Trait compensation and cospecialization in a freshwater snail: size, shape and antipredator behaviour

HABITAT-SPECIFIC VARIATION AND PERFORMANCE TRADE-OFFS IN SHELL ARMATURE OF NEW ZEALAND MUDSNAILS

Studies documenting phenotypic variation among populations show that ecological performance in one activity is sometimes traded off against another. Identifying environment-specific costs and benefits associated with performance trade-offs is fundamental to knowing how conflicting selection pressures shape phenotype-environment matching in populations. We studied phenotypic variation in shell armature (spininess) of the New Zealand mudsnail, Potamopyrgus antipodarum (Gray), and explored how this variability relates to performance trade-offs between flow resistance and predator deterrence. Smooth- and spiny-shell morphotypes exist in populations in New Zealand streams and lakes, but the patterns and correlates of spatial variation of these phenotypes, and the possible hydrodynamical constraints and antipredatory benefits associated with spiny shell armature, are unknown. Samples from 11 rivers and nine lakes on the South Island showed that, on average, nearly 70% of snails in streams were smooth-shelled, whereas >80% of snails in lakes were spiny, suggesting dissimilar selective pressures between habitats. A laboratory flume experiment revealed that spines collected seston (i.e., suspended algae) at current speeds <40 cm/s, making spiny morphs more prone to flow-induced dislodgment than smooth morphs. However, a fish feeding experiment showed that one benefit of spines on shells was a decrease in predation risk from the common bully (Gobiomorphus cotidianus), a widespread predator of mudsnails in both streams and lakes. All snails egested by bullies were dead, further suggesting that these fishes may exert strong lethal effects on mudsnail populations in nature. Spine expression in lakes also appeared to be temperature related. We conclude that functional trade-offs between risk of flow-induced dislodgment and risk of fish predation affect shell armature frequencies of Potamopyrgus in freshwater habitats.

In utero predator-induced responses in the viviparid snail Bellamya chinensis

Predator-induced defenses are well-known adaptive responses found in many marine and freshwater organisms. These responses usually occur in juveniles or actively growing adults and reduce the ability of a predator to consume its prey. We report a predator-induced response occurring in utero in the viviparid snail Bellamya chinensis (Gray, 1863) that results in significant changes in offspring number, shell size variability of released juveniles, and juvenile shell organic content. Water-borne cues derived from a predatory crayfish ( Orconectes limosus (Rafinesque, 1817)) induced a doubling of the number of offspring released; these newly born snails were generally smaller, though more variable in size, and possessed a greater shell organic content than controls. The generally smaller size of juveniles released in the presence of a predator reflects a faster rate of generation and passage through the uterus. These responses to the presence of a predator could be adaptive by decreasing predation pressure within a population or by reducing mortality following failed attacks.

Costs and limits of dosage response to predation risk: to what extent can tadpoles invest in anti-predator morphology?

Inducible defences have long been considered as a polyphenism opposing defended and undefended morphs. However, in nature, preys are exposed to various levels of predation risk and scale their investment in defence to actual predation risk. Still, among the traits that are involved in the defence, some are specific to one predator type while others act as a more generalised defence. The existence of defence costs could prevent an individual investing in all these traits simultaneously. In this study, we investigate the impact of an increasing level of predator density (stickleback, Gasterosteus aculeatus) on the expression of morphological inducible defences in tadpoles of Rana dalmatina. In this species, investment in tail length and tail muscle is a stickleback-specific response while increased tail fin depth is a more general defence. As expected, we found a relationship between investment in defence and level of risk through the responses of tail fin depth and tail length. We also found an exponential increase of defence cost, notably expressed by convex decrease of growth and developmental rates. We found a relative independence of investment in the different traits that compose the defence, revealing a high potential for fine tuning the expression of defended phenotypes with respect to local ecological conditions.

Putting prey back together again: integrating predator-induced behavior, morphology, and life history

The last decade has seen an explosion in the number of studies exploring predator-induced plasticity. Recently, there has been a call for more comprehensive approaches that can identify functional relationships between traits, constraints on phenotypic responses, and the cost and benefits of alternative phenotypes. In this study, we exposed Helisoma trivolvis, a freshwater snail, to a factorial combination of three resource levels and five predator environments (no predator, one or two water bugs, and one or two crayfish) and examined ten traits including behavior, morphology, and life history. Each predator induced a unique suite of behavioral and morphological responses. Snails increased near-surface habitat use with crayfish but not with water bugs. Further, crayfish induced narrow and high shells whereas water bugs induced wide shells and wide apertures. In terms of life history, both predators induced delayed reproduction and greater mass at reproduction. However, crayfish induced a greater delay in reproduction that resulted in reduced fecundity whereas water bugs did not induce differences in fecundity. Resource levels impacted the morphology of H. trivolvis; snails reared with greater resource levels produced higher shells, narrower shells, and wider apertures. Resource levels also impacted snail life history; lower resources caused longer times to reproduction and reduced fecundity. Based on an analysis of phenotypic correlations, the morphological responses to each predator most likely represent phenotypic trade-offs. Snails could either produce invasion-resistant shells for defense against water bugs or crush-resistant shells for defense against crayfish, but not both. Our use of a comprehensive approach to examine the responses of H. trivolvis has provided important information regarding the complexity of phenotypic responses to different environments, the patterns of phenotypic integration across environments, and the potential costs and benefits associated with plastic traits.

Behavioral Syndromes: An Integrative Overview

A behavioral syndrome is a suite of correlated behaviors expressed either within a given behavioral context (e.g., correlations between foraging behaviors in different habitats) or across different contexts (e.g., correlations among feeding, antipredator, mating, aggressive, and dispersal behaviors). For example, some individuals (and genotypes) might be generally more aggressive, more active or bold, while others are generally less aggressive, active or bold. This phenomenon has been studied in detail in humans, some primates, laboratory rodents, and some domesticated animals, but has rarely been studied in other organisms, and rarely examined from an evolutionary or ecological perspective. Here, we present an integrative overview on the potential importance of behavioral syndromes in evolution and ecology. A central idea is that behavioral correlations generate tradeoffs; for example, an aggressive genotype might do well in situations where high aggression is favored, but might be inappropriately aggressive in situations where low aggression is favored (and vice versa for a low aggression genotype). Behavioral syndromes can thereby result in maladaptive behavior in some contexts, and potentially maintain individual variation in behavior in a variable environment. We suggest terminology and methods for studying behavioral syndromes, review examples, discuss evolutionary and proximate approaches for understanding behavioral syndromes, note insights from human personality research, and outline some potentially important ecological implications. Overall, we suggest that behavioral syndromes could play a useful role as an integrative bridge between genetics, experience, neuroendocrine mechanisms, evolution, and ecology.

The influence of predator threat on the timing of a life-history switch point: predator-induced hatching in the southern leopard frog (Rana sphenocephala)

We tested the hypotheses that potential egg predators, crayfish Procambarus nigrocinctus and dytiscid Cybister sp. larvae, would accelerate the timing of hatching and that a larval predator, dragonfly naiad Anax junius, would delay hatching in the southern leopard frog (Rana sphenocephala). We also tested the hypothesis that differences in response would be proportional to predator lethality. Our results indicate that our hypotheses were partially supported. The presence of an efficient egg predator (crayfish) induces hatching faster than a less efficient predator (dytiscid larvae). However, the presence of a larval predator (naiads) did not delay hatching. Eggs that developed in the presence of egg predators produced hatchlings that were shorter (total length) than those reared in the presence of larval predators or those reared in the absence of predators. We suggest that earlier hatching times should decrease vulnerability to egg predators but result in shorter hatchlings.

Color change and color-dependent behavior in response to predation risk in the salamander sister species Ambystoma barbouri and Ambystoma texanum

Although many organisms show multiple types of trait responses to predation risk (e.g., shifts in behavior, morphology, color, chemistry or life history), relatively few studies have examined how prey integrate these multiple responses. We studied the joint expression of color and behavioral responses to predation risk in two sister species of salamander larvae that live in habitats with different selection pressures. We examined responses to predation risk in three situations that differed in availability of refuge and substrate color heterogeneity, and thus availability of behavioral options for reducing risk. Relative to Ambystoma texanum, A. barbouri larvae were darker in color and showed a greater range of color change. With no variation in background color or refuge available, both species exhibited color change to better match the available background. The degree of color change showed by both species, however, did not depend on predation risk. Given the option to choose between light and dark substrates, A. texanum exhibited behavioral background matching (i.e., they preferred substrates that matched their own body color), while A. barbouri's substrate preferences did not depend on their initial body color. Instead, A. barbouri responded to risk by showing a strong preference for dark substrates, followed by a change to a darker body color. With refuge available, A. texanum's refuge use was color-dependent; larvae that were well camouflaged spent less time in refuge. In contrast, A. barbouri showed strong refuge use in response to risk, regardless of their body color. Overall, these results reflect how conflicting selection pressures (predation risk, habitat ephemerality, risk of UV damage) and species differences in mean color and ability to change color can govern the interplay of complementary and compensatory behavioral and color responses to predation risk.

Inter- and intraspecific trait compensation of defence mechanisms in freshwater snails

Trait compensation occurs when mechanically independent adaptations are negatively correlated. Here, we report the first study to demonstrate trait compensation in predator-defence adaptations across several species. Freshwater pulmonate snails exposed experimentally to predation chemical cues from fishes and crushed conspecifics showed clear interspecific differences in their behavioural avoidance responses, which were negatively correlated with shell crush resistance. The type of avoidance response varied between species: thin-shelled species (Lymnaea stagnalis and Physa fontinalis) moved to the water-line or out of the water, while those with thick shells moved under cover or showed a mixed response. There were also intraspecific size-linked differences, with an ontogenetic increase in shell strength accompanied by a decrease in behavioural avoidance. Such trait compensation in response to predation has important implications for interspecific interactions and food-web dynamics.