hydrogen bonds between silk fibroin and tannic acid, can easily spread on the irregular root surface and permeate into dentinal tubules. Upon contact with the surrounding water, urea diffuses, prompting the hydrogel re-formation and creating intimate attachments with micromechanical inlay locks. Meanwhile, BP increases the crosslinking of the re-formed hydrogel network, resulting in reinforced cohesion for robust wet adhesion to the tooth root. This process establishes a structured platform for effective antimicrobial phototherapy and dentin remineralization promotion. This water-responsive fluid-hydrogel conversion system adapts to the irregular root surface in the dynamic wet environment, holding promise for addressing root caries. STATEMENT OF SIGNIFICANCE: Root caries bring a heavy burden to the aging society, but the irregular root surface and dynamic moist oral environment always hinder non-surgical therapeutic effects. Here, we propose a water-responsive fluid-hydrogel conversion strategy aimed at mechanical self-regulation on the irregular and wet root interface to construct a functional structural platform. The liquid system (ST-BP-U) that prebreak intermolecular hydrogen bonds can easily spread on irregular surfaces and dentin tubules. When encountering water, hydrogen bonds re-form, and BP increases the crosslinking of the hydrogel formed in situ. Based on this firm wet-adhesion platform, it provides powerful phototherapy effects and promotes dentin remineralization. This fluid-hydrogel conversion system turns the disadvantages of wet environment into advantages, offering a promising strategy for root caries.

Fluid Ejections in Nature

From microscopic fungi to colossal whales, fluid ejections are universal and intricate phenomena in biology, serving vital functions such as animal excretion, venom spraying, prey hunting, spore dispersal, and plant guttation. This review delves into the complex fluid physics of ejections across various scales, exploring both muscle-powered active systems and passive mechanisms driven by gravity or osmosis. It introduces a framework using dimensionless numbers to delineate transitions from dripping to jetting and elucidate the governing forces. Highlighting the understudied area of complex fluid ejections, this review not only rationalizes the biophysics involved but also uncovers potential engineering applications in soft robotics, additive manufacturing, and drug delivery. By bridging biomechanics, the physics of living systems, and fluid dynamics, this review offers valuable insights into the diverse world of fluid ejections and paves the way for future bioinspired research across the spectrum of life.

Biphasic burrowing in Atlantic hagfish (Myxine limosa)

Myxine limosa is a burrowing species of hagfish that occurs in the western North Atlantic in areas with muddy substrate and at depths generally greater than 100 meters. Burrowing of M. limosa has been observed from submersibles, but little is known about the behavior of these animals within the substrate or the biomechanical mechanisms involved. Here, we investigated burrowing in M. limosa by observing individuals as they burrowed through transparent gelatin. A photoelastic setup using crossed polarizers allowed us to visualize stress development in the gelatin as the hagfish moved through it. We found that M. limosa created U-shaped burrows in gelatin using a stereotyped, two-phase burrowing behavior. In the first ('thrash') phase, hagfish drove their head and their anterior body into the substrate using vigorous sinusoidal swimming movements, with their head moving side-to-side. In the second ('wriggle') phase, swimming movements ceased, with propulsion coming exclusively from the anterior, submerged portion of body. The wriggle phase involved side-to-side head movements and movements of the submerged part of the body that resembled the internal concertina strategy used by caecilians and uropeltid snakes. The entire burrowing process took on average 7.6 min to complete and ended with the hagfish's head protruding from the substrate and the rest of its body generally concealed. Understanding the burrowing activities of hagfishes could lead to improved understanding of sediment turnover in marine benthic habitats, new insights into the reproductive behavior of hagfishes, or even inspiration for the design of burrowing robots.

Colonization of the ocean floor by jawless vertebrates across three mass extinctions

BACKGROUND

The deep (> 200 m) ocean floor is often considered to be a refugium of biodiversity; many benthic marine animals appear to share ancient common ancestry with nearshore and terrestrial relatives. Whether this pattern holds for vertebrates is obscured by a poor understanding of the evolutionary history of the oldest marine vertebrate clades. Hagfishes are jawless vertebrates that are either the living sister to all vertebrates or form a clade with lampreys, the only other surviving jawless fishes.

RESULTS

We use the hagfish fossil record and molecular data for all recognized genera to construct a novel hypothesis for hagfish relationships and diversification. We find that crown hagfishes persisted through three mass extinctions after appearing in the Permian ~ 275 Ma, making them one of the oldest living vertebrate lineages. In contrast to most other deep marine vertebrates, we consistently infer a deep origin of continental slope occupation by hagfishes that dates to the Paleozoic. Yet, we show that hagfishes have experienced marked body size diversification over the last hundred million years, contrasting with a view of this clade as morphologically stagnant.

CONCLUSION

Our results establish hagfishes as ancient members of demersal continental slope faunas and suggest a prolonged accumulation of deep sea jawless vertebrate biodiversity.

Fluid ejections in nature

From microscopic fungi to colossal whales, fluidic ejections are a universal and intricate phenomenon in biology, serving vital functions such as animal excretion, venom spraying, prey hunting, spore dispersal, and plant guttation. This review delves into the complex fluid physics of ejections across various scales, exploring both muscle-powered active systems and passive mechanisms driven by gravity or osmosis. We introduce a framework using dimensionless numbers to delineate transitions from dripping to jetting and elucidate the governing forces. Highlighting the understudied area of complex fluid ejections, this work not only rationalizes the biophysics involved but also uncovers potential engineering applications in soft robotics, additive manufacturing, and drug delivery. By bridging biomechanics, the physics of living systems, and fluid dynamics, this review offers valuable insights into the diverse world of fluid ejections and paves the way for future bioinspired research across the spectrum of life.

Epidermal threads reveal the origin of hagfish slime

When attacked, hagfishes produce a soft, fibrous defensive slime within a fraction of a second by ejecting mucus and threads into seawater. The rapid setup and remarkable expansion of the slime make it a highly effective and unique form of defense. How this biomaterial evolved is unknown, although circumstantial evidence points to the epidermis as the origin of the thread- and mucus-producing cells in the slime glands. Here, we describe large intracellular threads within a putatively homologous cell type from hagfish epidermis. These epidermal threads averaged ~2 mm in length and ~0.5 μm in diameter. The entire hagfish body is covered by a dense layer of epidermal thread cells, with each square millimeter of skin storing a total of ~96 cm threads. Experimentally induced damage to a hagfish's skin caused the release of threads, which together with mucus, formed an adhesive epidermal slime that is more fibrous and less dilute than the defensive slime. Transcriptome analysis further suggests that epidermal threads are ancestral to the slime threads, with duplication and diversification of thread genes occurring in parallel with the evolution of slime glands. Our results support an epidermal origin of hagfish slime, which may have been driven by selection for stronger and more voluminous slime.

Mechanisms of gill-clogging by hagfish slime

Hagfishes defend themselves from gill-breathing predators by producing large volumes of fibrous slime when attacked. The slime's effectiveness comes from its ability to clog predators' gills, but the mechanisms by which hagfish slime clogs are uncertain, especially given its remarkably dilute concentration of solids. We quantified the clogging performance of hagfish slime over a range of concentrations, measured the contributions of its mucous and thread components, and measured the effect of turbulent mixing on clogging. To assess the porous structure of hagfish slime, we used a custom device to measure its Darcy permeability. We show that hagfish slime clogs at extremely dilute concentrations like those found in native hagfish slime and displays clogging performance that is superior to three thickening agents. We report an extremely low Darcy permeability for hagfish slime, and an effective pore size of 10-300 nm. We also show that the mucous and thread components play distinct yet crucial roles, with mucus being responsible for effective clogging and low permeability and the threads imparting mechanical strength and retaining clogging function over time. Our results provide new insights into the mechanisms by which hagfish slime clogs gills and may inspire the development of ultra-soft materials with novel properties.

Vertebrate origins are informed by larval lampreys (ammocoetes): a response to Miyashita et al., 2021

This paper addresses a recent claim by Miyashita and co-authors that the filter-feeding larval lamprey is a new evolutionary addition to the lamprey life-cycle and does not provide information about early vertebrates, in contrast to the traditional view that this ammocoete stage resembles the first vertebrates. The evidence behind this revolutionary claim comes from fossil lampreys from 360–306 Mya that include young stages – even yolk-sac hatchlings – with adult (predacious) feeding structures. However, the traditional view is not so easily dismissed. The phylogeny on which the non-ammocoete theory is based was not tested in a statistically meaningful way. Additionally, the target article did not consider the known evidence for the traditional view, namely that the complex filter-feeding structures are highly similar in ammocoetes and the invertebrate chordates, amphioxus and tunicates. In further support of the traditional view, I show that ammocoetes are helpful for reconstructing the first vertebrates and the jawless, fossil stem gnathostomes called ostracoderms – their pharynx, oral cavity, mouth opening, lips and filter-feeding mode (but, ironically, not their mandibular/jaw region). From these considerations, I offer a scenario for the evolution of vertebrate life-cycles that fits the traditional, ammocoete-informed theory and puts filter feeding at centre stage.

Directed Self-Assembly of Heterologously Expressed Hagfish EsTKα and EsTKγ for Functional Hydrogel

Hagfish slime proteins have long been considered useful due to their potential applications in novel green, environmental, and functional bionic materials. The two main component proteins in the slime thread of hagfish, (opt)EsTKα and (opt)EsTKγ, were used as raw materials. However, the methods available to assemble these two proteins are time- and labor-intensive. The conditions affecting protein self-assembly, such as the pH of the assembly buffer, protein concentration, and the protein addition ratio, were the subject of the present research. Through a series of tests, the self-assembly results of a variety of assembly conditions were explored. Finally, a simplified protein self-assembly method was identified that allows for simple, direct assembly of the two proteins directly. This method does not require protein purification. Under the optimal assembly conditions obtained by exploration, a new gel material was synthesized from the hagfish protein through self-assembly of the (opt)EsTKα and (opt)EsTKγ. This assembly method has the benefits of being a simple, time-saving, and efficient. The self-assembled protein gel products were verified by SDS polyacrylamide gel electrophoresis (SDS-PAGE) and contained (opt)EsTKα and (opt)EsTKγ proteins. Scanning electron microscopy (SEM) was used to investigate the self-assembled protein gel after freeze-drying, and it was observed that the self-assembled protein formed a dense, three-dimensional porous network structure, meaning that it had good water retention. Evaluation of the gel with atomic force microscopy (AFM) indicated that the surface of the protein fiber skeleton show the network-like structure and relatively smooth. Characterization by circular dichroism (CD) and Fourier transform infrared spectroscopy (FT-IR) demonstrated that the two proteins were successfully assembled, and that the assembled protein had a secondary structure dominated by α-helices. The rheological properties of the self-assembled products were tested to confirm that they were indeed hydrogel property.

Breathing versus feeding in the Pacific hagfish

Hagfish represent the oldest extant connection to the ancestral vertebrates, but their physiology is not well understood. Using behavioural (video), physiological (respirometry, flow measurements), classical morphological (dissection, silicone injection) and modern imaging approaches (micro-MRI, DICE micro-CT) we examined the interface between feeding and the unique breathing mechanism (nostril, high frequency velum, low frequency gill pouches (24) and pharyngo-cutaneous duct,PCD) in the Pacific hagfish, Eptatretus stoutii. A video tour via micro-MRI is presented through the breathing and feeding passages. We have reconciled earlier disagreement as to the position of the velum chamber, which powers inhalation through the nostril, placing it downstream of the merging point of food and water passages, such that the oronasal septum terminates at the anterior end of the velum chamber. When feeding occurs by engulfment of large chunks by the dental plates, food movement through the chamber may transiently interfere with breathing. Swallowing is accelerated by peristaltic body undulation involving the ventral musculature, and is complete within 5 sec. After a large meal (anchovy, 20% body mass), hagfish remain motionless, defecating bones and scales at 1.7 days and an intestinal peritrophic membrane at 5 days. O2 consumption rate approximately doubles within 1 h after feeding, remaining elevated through 12-24 h. This is achieved by combinations of elevated O2 utilization and ventilatory flow, the latter caused by varying increases in velar frequency and stroke volume. Additional imaging casts light on the reasons for the trend for greater O2 utilization by more posterior pouches and PCD in fasted hagfish.

Hagfish to Illuminate the Developmental and Evolutionary Origins of the Vertebrate Retina

The vertebrate eye is a vital sensory organ that has long fascinated scientists, but the details of how this organ evolved are still unclear. The vertebrate eye is distinct from the simple photoreceptive organs of other non-vertebrate chordates and there are no clear transitional forms of the eye in the fossil record. To investigate the evolution of the eye we can examine the eyes of the most ancient extant vertebrates, the hagfish and lamprey. These jawless vertebrates are in an ideal phylogenetic position to study the origin of the vertebrate eye but data on eye/retina development in these organisms is limited. New genomic and gene expression data from hagfish and lamprey suggest they have many of the same genes for eye development and retinal neurogenesis as jawed vertebrates, but functional work to determine if these genes operate in retinogenesis similarly to other vertebrates is missing. In addition, hagfish express a marker of proliferative retinal cells (Pax6) near the margin of the retina, and adult retinal growth is apparent in some species. This finding of eye growth late into hagfish ontogeny is unexpected given the degenerate eye phenotype. Further studies dissecting retinal neurogenesis in jawless vertebrates would allow for comparison of the mechanisms of retinal development between cyclostome and gnathostome eyes and provide insight into the evolutionary origins of the vertebrate eye.

Permanent deformation of triangle weaver silk enables ultrafast tangle-free release of spider webs

Entanglements are common in both natural and artificial systems and can result in both beneficial and harmful effects. Most spider webs are static structures held under constant tension and do not tangle. However, many spiders actively load tension into their webs by coiling silk threads that are released to "fire" webs at prey. Here we test whether or not tangling occurs during the rapid release of webs built by the triangle spider Hyptiotes cavatus. We use high-speed videography to examine the release of the spider's web, looking for signs of tangling both visually on the videos and on acceleration graphs. The spider tenses the web by pulling on a silken anchor line using a leg-over-leg movement, deforming the silk into permanent coils and storing excess slack in a loose bundle between the spider's legs. This 1-3cm long bundle of coils straightens during the web's release in as few as 4ms. Though the messy silk coils are pressed closely together, the web's release is never impeded by catastrophic tangling. This lack of serious tangling is perhaps due to the permanent coils preventing random movement of the silk. The coils also compact the loose silk, preventing interference with the spider's movement. The ability to coil its anchor line allows H. cavatus to permanently restructure its silk, facilitating its active web-hunting behavior. Our findings broaden our knowledge of silk manipulation by spiders and may give insights into creating tangle-free systems through structural changes.

Evolution of a remarkable intracellular polymer and extreme cell allometry in hagfishes

The size of animal cells rarely scales with body size, likely due to biophysical and physiological constraints.^1,2 In hagfishes, gland thread cells (GTCs) each produce a silk-like proteinaceous fiber called a slime thread.^3,4 The slime threads impart strength to a hagfish's defensive slime and thus are potentially subject to selection on their function outside of the body.^5-8 Body size is of fundamental importance in predator-prey interactions, which led us to hypothesize that larger hagfishes produce longer and stronger slime threads than smaller ones.⁹ Here, by sampling a range of sizes of hagfish from 19 species, we systematically examined the scaling of GTC and slime-thread dimensions with body size within both phylogenetic and ontogenetic contexts. We found that the length of GTCs varied between 40 and 250 μm and scaled positively with body size, exhibiting an allometric exponent greater than those in other animal cells. Correspondingly, larger hagfishes produce longer and thicker slime threads and thus are equipped to defend against larger predators. With diameter and length varying 4-fold (0.7-4 μm and 5-22 cm, respectively) over a body-size range of 10-128 cm, the slime threads characterize the largest intracellular polymers known in biology. Our results suggest selection for stronger defensive slime in larger hagfishes has driven the evolution of extreme size and allometry of GTCs.

Forgotten but not gone: Particulate matter as contaminations of mucosal systems

A decade ago, environmental issues, such as air pollution and the contamination of the oceans with microplastic, were prominently communicated in the media. However, these days, political topics, as well as the ongoing COVID-19 pandemic, have clearly taken over. In spite of this shift in focus regarding media representation, researchers have made progress in evaluating the possible health risks associated with particulate contaminations present in water and air. In this review article, we summarize recent efforts that establish a clear link between the increasing occurrence of certain pathological conditions and the exposure of humans (or animals) to airborne or waterborne particulate matter. First, we give an overview of the physiological functions mucus has to fulfill in humans and animals, and we discuss different sources of particulate matter. We then highlight parameters that govern particle toxicity and summarize our current knowledge of how an exposure to particulate matter can be related to dysfunctions of mucosal systems. Last, we outline how biophysical tools and methods can help researchers to obtain a better understanding of how particulate matter may affect human health. As we discuss here, recent research has made it quite clear that the structure and functions of those mucosal systems are sensitive toward particulate contaminations. Yet, our mechanistic understanding of how (and which) nano- and microparticles can compromise human health via interacting with mucosal barriers is far from complete.

Ecology of Fear: Spines, Armor and Noxious Chemicals Deter Predators in Cancer and in Nature

In nature, many multicellular and unicellular organisms use constitutive defenses such as armor, spines, and noxious chemicals to keep predators at bay. These defenses render the prey difficult and/or dangerous to subdue and handle, which confers a strong deterrent for predators. The distinct benefit of this mode of defense is that prey can defend in place and continue activities such as foraging even under imminent threat of predation. The same qualitative types of armor-like, spine-like, and noxious defenses have evolved independently and repeatedly in nature, and we present evidence that cancer is no exception. Cancer cells exist in environments inundated with predator-like immune cells, so the ability of cancer cells to defend in place while foraging and proliferating would clearly be advantageous. We argue that these defenses repeatedly evolve in cancers and may be among the most advanced and important adaptations of cancers. By drawing parallels between several taxa exhibiting armor-like, spine-like, and noxious defenses, we present an overview of different ways these defenses can appear and emphasize how phenotypes that appear vastly different can nevertheless have the same essential functions. This cross-taxa comparison reveals how cancer phenotypes can be interpreted as anti-predator defenses, which can facilitate therapy approaches which aim to give the predators (the immune system) the upper hand. This cross-taxa comparison is also informative for evolutionary ecology. Cancer provides an opportunity to observe how prey evolve in the context of a unique predatory threat (the immune system) and varied environments.

Red and white muscle proportions and enzyme activities in mesopelagic sharks

In the last decade, there has been an increase in the study of the ecology of deep-sea organisms. One way to understand an organism's ecology is the study of its metabolism. According to literature, deep-sea sharks possess a lower anaerobic enzyme activity than their shallow-water counterparts, but no difference has been observed regarding their aerobic enzyme activities. These studies have suggested deep-sea sharks should be slow and listless swimmers. However, other studies based on video observations have revealed differences in cruise swimming speed between different species. The present study examined muscles of squaliform sharks, including both luminous and non-luminous species. We combined measurements of the relative amounts of red and white muscle with assays of enzymes that are used as markers for aerobic (citrate synthase, malate dehydrogenase) and anaerobic (lactate dehydrogenase) metabolism, searching for a relationship with cruising speeds. Non-luminous deep-sea species displayed lower aerobic enzyme activities but similar anaerobic enzyme activities than the benthic shallow-water counterpart (Squalus acanthias). Conversely, luminous Etmopteridae species were found to have similar aerobic enzyme activities to S. acanthias but displayed lower anaerobic enzyme activities. Analyses revealed that red muscle proportion and aerobic enzyme activities were positively related to the cruise swimming speed. In contrast, Dalatias licha, which swims at the slowest cruise swimming speed ever recorded, presented a very low aerobic metabolic phenotype (lower aerobic marker enzymes and less red muscle). Finally, the values obtained for white muscle proportion and anaerobic metabolic phenotype suggested a high burst capacity for D. licha and non-luminous sharks.

From reductionism to synthesis: The case of hagfish slime

Reductionist strategies aim to understand the mechanisms of complex systems by studying individual parts and their interactions. In this review, we discuss how reductionist approaches have shed light on the structure, function, and production of a complex biomaterial - hagfish defensive slime. Hagfish slime is an extremely dilute hydrogel-like material composed of seawater, mucus, and silk-like proteins that can deploy rapidly. Despite being composed almost entirely of water, hagfish slime has remarkable physical properties, including high strength and toughness. While hagfish slime has a promising future in biomimetics, including the development of eco-friendly high-performance fibers, recreating hagfish slime in the lab has been a difficult challenge. Over the past two decades, reductionist experiments have provided a wealth of information about the individual components of hagfish slime. However, a reductionist approach provides a limited understanding because hagfish defensive slime, like most biological phenomena, is more than just the sum of its parts. We end by providing some thoughts about how the knowledge generated in the last few decades might be synthesized into a working model that can explain hagfish slime structure and function.

Complex fluids in animal survival strategies

Animals have evolved distinctive survival strategies in response to constant selective pressure. In this review, we highlight how animals exploit flow phenomena by manipulating their habitat (exogenous) or by secreting (endogenous) complex fluids. Ubiquitous endogenous complex fluids such as mucus demonstrate rheological versatility and are therefore involved in many animal behavioral traits ranging from sexual reproduction to protection against predators. Exogenous complex fluids such as sand can be used either for movement or for predation. In all cases, time-dependent rheological properties of complex fluids are decisive for the fate of the biological behavior and vice versa. To exploit these rheological properties, it is essential that the animal is able to sense the rheology of their surrounding complex fluids in a timely fashion. As timing is key in nature, such rheological materials often have clearly defined action windows matching the time frame of their direct biological behavior. As many rheological properties of these biological materials remain poorly studied, we demonstrate with this review that rheology and material science might provide an interesting quantitative approach to study these biological materials in particular in context towards ethology and bio-mimicking material design.

Vertebrate Evolution Conserves Hindbrain Circuits despite Diverse Feeding and Breathing Modes

Feeding and breathing are two functions vital to the survival of all vertebrate species. Throughout the evolution, vertebrates living in different environments have evolved drastically different modes of feeding and breathing through using diversified orofacial and pharyngeal (oropharyngeal) muscles. The oropharyngeal structures are controlled by hindbrain neural circuits. The developing hindbrain shares strikingly conserved organizations and gene expression patterns across vertebrates, thus begs the question of how a highly conserved hindbrain generates circuits subserving diverse feeding/breathing patterns. In this review, we summarize major modes of feeding and breathing and principles underlying their coordination in many vertebrate species. We provide a hypothesis for the existence of a common hindbrain circuit at the phylotypic embryonic stage controlling oropharyngeal movements that is shared across vertebrate species; and reconfiguration and repurposing of this conserved circuit give rise to more complex behaviors in adult higher vertebrates.

Comparative biomechanics of hagfish skins: diversity in material, morphology, and movement

The baggy skins of hagfishes confer whole-body flexibility that enables these animals to tie themselves into knots without injury. The skin's looseness is produced by a subcutaneous blood sinus that decouples the skin and body core and permits the core to contort dramatically without loading the skin in tension or shear. Hagfish skin represents a biological composite material comparable in strength and stiffness to the conventionally taut skins of other fishes. However, our understanding of hagfish skin is restricted to only one of 78 species: The Pacific hagfish Eptatretus stoutii. To determine if other hagfish share similar characteristics with E. stoutii, we measured material properties and compared histological data sets from the skins of four hagfish species: E. springeri, E. stoutii, Myxine glutinosa, and M. hubbsi. We also compared these material properties data with skins from the American eel, Anguilla rostrata. We subjected skin samples from all species to uniaxial tensile tests in order to measure strength, stiffness, extensibility, and toughness of skins stretched along longitudinal and circumferential axes. We also used a series of equibiaxial tensile tests on skin samples from E. stoutii, M. glutinosa, and A. rostrata to measure stiffness of skins simultaneously strained along both axes. Significant results of uniaxial and biaxial tests show that the skins from Eptatretus are anisotropic, being stiffer in the longitudinal axis, and more extensible than the isotropic skins of Myxine. Skins of A. rostrata were stiffer in the circumferential axis and they were stronger, tougher, and stiffer than all hagfish skins examined. The skins of Eptatretus are histologically distinct from Myxine skins and possess arrays of fibers that stain like muscle. These interspecific differences across hagfish skins show a phylogenetic pattern with knotting kinematics and flexibility; both genera belong to distinct but major subfamilies within the Myxinidae, and Eptatretus is known for creating and manipulating a greater diversity of knotting styles than Myxine.

On the fit of skins with a particular focus on the biomechanics of loose skins of hagfishes

There is a considerable diversity in how skins fit. Here, we review the function of both tight and loose skins and note that the latter are poorly understood. Analysis of loose skin examples suggest five functional categories: (I) freedom of movement, (II) surface area enhancement, (III) increased structural extensibility, (IV) lubrication, and (V) maladaptive examples arising through sexual or artificial selection. We investigate the skins of hagfishes as a model for understanding loose skin function by examining its structure using histology, standardized puncture resistance testing using the ASTM F1306 protocol, and the effect of internal pressure using a simple inflated balloon model. Skins of hagfishes are composed of multiple layers of cross-helically wound connective tissue fibers of a 45° angle to the longitudinal axis, resulting in a skin that functions as fabric cut “on the bias”. Hagfish skins are relatively yielding; however, skin looseness adds a “structural extensibility” that may allow hagfishes to compensate for low puncture resistance. Physical balloon models, with stiff cores that limit length changes, show that only low pressures allow short loop radii without local buckling. Hagfishes represent ideal organisms for studying loose skin function because their skins seem to fit in all functionally adaptive categories.

The use of Atlantic hagfish (Myxine glutinosa) as a bioindicator species for studies on effects of dumped chemical warfare agents in the Skagerrak. 2. Biochemical biomarkers

The sea bottom of the Skagerrak Strait (North Sea) contains munitions loaded with chemical warfare agents (CWA), mostly stored in shipwrecks scuttled intentionally after the end of the World War II. The munition shells inside the wrecks are in different states of deterioration and corrosion and their environmental risk potential is unknown. The Atlantic hagfish (Myxine glutinosa), a sediment-dwelling chordate, was used as a model organism to study the potential impact of dumped CWA on the local ecosystem by using biochemical biomarkers. The hagfish were collected in 2017 and 2018 at three sampling sites: in the immediate vicinity of a wreck with CWA in the Skagerrak, a few kilometres from the wreck, and a reference site 21 km from the wreck, considered to be free of CWA. Significant differences were observed between the wreck site and the reference sites in the activities of glutathione reductase, superoxide dismutase and glutathione S-transferase, while the activity levels of catalase and acetylcholinesterase were identical at all sites. The recorded differences demonstrated negative biological effects in the hagfish sampled close to the dumped chemical munitions. Due to the limited knowledge of hagfish biology and of the extent of CWA contamination in Skagerrak, the results presented here warrant more research to further elucidate the potential environmental risks of the scuttled wrecks. The usefulness of the species as a bioindicator organism is further discussed.

Recent innovations in artificial skin

The skin is a "smart", multifunctional organ that is protective, self-healing and capable of sensing and many forms of artificial skins have been developed with properties and functionalities approximating those of natural skin. Starting from specific commercial products for the treatment of burns, progress in two fields of research has since allowed these remarkable materials to be viable skin replacements for a wide range of dermatological conditions. This review maps out the development of bioengineered skin replacements and synthetic skin substitutes, including electronic skins. The specific behaviors of these skins are highlighted, and the performances of both types of artificial skins are evaluated against this. Moving beyond mere replication, highly advanced artificial skin materials are also identified as potential augmented skins that can be used as flexible electronics for health-care monitoring and other applications.

A New Model of Hagfish Slime Mucous Vesicle Stabilization and Deployment

Hagfishes thwart predators by releasing large volumes of gill-clogging slime, which consists of mucus and silk-like fibers. The mucous fraction originates within gland mucous cells, which release numerous vesicles that swell and rupture when ejected into seawater. Several studies have examined the function of hagfish slime mucous vesicles in vitro, but a comprehensive model of their biophysics is lacking. Here, we tested the hypothesis that vesicles contain polyanionic glycoproteins stabilized by divalent cations and deploy in seawater via exchange of divalent for monovalent cations. We also tested the hypothesis that vesicle swelling and stabilization are governed by "Hofmeister effects". We found no evidence for either hypothesis. Our results show that hagfish mucous granules are only stabilized by multivalent anions, and pH titration experiments underscore these results. Our results lead us to the conclusion that the hagfish slime mucous gel is in fact polycationic in nature.

Concentration effects of three common fish anesthetics on Pacific hagfish (Eptatretus stoutii)

The efficacy of three common fish anesthetics (clove oil, 2-phenoxyethanol, and tricaine methanesulfonate) was evaluated in the Pacific hagfish (Eptatretus stoutii). The overarching aim of our study was to identify the best anesthetic and concentration for the purposes of routine laboratory use of Pacific hagfish (i.e., short and consistent induction and recovery times and minimized stress and safety risk to hagfish). The objectives of our study were fourfold: (1) identify anesthetic stages of Pacific hagfish using clove oil anesthesia; (2) establish standardized anesthesia preparation procedures; (3) determine the optimal anesthetic and concentration for safely achieving stage V anesthesia; and (4) investigate the effects of repeatedly exposing Pacific hagfish to anesthesia. Experimental concentrations, ranging from 50 to 400 mg/L, of each anesthetic were tested on at least three Pacific hagfish individuals. We found the following: (1) Pacific hagfish exhibited similar stages of anesthesia to those described for bony fishes; (2) sufficient mixing of clove oil with seawater had a considerable effect on the consistency and timing of anesthetic induction; (3) concentration and anesthetic significantly impacted induction and recovery timing, whereas body mass had no impact on anesthetic trends; and (4) repeatedly exposing Pacific hagfish to optimal concentrations of clove oil or MS-222 had no effect on induction or recovery timing, whereas exposure number significantly impacted induction timing when using 2-PE. Due to consistent induction and recovery times, low risk of accidental overdose, and high safety margins for both handler and hagfish, we recommend 175 mg/L of clove oil as the ideal anesthetic and concentration for the routine laboratory use of Pacific hagfish.

Unravelling hagfish slime

Hagfish slime is a unique predator defence material containing a network of long fibrous threads each ∼10 cm in length. Hagfish release the threads in a condensed coiled state known as skeins (∼100 µm), which must unravel within a fraction of a second to thwart a predator attack. Here we consider the hypothesis that viscous hydrodynamics can be responsible for this rapid unravelling, as opposed to chemical reaction kinetics alone. Our main conclusion is that, under reasonable physiological conditions, unravelling due to viscous drag can occur within a few hundred milliseconds, and is accelerated if the skein is pinned at a surface such as the mouth of a predator. We model a single skein unspooling as the fibre peels away due to viscous drag. We capture essential features by considering simplified cases of physiologically relevant flows and one-dimensional scenarios where the fibre is aligned with streamlines in either uniform or uniaxial extensional flow. The peeling resistance is modelled with a power-law dependence on peeling velocity. A dimensionless ratio of viscous drag to peeling resistance appears in the dynamical equations and determines the unraveling time scale. Our modelling approach is general and can be refined with future experimental measurements of peel strength for skein unravelling. It provides key insights into the unravelling process, offers potential answers to lingering questions about slime formation from threads and mucous vesicles, and will aid the growing interest in engineering similar bioinspired material systems.

The complete mitochondrial genome of a new deep-sea hagfish Eptatretus sp. Nan-Hai (Myxinidae: Eptatretus) from the South China Sea

In this study, the complete mitochondrial DNA sequence of a hagfish Eptatretus sp. Nan-Hai from a depth of 1000 m is presented. The complete sequence was determined using next-generation sequencing and long PCRs. The mitochondrial genome of Eptatretus sp. Nan-Hai is 17,538 bps in size and composed of 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNA genes, and one control region (D-loop). The base composition of mitochondrial genome is biased toward A + T content, at 67.21%, with GC skew of −0.35 and AT skew of −0.03. A phylogenetic tree revealed that within the genus Eptatretus, Eptatretus sp. Nan-Hai is closely related to Eptatretus atami.

Acquisition of alanyl-alanine in an Agnathan: Characteristics of dipeptide transport across the hindgut of the Pacific hagfish Eptatretus stoutii

This study used ³ H-_L -alanyl-_L -alanine to demonstrate dipeptide uptake using in vitro gut sacs prepared from the hindgut of the Pacific hagfish Eptatretus stoutii. Concentration-dependent kinetic analysis resulted in a sigmoidal distribution with a maximal (± SE) uptake rate (J_max -like) of 70 ± 3 nmol cm^-2 h^-1 and an affinity constant (K_m -like) of 1072 ± 81 μM. Addition of high alanine concentrations to transport assays did not change dipeptide transport rates, indicating that hydrolysis of the dipeptide in mucosal solutions and subsequent uptake via apical amino acid transporters was not occurring, which was further supported by a K_m distinct from that of amino acid transport. Transport occurred independent of mucosal pH, but uptake was reduced by 42% in low mucosal sodium. This may implicate cooperation between peptide transporters and sodium-proton exchangers, previously demonstrated in several mammalian and teleost species. Finally, apical _L -alanyl-_L -alanine uptake rates (i.e., mucosal disappearance) were significantly increased following a meal, demonstrating regulation of uptake. Overall, this examination of dipeptide acquisition in the earliest extant Agnathan suggests evolutionarily conserved mechanisms of transport between hagfish and later-diverging vertebrates such as teleosts and mammals.

High concentrations of trimethylamines in slime glands inhibit skein unraveling in Pacific hagfish

Hagfish defend themselves from fish predators by producing large volumes of gill-clogging slime when they are attacked. The slime consists of seawater and two major components that are ejected from the slime glands: mucus and threads. The threads are produced within specialized cells and packaged into intricately coiled bundles called skeins. Skeins are kept from unraveling via a protein adhesive that dissolves when the skeins are ejected from the slime glands. Previous work revealed that hagfish slime glands have high concentrations of methylamines including trimethylamine N-oxide (TMAO), trimethylglycine (betaine) and dimethylglycine (DMG); however, the function of these compounds in the slime glands is unknown. We hypothesized that methylamines have stabilizing effects on the skeins that prevent premature unraveling in the gland. To test this hypothesis, we quantified the effect of methylamines on skein unraveling in Pacific hagfish and found that TMAO and betaine have inhibitory effects on skein unraveling in vitro Furthermore, we found that TMAO is a more effective inhibitor of unraveling than betaine, but the presence of TMAO synergistically boosts the inhibitory action of betaine. Glycine and DMG were far less effective inhibitors of unraveling at natural concentrations. Our results support the hypothesis that high levels of trimethylamines in the slime glands may act to hold the coiled thread skeins together within gland thread cells, and they may do so by stabilizing adhesive proteins. These results advance our knowledge of skein stabilization and deployment and provide yet another example of trimethylamines functioning to stabilize proteins in a marine organism.

Structure and dynamics of hagfish mucin in different saline environments

The defense mechanism of hagfish against predators is based on its ability to form slime within a few milliseconds. Hagfish slime consists of two main components, namely mucin-like glycoproteins and long protein threads, which together entrap vast amounts of water and thus form a highly dilute hydrogel. Here, we investigate the mucin part of this hydrogel, in particular the role of the saline marine environment on the viscoelasticity and structure. By means of dynamic light scattering (DLS), shear and extensional rheology we probe the diffusion dynamics, the flow behavior, and the longest filament breaking time of hagfish mucin solutions. Using DLS we find a concentration-independent diffusion coefficient - characteristic for polyelectrolytes - up to the entanglement regime of 0.2 mg ml-1, which is about ten times higher than the natural concentration of hagfish mucin in hagfish slime. We also observe a slow relaxation process associated with clustering, probably due to electrostatic interactions. Shear rheology further revealed that hagfish mucin possesses pronounced viscoelastic properties at high concentrations (3 mg ml-1), showing that mucin alone achieves mechanical properties similar to those of natural hagfish slime (mucins and protein threads). The main effects of added seawater salts, and predominantly CaCl2 is to reduce the intensity of the slow relaxation process, which suggests that calcium ions lead to an ionotropic gelation of hagfish mucins.

Behavioural responses of the hagfish Eptatretus stoutii to nutrient and noxious stimuli

The suitability of a traditional testing paradigm (e.g. choice chamber) for assessing chemosensory behaviour in the Pacific hagfish, Eptatretus stoutii, was examined. Actively-swimming hagfish, tested at night, showed no preference for any region of a T-maze in the absence of a stimulus, but in the presence of an olfactory food cue, spent significantly more time in the zone where the cue was placed. Conversely, hagfish avoided spending time in the zone the fish anaesthetic 3-amino benzoic acid ethylester (MS-222) was placed, and demonstrated significantly more reversal responses in which the fish moved its body backwards. These data suggest that hagfish are an amenable model species for laboratory testing of behaviour.

The good, the bad and the slimy: experimental studies of hagfish digestive and nutritional physiology

The hagfishes provide valuable insight into the physiology of feeding, digestion and nutrient absorption by virtue of unusual and unique features of their biology. For example, members of this group undergo long periods of fasting, and are the only vertebrates known to absorb organic nutrients across their epidermal surface. Such properties engender significant attention from researchers interested in feeding and feeding-related processes; however, the practical realities of employing the hagfish as an experimental organism can be challenging. Many of the key tools of the experimental biologist are compromised by a species that does not readily feed in captivity, is difficult to instrument and which produces copious quantities of slime. This Commentary provides critical insight into the key aspects of hagfish feeding and digestive processes, and highlights the pitfalls of this group as experimental organisms. We also suggest key research gaps that, if filled, will lead to better understanding of hagfishes, and we consider how this group may advance our knowledge of feeding, digestion and nutrient absorption processes.

Activity and post-prandial regulation of digestive enzyme activity along the Pacific hagfish (Eptatretus stoutii) alimentary canal

Hagfishes are living representatives of the earliest-diverging vertebrates and are thus useful for the study of early vertebrate physiology. It has been previously postulated that digestive enzymes account for the majority of digestion because hagfish are agastric with notable zymogen granules in specialized cells of the hindgut. While the presence of some digestive enzymes (amylase, lipase and leucinaminopeptidase) have been confirmed with histochemistry, quantification of enzymatic activity is limited. This study sought to biochemically quantify the tissue activity of six digestive enzymes (α-amylase, maltase, lipase, trypsin, aminopeptidase and alkaline phosphatase) along the length of the Pacific hagfish (Eptatretus stoutii) alimentary canal. In addition, the effect of feeding on the rate of enzyme activity was examined. Overall, maltase and trypsin activities were unchanging with respect to location or feeding status, while the activities of α-amylase and alkaline phosphatase decreased substantially following feeding, but were consistent along the length. Lipase and aminopeptidase activities were elevated in the anterior region of the alimentary canal in comparison to the more posterior regions, but were not altered with feeding. This study indicates hagfish have an assortment of digestive enzymes that likely are the result of a varied diet. The differential expression of these enzymes along the tract and in regards to feeding may be indications of early compartmentalization of digestive function.

Unraveling inter-species differences in hagfish slime skein deployment

Hagfishes defend themselves from fish predators by producing defensive slime consisting of mucous and thread components that interact synergistically with seawater to pose a suffocation risk to their attackers. Deployment of the slime occurs in a fraction of a second and involves hydration of mucous vesicles as well as unraveling of the coiled threads to their full length of ∼150 mm. Previous work showed that unraveling of coiled threads (or ‘skeins’) in Atlantic hagfish requires vigorous mixing with seawater as well as the presence of mucus, whereas skeins from Pacific hagfish tend to unravel spontaneously in seawater. Here, we explored the mechanisms that underlie these different unraveling modes, and focused on the molecules that make up the skein glue, a material that must be disrupted for unraveling to proceed. We found that Atlantic hagfish skeins are also held together with a protein glue, but compared with Pacific hagfish glue, it is less soluble in seawater. Using SDS-PAGE, we identified several soluble proteins and glycoproteins that are liberated from skeins under conditions that drive unraveling in vitro. Peptides generated by mass spectrometry of five of these proteins and glycoproteins mapped strongly to 14 sequences assembled from Pacific hagfish slime gland transcriptomes, with all but one of these sequences possessing homologs in the Atlantic hagfish. Two of these sequences encode unusual acidic proteins that we propose are the structural glycoproteins that make up the skein glue. These sequences have no known homologs in other species and are likely to be unique to hagfishes. Although the ecological significance of the two modes of skein unraveling described here are unknown, they may reflect differences in predation pressure, with selection for faster skein unraveling in the Eptatretus lineage leading to the evolution of a glue that is more soluble.

Concentration-independent mechanics and structure of hagfish slime

The defense mechanism of hagfish slime is remarkable considering that hagfish cannot control the concentration of the resulting gel directly; they simply exude a concentrated material into a comparably "infinite" sea of water to form a dilute, sticky, cohesive elastic gel. This raises questions about the robustness of gel formation and rheological properties across a range of concentrations, which we study here for the first time. Across a nearly 100-fold change in concentration, we discover that the gel has similar viscoelastic time-dependent properties with constant power-law exponent (α=0.18±0.01), constant relative damping tanδ=G^''/G^'≈0.2-0.3, and varying overall stiffness that scales linearly with the concentration (∼c^0.99±0.05). The power-law viscoelasticity (fit by a fractional Kelvin-Voigt model) is persistent at all concentrations with nearly constant fractal dimension. This is unlike other materials and suggests that the underlying material structure of slime remains self-similar irrespective of concentration. This interpretation is consistent with our microscopy studies of the fiber network. We derive a structure-rheology model to test the hypothesis that the origins of ultra-soft elasticity are based on bending of the fibers. The model predictions show an excellent agreement with the experiments. Our findings illustrate the unusual and robust properties of slime which may be vital in its physiological use and provide inspiration for the design of new engineered materials.

STATEMENT OF SIGNIFICANCE

Hagfish produce a unique gel-like material to defend themselves against predator attacks. The successful use of the defense gel is remarkable considering that hagfish cannot control the concentration of the resulting gel directly; they simply exude a small quantity of biomaterial which then expands by a factor of 10,000 (by volume) into an "infinite" sea of water. This raises questions about the robustness of gel formation and properties across a range of concentrations. This study provides the first ever understanding of the mechanics of hagfish slime over a very wide range of concentration. We discover that some viscoelastic properties of slime are remarkably constant regardless of its concentration. Such a characteristic is uncommon in most known materials.

Fish associated with a subsea pipeline and adjacent seafloor of the North West Shelf of Western Australia

Information on the potential ecological value of offshore oil and gas infrastructure is required as it reaches the end of its operational life and decisions must be made regarding the best practice option for decommissioning. This study uses baited remote underwater stereo-video systems to assess fish assemblages along an offshore subsea pipeline and in adjacent natural seabed habitats at ∼140 m depth on the North West Shelf of Western Australia. A total of 955 fish from 40 species and 25 families were recorded. Species richness was, on average 25% higher on the pipeline (6.48 ± 0.37 SE) than off (4.81 ± 0.28 SE) while relative abundance of fish was nearly double on the pipeline (20.38 ± 2.81 SE) than in adjacent natural habitats (10.97 ± 1.02 SE). The pipeline was characterised by large, commercially important species known to associate with complex epibenthic habitat and, as such, possessed a biomass of commercial fish ca 7.5 × higher and catch value ca. 8.6 × ($65.11 ± $11.14 SE) than in adjacent natural habitats ($7.57 ± $2.41 SE). This study has added to the knowledge of fish assemblage associations with subsea infrastructure and provides a greater understanding of the ecological and fisheries implications of decommissioning, helping to better inform decision-making on the fate of infrastructure.

Flaccid skin protects hagfishes from shark bites

Hagfishes defend themselves from fish predators by releasing large volumes of gill-clogging slime when they are attacked. Slime release is not anticipatory, but is only released after an attack has been initiated, raising the question of how hagfishes survive the initial attack, especially from biting predators such as sharks. We tested two hypotheses that could explain how hagfishes avoid damage from shark bites: puncture-resistant skin, and a loose and flaccid body design that makes it difficult for teeth to penetrate body musculature and viscera. Based on data from skin puncture tests from 22 fish species, we found that hagfish skin is not remarkably puncture resistant. Simulated shark bites on hagfish and their closest living relatives, lamprey, as well as whole animal inflation tests, revealed that the loose attachment of hagfish skin to the rest of the body and the substantial 'slack volume' in the subcutaneous sinus protect hagfish musculature and viscera from penetrating teeth. While recent work has found evidence that the capacious subcutaneous sinus in hagfishes is important for behaviours such as knot-tying and burrowing, our work demonstrates that it also plays a role in predator defence.

Effect of ionic strength and seawater cations on hagfish slime formation

The defensive slime of hagfish consists of a polyanionic mucin hydrogel that synergistically interacts with a fiber network forming a coherent and elastic hydrogel in high ionic strength seawater. In seawater, the slime deploys in less than a second entrapping large quantities of water by a well-timed thread skein unravelling and mucous gel swelling. This rapid and vast hydrogel formation is intriguing, as high ionic strength conditions generally counteract the swelling speed and ratio of polyelectrolyte hydrogels. In this work we investigate the effect of ionic strength and seawater cations on slime formation dynamics and functionality. In the absence of ionic strength skeins swell radially and unravel uncontrolled, probably causing tangling and creating a confined thread network that entraps limited water. At high ionic strength skeins unravel, but create a collapsed and dense fiber network. High ionic strength conditions therefore seem crucial for controlled skein unraveling, however not sufficient for water retention. Only the presence of naturally occurring Ca2+ or Mg2+-ions allowed for an expanded network and full water retention probably due to Ca2+-mediated vesicle rupture and cross-linking of the mucin. Our study demonstrates that hagfish slime deployment is a well-timed, ionic-strength, and divalent-cation dependent dynamic hydrogel formation process.

Emptying and refilling of slime glands in Atlantic (Myxine glutinosa) and Pacific (Eptatretus stoutii) hagfishes

Hagfishes are known for their unique defensive slime, which they use to ward off gill breathing predators. While much is known about the slime cells (gland thread cells and gland mucous cells), little is known about how long slime gland refilling takes, or how slime composition changes with refilling or repeated stimulation of the same gland. Slime glands can be individually electro-stimulated to release slime, and this technique was used to measure slime gland refilling times for Atlantic and Pacific hagfish. The amount of exudate produced, the composition of exudate, and the morphometrics of slime cells were analyzed during refilling, and as a function of stimulation number when full glands were stimulated in rapid succession. Complete refilling of slime glands for both species took three to four weeks, with Pacific hagfish achieving faster absolute rates exudate recovery than Atlantics. We found significant changes in composition of exudate and morphometrics of slime cells from Pacific hagfish during refilling. Over successive stimulations of full Pacific glands, multiple boluses of exudate were released, with exudate composition, but not thread cell morphometrics, changing significantly. Finally, histological examination of slime glands revealed slime cells retained in glands after exhaustion. Discrepancies in volume of cells released that can be explained by contraction of striated muscle alone suggests other mechanisms may be involved in the exudate ejection. Our results provide a first look at the process and timing of slime gland refilling in hagfishes, and raise new questions about how refilling is achieved at the cellular level.

Cellular mechanisms of slime gland refilling in Pacific hagfish (Eptatretus stoutii)

Hagfishes use their defensive slime to ward off gill-breathing predators. Slime gland refilling is a surprisingly slow process, and previous research has shown that the composition of the slime exudate changes significantly during refilling, which likely has consequences for the functionality of the slime. This study set out to expand our understanding of slime gland refilling by examining the cellular processes involved in refilling of the glands, as well as determining where in the gland the main slime cells, the gland thread cells and gland mucous cells, arise. Slime glands were electro-stimulated to exhaust their slime stores, left to refill for set periods of time, and harvested for histological and immunohistochemical examination. Whole slime glands, gland thread cell morphometrics and slime cell proportions were examined over the refilling cycle. Slime glands decreased significantly in size after exhaustion, but steadily increased in size over refilling. Gland thread cells were the limiting factor in slime gland refilling, taking longer to replenish and mature than gland mucous cells. Newly produced gland thread cells underwent most of their growth near the edge of the gland, and larger cells were found farthest from the edge of the gland. Immunohistochemical analysis also revealed proliferating cells only within the epithelial lining of the slime gland, suggesting that new slime cells originate from undifferentiated cells lining the gland. Our results provide an in-depth look at the cellular dynamics of slime gland refilling in Pacific hagfish, and provide a model for how slime glands refill at the cellular level.