Push or Pull? The light-weight architecture of the Daphnia pulex carapace is adapted to withstand tension, not compression

Distinct cell proliferation patterns underlying the development of defensive crests in Daphnia longicephala

The freshwater crustacean Daphnia is well known for its expression of morphological defenses in the presence of predators. Research into this phenomenon has mostly centered on the ecology and evolution of Daphnia defenses; information is limited on the cellular mechanisms that underlie site-specific tissue growth. We aimed to determine these cellular mechanisms, specifically those associated with the development of defensive crests in D. longicephala. With the help of a cell-proliferation assay we monitored changes in the epidermal tissue of naïve and predator-exposed D. longicephala. Based on our results, we propose that cell division is delayed in favor of cell growth, which results in crest formation. Further, we identify specific regions of proliferative activity in a time-dependent manner. Defense development starts in the ventral region, before extending in the cranial and then dorsal directions. We demonstrate that these cellular changes begin as early as 2 h after predator exposure. Our results provide new insights into the cellular processes underlying morphological defense expression in Daphnia.

Facing the Green Threat: A Water Flea's Defenses against a Carnivorous Plant

Every ecosystem shows multiple levels of species interactions, which are often difficult to isolate and to classify regarding their specific nature. For most of the observed interactions, it comes down to either competition or consumption. The modes of consumption are various and defined by the nature of the consumed organism, e.g., carnivory, herbivory, as well as the extent of the consumption, e.g., grazing, parasitism. While the majority of consumers are animals, carnivorous plants can also pose a threat to arthropods. Water fleas of the family Daphniidae are keystone species in many lentic ecosystems. As most abundant filter feeders, they link the primary production to higher trophic levels. As a response to the high predatory pressures, water fleas have evolved various inducible defenses against animal predators. Here we show the first example, to our knowledge, in Ceriodaphnia dubia of such inducible defenses of an animal against a coexisting plant predator, i.e., the carnivorous bladderwort (Utricularia x neglecta Lehm, Lentibulariaceae). When the bladderwort is present, C. dubia shows changes in morphology, life history and behavior. While the morphological and behavioral adaptations improve C. dubia's survival rate in the presence of this predator, the life-history parameters likely reflect trade-offs for the defense.

Pricklier with the proper predator? Predator-induced small-scale changes of spinescence in Daphnia

Phenotypic plasticity in defensive traits is a common response of prey organisms to variable and unpredictable predation regimes and risks. Cladocerans of the genus Daphnia are keystone species in the food web of lentic freshwater bodies and are well known for their ability to express a large variety of inducible morphological defenses in response to invertebrate and vertebrate predator kairomones. The developed defenses render the daphnids less susceptible to predation. So far, primarily large-scale morphological defenses, like helmets, crests, and tail-spines, have been documented. However, less is known on whether the tiny spinules, rather inconspicuous traits which cover many Daphnia's dorsal and ventral carapace margins, respond to predator kairomones, as well. For this reason, we investigated two Daphnia species (D. magna and D. longicephala) concerning their predator kairomone-induced changes in dorsal and ventral spinules. Since these small, inconspicuous traits may only act as a defense against predatory invertebrates, with fine-structured catching apparatuses, and not against vertebrate predators, we exposed them to both, an invertebrate (Triops cancriformis or Notontecta maculata) and a vertebrate predator (Leucaspius delineatus). Our results show that the length of these spinules as well as spinules-covered areas vary, likely depending on the predator the prey is exposed to. We further present first indications of a Daphnia species-specific elongation of the spinules and an increase of the spinules-bearing areas. Although we cannot exclude that spinescence is altered because it is developmentally connected to changes in body shape in general, our results suggest that the inducible alterations to the spinule length and spinules-covered areas disclose another level of predator-induced changes in two common Daphnia species. The predator-induced changes on this level together with the large-scale and ultrastructural defensive traits may act as the overall morphological defense, adjusted to specific predator regimes in nature.

Uncovering the chemistry behind inducible morphological defences in the crustacean Daphnia magna via micro-Raman spectroscopy

The widespread distribution of Crustacea across every aquatic ecological niche on Earth is enabled due to their exoskeleton's versatile properties. Especially mineralization of the exoskeleton provides protection against diverse environmental threats. Thereby, the exoskeleton of some entomostracans is extremely phenotypically plastic, especially in response to predators. For instance, the freshwater zooplankton Daphnia forms conspicuous inducible morphological defenses, such as helmets, and can increase the stability of its exoskeleton, which renders them less vulnerable to predation. In this study, we reveal for the first time the chemical composition of the exoskeleton of Daphnia magna, using Raman spectroscopy, to be composed of α-chitin and proteins with embedded amorphous calcium carbonate (ACC). Furthermore, we reveal the exoskeleton's chemical changes associated with inducible defense mechanisms in the form of more substantial mineralization, which is probably correlated with enhanced carapace stability. We, therefore, highlight the importance of calcium-biominerals for inducible morphological defenses in Daphnia.

Ontogeny and potential function of poacher armor (Actinopterygii: Agonidae)

Many vertebrates are armored over all or part of their body. The armor may serve several functional roles including defense, offense, visual display, and signal of experience/capability. Different roles imply different tradeoffs; for example, defensive armor usually trades resistance to attack for maneuverability. The poachers (Agonidae), 47 species of scorpaeniform fishes, are a useful system for understanding the evolution and function of armor due to their variety and extent of armoring. Using publically available CT-scan data from 27 species in 16 of 21 genera of poachers we compared the armor to axial skeletal in the mid body region. The ratio of average armor density to average skeleton density ranged from 0.77 to 1.17. From a defensive point of view, the total investment in mineralization (volume * average density) is more interesting. There was 10 times the material invested in the armor as in the endoskeleton in some small, smooth plated species, like Aspidophoroides olrikii. At the low end, some visually arresting species like Percis japonica, had ratios as low as 2:1. We categorized the extent and type (impact vs. abrasion) in 34 Agonopsis vulsa across all 35+ plates in the eight rows along the body. The ventral rows show abrasive damage along the entire length of the fish that gets worse with age. Impact damage to head and tail plates gets more severe and occurs at higher rates with age. The observed damage rates and the large investment in mineralization of the armor suggest that it is not just for show, but is a functional defensive structure. We cannot say what the armor is defense against, but the abrasive damage on the ventrum implies their benthic lifestyle involves rubbing on the substrate. The impact damage could result from predatory attacks or from intraspecific combat.

Knowing the Enemy: Inducible Defences in Freshwater Zooplankton

Phenotypic plasticity in defensive traits is an appropriate mechanism to cope with the variable hazard of a frequently changing predator spectrum. In the animal kingdom these so-called inducible defences cover the entire taxonomic range from protozoans to vertebrates. The inducible defensive traits range from behaviour, morphology, and life-history adaptations to the activation of specific immune systems in vertebrates. Inducible defences in prey species play important roles in the dynamics and functioning of food webs. Freshwater zooplankton show the most prominent examples of inducible defences triggered by chemical cues, so-called kairomones, released by predatory invertebrates and fish. The objective of this review is to highlight recent progress in research on inducible defences in freshwater zooplankton concerning behaviour, morphology, and life-history, as well as difficulties of studies conducted in a multipredator set up. Furthermore, we outline costs associated with the defences and discuss difficulties as well as the progress made in characterizing defence-inducing cues. Finally, we aim to indicate further possible routes in this field of research and provide a comprehensive table of inducible defences with respect to both prey and predator species.

Biomechanical analysis of prey capture in the carnivorous Southern bladderwort (Utricularia australis)

We recorded capture events (CEs) of the daphniid Ceriodaphnia dubia by the carnivorous Southern bladderwort with suction traps (Utricularia australis). Independent to orientation and behavior during trap triggering, the animals were successfully captured within 9 ms on average and sucked in with velocities of up to 4 m/s and accelerations of up to 2800 g. Phases of very high acceleration during onsets of suction were immediately followed by phases of similarly high deceleration (max.: -1900 g) inside the bladders, leading to immobilization of the prey which then dies. We found that traps perform a 'forward strike' during suction and that almost completely air-filled traps are still able to perform suction. The trigger hairs on the trapdoors can undergo strong bending deformation, which we interpret to be a safety feature to prevent fracture. Our results highlight the elaborate nature of the Utricularia suction traps which are functionally resilient and leave prey animals virtually no chance to escape.