Epidermal microstructures on the paired fins of marine sculpins suggest new functional hypotheses supporting benthic station-holding

Harsh environments, such as those with breaking waves and turbulent flows, present extreme challenges to organismal survival. Many animals exploiting these habitats possess adaptations to maintain position under dynamic flow conditions, such as reversible or permanent attachment systems. However, some station-holding fishes (e.g. sculpins) instead rely on morphological and behavioural modifications of their pectoral fins to increase friction with the substrate and combat drag. Despite epidermal microstructures on the fins of other benthic fishes, little exploration of pectoral fin surfaces at the microscopic scale has been undertaken in sculpins. Using scanning electron microscopy, we discovered microscopic, fibrillar projections contained within single cells on the ventral surfaces of the paired fin rays of two intertidal and two subtidal species of marine sculpins. In contrast to subtidal species, the intertidal species possessed epidermal cells with discrete channels separating groups of fibrillar projections. These features bear a striking resemblance to epidermal microstructures described in other fishes but have distinct morphological differences. We suggest the hypothesis that these previously overlooked features contribute to sculpin station-holding performance via enhanced mechanical interactions with the substrate, suggesting new taxa within which to explore potential mechanisms of underwater friction enhancement and adhesion.

Nature-Inspired Wet Drug Delivery Platforms

Nature has created various organisms with unique chemical components and multi-scale structures (e.g., foot proteins, toe pads, suckers, setose gill lamellae) to achieve wet adhesion functions to adapt to their complex living environments. These organisms can provide inspirations for designing wet adhesives with mediated drug release behaviors in target locations of biological surfaces. They exhibit conformal and enhanced wet adhesion, addressing the bottleneck of weaker tissue interface adhesion in the presence of body fluids. Herein, it is focused on the research progress of different wet adhesion and bioinspired fabrications, including adhesive protein-based adhesion and inspired adhesives (e.g., mussel adhesion); capillarity and Stefan adhesion and inspired adhesive surfaces (e.g., tree frog adhesion); suction-based adhesion and inspired suckers (e.g., octopus' adhesion); interlocking and friction-based adhesion and potential inspirations (e.g., mayfly larva and teleost adhesion). Other secreted protein-induced wet adhesion is also reviewed and various suckers for other organisms and their inspirations. Notably, one representative application scenario of these bioinspired wet adhesives is highlighted, where they function as efficient drug delivery platforms on target tissues and/or organs with requirements of both controllable wet adhesion and optimized drug release. Finally, the challenges of these bioinspired wet drug delivery platforms in the future is presented.

Nature-inspired adhesive systems

Many organisms in nature thrive in intricate habitats through their unique bio-adhesive surfaces, facilitating tasks such as capturing prey and reproduction. It's important to note that the remarkable adhesion properties found in these natural biological surfaces primarily arise from their distinct micro- and nanostructures and/or chemical compositions. To create artificial surfaces with superior adhesion capabilities, researchers delve deeper into the underlying mechanisms of these captivating adhesion phenomena to draw inspiration. This article provides a systematic overview of various biological surfaces with different adhesion mechanisms, focusing on surface micro- and nanostructures and/or chemistry, offering design principles for their artificial counterparts. Here, the basic interactions and adhesion models of natural biological surfaces are introduced first. This will be followed by an exploration of research advancements in natural and artificial adhesive surfaces including both dry adhesive surfaces and wet/underwater adhesive surfaces, along with relevant adhesion characterization techniques. Special attention is paid to stimulus-responsive smart artificial adhesive surfaces with tunable adhesive properties. The goal is to spotlight recent advancements, identify common themes, and explore fundamental distinctions to pinpoint the present challenges and prospects in this field.

Holding in the stream: convergent evolution of suckermouth structures in Loricariidae (Siluriformes)

Suckermouth armoured catfish (Loricariidae) are a highly speciose and diverse freshwater fish family, which bear upper and lower lips forming an oral disc. Its hierarchical organisation allows the attachment to various natural surfaces. The discs can possess papillae of different shapes, which are supplemented, in many taxa, by small horny projections, i.e. unculi. Although these attachment structures and their working mechanisms, which include adhesion and interlocking, are rather well investigated in some selected species, the loricariid oral disc is unfortunately understudied in the majority of species, especially with regard to comparative aspects of the diverse oral structures and their relationship to the ecology of different species. In the present paper, we investigated the papilla and unculi morphologies in 67 loricariid species, which inhabit different currents and substrates. We determined four papilla types and eight unculi types differing by forms and sizes. Ancestral state reconstructions strongly suggest convergent evolution of traits. There is no obvious correlation between habitat shifts and the evolution of specific character states. From handling the structures and from drying artefacts we could infer some information about their material properties. This, together with their shape, enabled us to carefully propose hypotheses about mechanisms of interactions of oral disc structures with natural substrates typical for respective fish species.

Maneuverable gait selection for a novel fish-inspired robot using a CMA-ES-assisted workflow

Among underwater vehicles, fish-inspired designs are often selected for their efficient gaits; these designs, however, remain limited in their maneuverability, especially in confined spaces. This paper presents a new design for a fish-inspired robot with two degree-of-freedom pectoral fins and a single degree-of-freedom caudal fin. This robot has been designed to operate in open-channel canals in the presence of external disturbances. With the complex interactions of water in mind, the composition of goal-specific swimming gaits is trained via a machine learning workflow in which automated trials in the lab are used to select a subset of potential gaits for outdoor trials. The goal of this process is to minimize the time cost of outdoor experimentation through the identification and transfer of high-performing gaits with the understanding that, in the absence of complete replication of the intended target environment, some or many of these gaits must be eliminated in the real world. This process is motivated by the challenge of balancing the optimization of complex, high degree-of-freedom robots for disturbance-heavy, random, niche environments against the limitations of current machine learning techniques in real-world experiments, and has been used in the design process as well as across a number of locomotion goals. The key contribution of this paper involves finding strategies that leverage online learning methods to train a bio-inspired fish robot by identifying high-performing gaits that have a consistent performance both in the laboratory experiments and the intended operating environment. Using the workflow described herein, the resulting robot can reach a forward swimming speed of 0.385 m s^-1(0.71 body lengths per second) and can achieve a near-zero turning radius.

Hydraulic flow resistance of epigean and hypogean fish of the family Trichomycteridae (Ostariophysi, Siluriformes)

Critical swimming speeds of four trichomycterid fish species from epigean and hypogean environments were analyzed and compared: Trichomycterus itacarambiensis and Ituglanis passensis, both troglobitic from underground rivers; Trichomycterus brasiliensis, from epigean rivers; and Ituglanis sp., an undescribed troglophilic species from an underground stream. Swimming tests were conducted with a non-volitional apparatus in which fish swim against a progressive incremental water velocity until they longer resist the flow. Total length was significantly related to critical speed for only T. itacarambiensis. The critical speed obtained by each species, in decreasing order, with values in lengths per second (lengths/s), were: I. passensis (3.61), T. itacarambiensis (3.49), T. brasiliensis (3.11) and Ituglanis sp. (1.89). Swimming performance differed between the congeners T. itacarambiensis and T. brasiliensis, but did not differed between I. passensis and Ituglanis sp. The greater speed for the troglobitic species compared to that of the troglophilic and epigean species is probably related to seasonal flooding pulses that can be extremely severe in caves. Furthermore, during the tests, fish were observed using their mouth and/or barbels to fasten themselves to the substrate to avoid high flows.

Learning from Northern clingfish (Gobiesox maeandricus): bioinspired suction cups attach to rough surfaces

While artificial suction cups only attach well to smooth surfaces, the Northern clingfish can attach to surfaces ranging from nanoscale smooth to rough stone. This ability is highly desirable for technical applications. The morphology of the fish's suction disc and its ability to attach to rough and slimy surfaces have been described before, and here we aim to close gaps in the biomechanical understanding, and transfer the biomechanical principles to technical suction cups. We demonstrate that the margin of the suction disc is the critical feature enabling attachment to rough surfaces. Second, friction measurements show that friction of the disc rim is increased on rough substrates and contributes to high tenacity. Increased friction causes a delay in failure of the suction cup and increases the attachment force. We were able to implement these concepts to develop the first suction cups bioinspired by Northern clingfish. These cups attach with tenacities up to 70 kPa on surfaces as rough as 270 µm grain size. The application of this technology is promising in fields such as surgery, industrial production processes and whale tagging. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

The effect of substratum type on aspects of swimming performance and behaviour in shortnose sturgeon Acipenser brevirostrum

The swimming performance and associated swimming behaviour (i.e. substratum-skimming, station-holding and free swimming) were assessed in shortnose sturgeon Acipenser brevirostrum during critical swimming and endurance swimming tests over a rough and a smooth substratum. It was hypothesized that the addition of a rough substratum in the swimming flume may provide a surface for the A. brevirostrum to grip and offer an energetic advantage. Substratum type did not affect the critical swimming performance, but A. brevirostrum consistently performed more bottom behaviours (i.e. substratum-skimming and station-holding) while on a smooth substratum. Acipenser brevirostrum had little contact with the rough substratum until the velocity was >1 body length s^-1 . Endurance swimming time was significantly lower for A. brevirostrum over the rough bottom at the highest velocity (30 cm s^-1 ) which may be attributed to the observed increase in free swimming and decrease in bottom behaviours. During endurance swimming, the rough substratum was mainly used at intermediate velocities, suggesting that there may be a stability cost associated with being in contact with the rough substratum at certain velocities.

Attachment to challenging substrates--fouling, roughness and limits of adhesion in the northern clingfish (Gobiesox maeandricus)

Northern clingfish use a ventral suction disc to stick to rough substrates in the intertidal zone. Bacteria, algae and invertebrates grow on these surfaces (fouling) and change the surface properties of the primary substrate, and therefore the attachment conditions for benthic organisms. In this study, we investigate the influence of fouling and surface roughness on the adhesive strength of northern clingfish, Gobiesox maeandricus. We measured clingfish tenacity on unfouled and fouled substrates over four surface roughnesses. We exposed surfaces for 6 weeks in the Pacific Ocean, until they were covered with periphyton. Clingfish tenacity is equivalent on both fouled and unfouled smooth substrates; however, tenacity on fouled rough surfaces is less compared with tenacity on unfouled ones. We hypothesize that parts of biofilm may act as a lubricant and decrease friction of the disc margin, thereby making disc margins slip inwards and fail at lower tenacities. Nevertheless, even on fouled surfaces the adhesive forces are approximately 150 times the body weight of the fish. To identify the upper threshold of surface roughness the fish can cling to, we tested seven unfouled substrates of increasing surface roughness. The threshold roughness at which northern clingfish failed increased with specimen size. We hypothesize that because of the elastic properties of the disc margin, a larger disc can adapt to larger surface irregularities. The largest specimens (length 10-12 cm) were able to cling to surfaces with 2-4 mm grain size. The fish can attach to surfaces with roughness between 2 and 9% of the suction disc width.

Aquatic versus terrestrial attachment: Water makes a difference

Animal attachment to a substrate is very different in terrestrial and aquatic environments. We discuss variations in both the forces acting to detach animals and forces of attachment. While in a terrestrial environment gravity is commonly understood as the most important detachment force, under submerged conditions gravity is nearly balanced out by buoyancy and therefore matters little. In contrast, flow forces such as drag and lift are of higher importance in an aquatic environment. Depending on the flow conditions, flow forces can reach much higher values than gravity and vary in magnitude and direction. For many of the attachment mechanisms (adhesion including glue, friction, suction and mechanical principles such as hook, lock, clamp and spacer) significant differences have to be considered under water. For example, the main principles of dry adhesion, van der Waals forces and chemical bonding, which make a gecko stick to the ceiling, are weak under submerged conditions. Capillary forces are very important for wet adhesion, e.g., in terrestrial beetles or flies, but usually do not occur under water. Viscous forces are likely an important contributor to adhesion under water in some mobile animals such as torrent frogs and mayflies, but there are still many open questions to be answered. Glue is the dominant attachment mechanism of sessile aquatic animals and the aquatic realm presents many challenges to this mode of attachment. Viscous forces and the lack of surface tension under submerged conditions also affect frictional interactions in the aquatic environment. Moreover, the limitation of suction to the pressure difference at vacuum conditions can be ameliorated under water, due to the increasing pressure with water depth.

Evolutionary novelty versus exaptation: oral kinematics in feeding versus climbing in the waterfall-climbing Hawaiian Goby Sicyopterus stimpsoni

Species exposed to extreme environments often exhibit distinctive traits that help meet the demands of such habitats. Such traits could evolve independently, but under intense selective pressures of extreme environments some existing structures or behaviors might be coopted to meet specialized demands, evolving via the process of exaptation. We evaluated the potential for exaptation to have operated in the evolution of novel behaviors of the waterfall-climbing gobiid fish genus Sicyopterus. These fish use an “inching” behavior to climb waterfalls, in which an oral sucker is cyclically protruded and attached to the climbing surface. They also exhibit a distinctive feeding behavior, in which the premaxilla is cyclically protruded to scrape diatoms from the substrate. Given the similarity of these patterns, we hypothesized that one might have been coopted from the other. To evaluate this, we filmed climbing and feeding in Sicyopterus stimpsoni from Hawai’i, and measured oral kinematics for two comparisons. First, we compared feeding kinematics of S. stimpsoni with those for two suction feeding gobiids (Awaous guamensis and Lentipes concolor), assessing what novel jaw movements were required for algal grazing. Second, we quantified the similarity of oral kinematics between feeding and climbing in S. stimpsoni, evaluating the potential for either to represent an exaptation from the other. Premaxillary movements showed the greatest differences between scraping and suction feeding taxa. Between feeding and climbing, overall profiles of oral kinematics matched closely for most variables in S. stimpsoni, with only a few showing significant differences in maximum values. Although current data cannot resolve whether oral movements for climbing were coopted from feeding, or feeding movements coopted from climbing, similarities between feeding and climbing kinematics in S. stimpsoni are consistent with evidence of exaptation, with modifications, between these behaviors. Such comparisons can provide insight into the evolutionary mechanisms facilitating exploitation of extreme habitats.

Pelvic fins in teleosts: structure, function and evolution

The pelvic fins of teleosts are paired appendages that are considered to be homologous to the hind limbs of tetrapods. Because they are less important for swimming, their morphology and function can be flexibly modified, and such modifications have probably facilitated the adaptations of teleosts to various environments. Recently, among these modifications, pelvic-fin loss has gained attention in evolutionary developmental biology. Pelvic-fin loss, however, has only been investigated in a few model species, and various biological aspects of pelvic fins in teleosts in general remain poorly understood. This review summarizes the current state of knowledge regarding pelvic fins, such as their structure, function and evolution, to elucidate their contribution to the considerable diversity of teleosts. This information could be invaluable for future investigations into various aspects of pelvic fins, which will provide clues to understanding the evolution, diversity and adaptations of teleosts.

Behaviour and performance of juvenile shortnose sturgeon Acipenser brevirostrum at different water velocities

Critical swimming speeds (mean +/-s.e.) for juvenile shortnose sturgeon Acipenser brevirostrum were 34.4 cm s(-1)+/- 1.7 (2.18 +/- 0.09 body lengths, BL s(-1)). Swimming challenges at 10, 20 and 30 cm s(-1) revealed that juvenile A. brevirostrum are relatively poor swimmers, and that the fish did not significantly modify their swimming behaviour, although they spent more time substratum skimming (i.e. contact with flume floor) at 30 cm s(-1) relative to 10 cm s(-1). When present, these behavioural responses are probably related to morphological features, such as flattened rostrum, large pectoral fins, flattened body shape and heterocercal tail, and may be important to reduce the costs of swimming.

Pacific lamprey climbing behavior

New lamprey-friendly fishways feature inclined ramps that facilitate passage of Pacific lampreys ( Lampetra tridentata (Richardson, 1836)) over Bonneville Dam on the Columbia River, USA. We observed the lampreys moving against water at two flow volumes and on two ramps of 45° and 18° angles relative to horizontal. We documented climbing movements using high-speed video (125 frames/s). Lampreys advanced on the ramps by repeated cycles of attaching to the ramps by their sucker mouths (resting phase), bending their bodies into a W shape (stage II), and then, rapidly straightening the body to propel themselves up the ramp, with simultaneous brief (20–140 ms) release of suction (stage III). We inferred that lampreys were using burst swimming to propel themselves up the ramp, because we observed inflection points in the body curvature traveling toward the posterior of the body and the center of mass moving up, during stage III. This climbing behavior is not described for any other fish species. Vertical motion, relative to the ground, during each cycle of movement was greatest in the 45° ramp – low water flow volume treatment (mean of 0.07 L/cycle), but the movement upstream along the ramp plane was greatest on the 18° ramp, regardless of flow volume. These findings can be used to develop ramp designs that maximize lamprey climbing performance.