Risks for larvae mediated by plasticity in hatching

Risks associated with benthic and planktonic development can be mediated by plasticity in hatching. The ability to delay hatching while awaiting favorable planktonic conditions, combined with the ability to accelerate hatching when encapsulated offspring are at risk, should be advantageous. We tested this predicted association of hatching plasticities with a barnacle. In the winter, broods of barnacles (Balanus glandula) reached hatching‐capable stages at widely varying times, but these broods hatched in the spring within about 2 weeks, consistent with a synchronizing environmental stimulus for hatching. In contrast, the same adults held subsequent broods (during later spring and summer) briefly. Either an environmental stimulus for hatching was not needed later in the season, or it was more frequently present. Dissections of brood lamellae that scattered smaller clumps of the encapsulated nauplii induced hatching. Crabs eating brooding adults had a similar effect: crabs broke the barnacles' tests, and many nauplii hatched. In contrast, when whelks ate barnacles, they left the barnacles' wall plates and opercula in place, and few nauplii were released. In some cases, numerous hatched nauplii were trapped within the test of the killed mother. At a field site with abundant whelks, many dead barnacles had opercular plates in place. Plasticity in hatching of broods adjusted risks for planktonic larvae against risks of death of the parent before release of embryos, but escape or death of brooded offspring depended on the kind of damage to the brooding mother and thus on the kind of predator. Although both predators killed brooding parents, subtle snails imposed a greater risk than crushing crabs.     

Hatching plasticity in the tropical gastropod Nerita scabricosta

Hatching plasticity has been documented in diverse terrestrial and freshwater taxa, but in few marine invertebrates. Anecdotal observations over the last 80 years have suggested that intertidal neritid snails may produce encapsulated embryos able to significantly delay hatching. The cause for delays and the cues that trigger hatching are unknown, but temperature, salinity, and wave action have been suggested to play a role. We followed individual egg capsules of Nerita scabricosta in 16 tide pools to document the variation in natural time to hatching and to determine if large delays in hatching occur in the field. Hatching occurred after about 30 d and varied significantly among tide pools in the field. Average time to hatching in each pool was not correlated with presence of potential predators, temperature, salinity, or pool size. We also compared hatching time between egg capsules in the field to those kept in the laboratory at a constant temperature in motionless water, and to those kept in the laboratory with sudden daily water motion and temperature changes. There was no significant difference in the hatching rate between the two laboratory treatments, but capsules took, on average, twice as long to hatch in the laboratory as in the field. Observations of developing embryos showed that embryos in the field develop slowly and continuously until hatching, but embryos in the laboratory reach the hatching stage during the first month of development and remain in stasis after that. Instances of hatching plasticity in benthic marine invertebrates, like the one in N. scabricosta, could greatly enhance our ability to investigate the costs and benefits of benthic versus planktonic development, a long‐standing area of interest for invertebrate larval biologists.     

Wasp predation and wasp-induced hatching of red-eyed treefrog eggs

Eggs often suffer high levels of predation and, compared with older animals, embryos have few options available for antipredator defence. None the less, hatchlings can escape from many predators to which eggs are vulnerable. I studied early hatching as an antipredator defence of red-eyed treefrog embryos, Agalychnis callidryas, in response to egg predation by social wasps (Polybia rejecta). Red-eyed treefrogs attach their eggs to vegetation overhanging water, where they are exposed to arboreal and aerial predators. Wasps attacked half the egg clutches and killed almost a quarter of the eggs I monitored at a natural breeding site in Panama. Hatching tadpoles fall into the water, where they face aquatic predators. As predicted from improved survival of older hatchlings with aquatic predators, most undisturbed eggs hatched relatively late. However, many younger embryos directly attacked by wasps hatched immediately. Embryos attacked by wasps hatched as much as a third younger than the peak undisturbed hatching age, and most hatching embryos escaped. Thus hatching is an effective defence against wasp predation, and plasticity in hatching stage allows embryos to balance risks from stage-specific egg and larval predators. Wasp-induced hatching is behaviourally similar to the snake-induced hatching previously described in A. callidryas, but occurs in fewer eggs at a time, congruent with the scale of the risk. Individual embryos hatch in response to wasps, which take single eggs, whereas whole clutches hatch in response to snakes, which consume entire clutches. Embryos of A. callidryas thus respond appropriately to graded variation in mortality risks. Copyright 2000 The Association for the Study of Animal Behaviour.     

Oxygen, gills, and embryo behavior: mechanisms of adaptive plasticity in hatching.

Many species alter the timing of hatching in response to egg or larval predators, pathogens, or physical risks. This plasticity depends on separation between the onset of hatching competence and physiological limits to embryonic development. I present a framework based on heterokairy to categorize developmental mechanisms and identify traits contributing to and limiting hatching plasticity, then apply it to a case of predator-induced hatching. Red-eyed treefrogs have arboreal eggs, and tadpoles fall into ponds upon hatching. Egg and tadpole predators select for earlier and later hatching, respectively. Embryos hatch up to 30% early in predator attacks, and later if undisturbed. They maintain large external gills throughout the plastic hatching period, delaying gill regression while development otherwise continues. Rapid gill regression occurs upon hatching. Prolonged embryonic development depends on external gills; inducing gill regression causes hatching. External hypoxia retards development, kills eggs, and induces hatching. Nonetheless, embryos develop synchronously and without hatching prematurely across a broad range of perivitelline PO2, from 0.5-12.5 kPa. Embryos exploit spatial variation of PO2 within eggs by positioning gills against patches of air-exposed surface. Respiratory plasticity and oxygen-sensitive behavior appear critical for the hatching plasticity that balances a predation risk trade-off across life stages.     

Oxygen, gills, and embryo behavior: mechanisms of adaptive plasticity in hatching

Many species alter the timing of hatching in response to egg or larval predators, pathogens, or physical risks. This plasticity depends on separation between the onset of hatching competence and physiological limits to embryonic development. I present a framework based on heterokairy to categorize developmental mechanisms and identify traits contributing to and limiting hatching plasticity, then apply it to a case of predator-induced hatching. Red-eyed treefrogs have arboreal eggs, and tadpoles fall into ponds upon hatching. Egg and tadpole predators select for earlier and later hatching, respectively. Embryos hatch up to 30% early in predator attacks, and later if undisturbed. They maintain large external gills throughout the plastic hatching period, delaying gill regression while development otherwise continues. Rapid gill regression occurs upon hatching. Prolonged embryonic development depends on external gills; inducing gill regression causes hatching. External hypoxia retards development, kills eggs, and induces hatching. Nonetheless, embryos develop synchronously and without hatching prematurely across a broad range of perivitelline PO₂, from 0.5–12.5 kPa. Embryos exploit spatial variation of PO₂ within eggs by positioning gills against patches of air-exposed surface. Respiratory plasticity and oxygen-sensitive behavior appear critical for the hatching plasticity that balances a predation risk trade-off across life stages.     

Environmental context shapes immediate and cumulative costs of risk-induced early hatching

In animals with complex life cycles, fitness trade-offs across life stages determine the optimal time for transitions between stages. If these trade-offs vary predictably, adaptive plasticity in the timing of life history transitions may evolve. For instance, embryos of many species are capable of accelerating hatching to escape from egg predation and other hazards, but for plasticity in hatching timing to be selectively maintained, early hatching must also entail costs, probably in subsequent life stages. However the post-hatching environment, which influences this cost, is variable in nature. We assessed how two elements of the post-hatching environment, predator species and age structure created by hatching age plasticity, affect costs of hatching early in red-eyed treefrogs, Agalychnis callidryas. Red-eyed treefrog embryos were induced to hatch at the onset of hatching competence or near the peak of spontaneous hatching and exposed to one of three insect predators in single or mixed hatching-age treatments. Age structure created by hatching-age plasticity did not affect tadpole survivorship or growth; however, the consequences of hatching timing depended on predator species and foraging mode. Tadpoles that were induced to hatch early experienced initially higher mortality rates only with the more actively foraging predator. Nonetheless, mortality costs of accelerated hatching were apparent with all predators once we factored in the longer duration of exposure that early hatchlings experience in nature. This study suggests that extended exposure of young larvae to predators may be a general cost of early hatching, explaining why spontaneous hatching occurs later in life across variable environmental contexts.     

Constrained plasticity in switching across life stages: pre- and post-switch predators elicit early hatching

The timing of many life history events shows phenotypic plasticity in response to the risk of predation. Theory predicts that increased risk of mortality in an early stage should select for switching earlier, while a higher risk after the transition should select for switching later. Here we examined the effects of stage-specific predation risk on the timing of hatching of Rana temporaria. Embryos were exposed to chemical cues from either an egg predator (Haemopis sanguisuda) or a tadpole predator (Aeshna cyanea) to evaluate three specific hypotheses: (1) a fixed intermediate response, (2) a ‘fixed predator’ response (i.e., either anticipation or delay), and (3) a specific predator response (both anticipation and delay). Rana temporaria embryos did not discern between pre- and post-hatching specific predators, and they hatched prematurely regardless predator type. These results suggest that R. temporaria embryos respond to predation risk in a fixed way by hatching early, and that they use cues stemming from injured conspecifics, which provides a simple, conservative mechanism of risk assessment. In conclusion, our data are not anticipated by the theoretical consideration that organisms should spend less time in more dangerous environments, but they confirm an invariable adjustment of hatching time in response to an inscrutable predation risk (response to a fixed-predator) in connection with a consistent mechanism mediating the perception of predation risk.     

Conspecific density determines the magnitude and character of predator-induced phenotype

The benefits in survival gained from predator-induced phenotypes often come at a cost to other components of fitness. Therefore, the level of expression of an induced phenotype should mirror the level of risk in the environment. When a predator exhibits a saturating functional response the risk of mortality to a given prey decreases as prey density increases. Therefore, for a given predator threat, investment in defense should be lower in prey at high density relative to those at low density. In this study, I test whether the magnitude of predator-induced morphological plasticity decreases with increasing conspecific density by exposing pine woods tree frog (Hyla femoralis) tadpoles at three different densities to predators (present or absent) in a factorial experiment. Tadpole morphology was not affected by changes in density in the absence of predators. However, predators had a significant, density-dependent effect on tadpole morphology. Specifically, the magnitude of morphological response was graded and larger for animals in the low density (high risk) environment. This study demonstrates that tadpoles can modulate phenotypic plasticity in response to mortality risk as a function of both the density of conspecifics and chemical cues from predators, which suggests that they are able to detect and respond to fine-scale changes in the threat environment. In addition, this study highlights the need for analytical approaches that allow morphological plasticity studies to elucidate allometric relationships in addition to simply quantifying size-corrected traits. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Predator-induced defenses in a salt-marsh gastropod

Predator-induced defenses are among the most ecologically important forms of phenotypic plasticity. Although predation and induced defenses are well documented in rocky-intertidal systems, they have received less attention in soft-bottom communities. Shell-crushing predators are common in soft-bottom, vegetated habitats, which often exhibit substantial spatial heterogeneity in predation intensity. We examined variations in shell morphology of the salt-marsh periwinkle, Littoraria irrorata, among marsh microhabitats in the northern Gulf of Mexico that vary in their accessibility to predatory blue crabs, Callinectes sapidus. Littoraria from high-predation sites exhibited more extensively calcified apertural lips and narrower apertural openings relative to snails from low-predation sites. Thick apertural lips generally increased the handling time required by Callinectes to breach Littoraria shells in laboratory experiments, although the method of shell entry used by crabs was dependent on the crab:snail size ratio. Apertural-lip thickness was not related to past predation events in field-collected snails. Snails exposed to water treated with the effluent of Callinectes and crushed conspecifics produced significantly thicker apertural lips than controls, with a response time and morphological extent comparable to that of their rocky-shore counterparts. This study underscores the widespread occurrence of predator-induced plasticity in marine gastropods and emphasizes its role in soft-bottom, vegetated marine habitats, where shell-crushing predation can be as prevalent a selective force as in the rocky intertidal.     

To hatch and hatch not: similar selective trade-offs but different responses to egg predators in two closely related, syntopic treefrogs

Risk-sensitive hatching is adaptive for species facing a trade-off between egg-stage and post-hatching risks, and environmental variation in one or both stages. Such plasticity has been found in amphibians, fishes, reptiles and spiders, with red-eyed treefrogs (Agalychnis callidryas) being the best-studied case. We assessed hatching plasticity and egg- and larval-stage risks in a closely related, syntopic species, the gliding leaf-frog (Agalychnis spurrelli). We found a lower hatching response to egg-eating snakes in A. spurrelli (9–28% of embryos escaped) than in A. callidryas (59–80% escaped). Levels of snake predation were similarly high for clutches of both species monitored at a pond in Costa Rica, and in fish predation experiments early-hatched A. spurrelli tadpoles were more vulnerable than later hatchlings, as has been shown for A. callidryas. A. spurrelli thus face a risk trade-off similar to A. callidryas, and likely would benefit from predator-induced hatching; their lower responsiveness to snakes appears nonadaptive. A. spurrelli embryos showed a stronger hatching response (57% hatched in 1 h) to submergence underwater than to snake attacks even though submergence is a less frequent risk. This suggests they have a greater capacity for early hatching than is expressed in the context of snake attacks, but have much lower sensitivity to snake cues than to flooding cues. Development in A. spurrelli is accelerated compared to syntopic A. callidryas, and spontaneous hatching is earlier and more synchronous. This is congruent with predictions based on selection by egg predators in the absence of a strong escape hatching response.     

Plasticity of hatching in amphibians: evolution, trade-offs, cues and mechanisms

Many species of frogs and salamanders, in at least 12 families, alter their timing of hatching in response to conditions affecting mortality of eggs or larvae. Some terrestrially laid or stranded embryos wait to hatch until they are submerged in water. Some embryos laid above water accelerate hatching if the eggs are dehydrating; others hatch early if flooded. Embryos can hatch early in response to predators and pathogens of eggs or delay hatching in response to predators of larvae; some species do both. The phylogenetic pattern of environmentally cued hatching suggests that similar responses have evolved convergently in multiple amphibian lineages. The use of similar cues, including hypoxia and physical disturbance, in multiple contexts suggests potential shared mechanisms underlying the capacity of embryos to respond to environmental conditions. Shifts in the timing of hatching often have clear benefits, but we know less about the trade-offs that favor plasticity, the mechanisms that enable it, and its evolutionary history. Some potentially important types of cued hatching, such as those involving embryo-parent interactions, are relatively unexplored. I discuss promising directions for research and the opportunities that the hatching of amphibians offers for integrative studies of the mechanisms, ecology and evolution of a critical transition between life-history stages.     

Phenotypically plastic responses of green frog embryos to conflicting predation risk

Predators have been shown to alter the timing of switch points between life history stages, but few studies have addressed switch point plasticity in prey exposed simultaneously to conflicting predation pressure. We tested hatching responses of green frog (Rana clamitans) embryos subject to perceived predation risk from chemical cues released by two stage-specific predators, predicting that these predators would elicit: (1) directional hatching responses when presented independently, and (2) intermediate phenotypic responses when presented simultaneously. R. clamitans embryos in outdoor exclosures were exposed to cues from an egg predator (freshwater leeches; Nephelopsis obscura), a larval predator (dragonfly nymphs, Aeschna canadensis), and both predators in a 2 × 2 factorial experiment, and changes in hatchling size, hatchling developmental stage, and hatching time were compared to those for control embryos. Leeches alone induced embryos to hatch at a smaller size and an earlier developmental stage than controls, while dragonfly nymphs elicited a delay in egg hatching time that was associated with larger size and later developmental stage at hatching. Embryos failed to respond to simultaneous exposure to both predators, implying that responses to each occurred concurrently and were therefore dampened. Our results indicate that prey under threat from conflicting predators may manifest intermediate defensive phenotypes. Such intermediate responses may result in elevated rates of prey mortality with possible consequences at the population level.     

Effects of hatching age on development and hatchling morphology in the red-eyed tree frog, Agalychnis callidryas

The red-eyed treefrog, Agalychnis callidryas , lays eggs on leaves overhanging ponds. Tadpoles hatch and enter the water at different ages, and late-hatched tadpoles survive aquatic predators better than do early-hatched tadpoles. Here I assess developmental consequences of hatching age through: (1) a morphological study of embryos and tadpoles through the plastic hatching period; (2) a behavioural assay for an effect of hatching age on feeding; and (3) a field experiment testing the effect of hatching age on growth to metamorphosis. Substantial development of feeding, digestive, respiratory and locomotor structures occurs in embryos over the plastic hatching period. Hatchling morphology thus varies with age, with consequences for behaviour and predation risk. Hatched tadpoles develop faster than embryos, and early-hatched tadpoles feed before late-hatched tadpoles. After all tadpoles have hatched, the effect of hatching age on size decreases. I found no evidence for an effect of hatching age on size at metamorphosis and only weak evidence for an effect on larval period. Hatching age affects the sequence of developmental change: early-hatched tadpoles lose external gills while otherwise more developed embryos maintain them. Plasticity in external gill resorption may be adaptive given differences in the respiratory environments of embryos and tadpoles. Early-hatched tadpoles also diverge from embryos in shape, growing relatively smaller tails. The study of functional morphology and developmental plasticity will contribute to understanding hatching as an ontogenetic niche shift.     

Predator‐induced phenotypic plasticity in tadpoles: extension or innovation?

Phenotypic plasticity, the ability of a trait to change as a function of the environment, is central to many ideas in evolutionary biology. A special case of phenotypic plasticity observed in many organisms is mediated by their natural predators. Here, we used a predator-prey system of dragonfly larvae and tadpoles to determine if predator-mediated phenotypic plasticity provides a novel way of surviving in the presence of predators (an innovation) or if it represents a simple extension of the way noninduced tadpoles survive predation. Tadpoles of Limnodynastes peronii were raised in the presence and absence of predation, which then entered a survival experiment. Induced morphological traits, primarily tail height and tail muscle height, were found to be under selection, indicating that predator-mediated phenotypic plasticity may be adaptive. Although predator-induced animals survived better, the multivariate linear selection gradients were similar between the two tadpole groups, suggesting that predator-mediated phenotypic plasticity is an extension of existing survival strategies. In addition, nonlinear selection gradients indicated a cost of predator-induced plasticity that may limit the ability of phenotypic plasticity to enhance survival in the presence of predators.     

Hatching responses of subsocial spitting spiders to predation risk

The carrying of eggs often renders parents vulnerable to predators due to increased conspicuousness or decreased mobility. Nonetheless, egg-carrying parents can escape from the predators to which they are vulnerable. Previous studies have demonstrated heavy predation by spider-eating jumping spiders (Portia labiata) on egg-carrying spitting spider (Scytodes pallida) females, but little predation on eggless females. If the timing of hatching is phenotypically plastic, then both S. pallida females and their eggs could reduce the risk of predation by hatching early. Hence, this study examines the hatching responses of S. pallida to chemical cues from P. labiata, both in the laboratory and in the field, and addresses the following questions. (i) Do cues from predatory P. labiata influence the hatching traits of S. pallida? (ii) Are the olfactory cues from predators sufficient for predator detection by S. pallida ? (iii) Are hatching responses to predatory P. labiata controlled by egg-carrying S. pallida females, or directly by their embryos? The study provides evidence of hatching as a life-history switch point, which shows an adaptive plasticity in response to predation risk in egg-carrying S. pallida. Egg-carrying S. pallida females, but not unattended eggs, adjust egg-hatching time (the interval between oviposition and hatching) in response to the threat of predation on both the female and her eggs by P. labiata. In the presence of P. labiata, eggs that are carried by females hatch sooner; the hatchlings of these eggs are therefore smaller than hatchlings born in the absence of P. labiata. Chemical cues that are released from the draglines of P. labiata are sufficient to elicit changes in the egg-hatching traits of S. pallida. Hatching early in response to this predator may benefit both females and their offspring. To my knowledge, this is the first direct experimental study to demonstrate predator-induced hatching plasticity in spiders and, in particular, in animals with parental care.     

Embryological aspects of inducible morphological defenses in Daphnia

Many cases of predator-induced morphological plasticity in daphnids are well studied examples of inducible defenses. However, little is known about the early development of these sometimes conspicuous traits. We compared for the first time in five different Daphnia species the embryonic development of predator-induced and noninduced animals using scanning electron microscopy (SEM). We observed significant morphological changes in the last embryonic stage in helmet formation in Daphnia cucullata and in neck-pedestal development in Daphnia pulex. In contrast, no morphological changes could be found during embryogenesis between induced and noninduced Daphnia lumholtzi, D. longicephala, and D. ambigua. Strategies for initiating the defensive traits differ among Daphnia species because of trade-offs between environmental requirements and developmental constraints. Some general features of Daphnia embryonic development are described using SEM. All Daphnia embryos have to shed at least three different membranes before leaving the brood pouch of the mother. After the embryos shed the third membrane, chemosensillae that are likely able to detect predator-released chemicals are exposed to the olfactory environment.     

Antipredator behaviour mediated by chemical cues: the role of conspecific alarm signalling and predator labelling in the avoidance response of a marine gastropod

Gastropods represent a challenge in the understanding of alarm signalling. We studied predator avoidance (climbing behaviour) of the marine snail Tegula funebralis in laboratory experiments. Snails were exposed to crude extract of conspecifics, and to water conditioned by actively feeding or non-feeding predatory crabs. Crabs had previously been maintained on different diets, and were accordingly labelled by chemical cues of various origins. Tegula -extract alone released climbing behaviour in May, but not in June. However, during both these months, snails responded to chemical cues from crabs that were actively feeding on Tegula . Crabs labelled by Tegula -diet, and actively feeding on Tegula , also caused more climbing responses compared to crabs labelled by other diets. Chemical cues derived from crabs actively feeding on another snail species, or from non-feeding crabs, did not induce snail climbing no matter the previous feeding history of the predators. When snails received Tegula -extract combined with water conditioned with a non-feeding, Tegula -labelled crab, no climbing occurred. However, when the non-feeding, Tegula -labelled crab was present in the solution of Tegula -extract, moderate climbing responses were obtained. The results imply that climbing responses of T. funebralis are in general caused by the action of a two-component system. This system seems to be a mixture of chemical cues leaking from the tissue of conspecifics when being eaten, and latent conspecific chemicals that are modified in crabs and presumably released with the urine of chemically labelled predators. The modified chemical labels appear to be fully released by crabs when feeding, and moderately released when detecting food. The responses obtained in May with crude extract alone may result from a seasonal change in alarm signalling properties, or a change in behavioural responsiveness of snails exposed to a variable predator regime.     

Plasticity of hatching and the duration of planktonic development in marine invertebrates

Plasticity in hatching potentially adjusts risks of benthic and planktonic development for benthic marine invertebrates. The proportionate effect of hatching plasticity on duration of larval swimming is greatest for animals that can potentially brood or encapsulate offspring until hatching near metamorphic competence. As an example, early hatching of the nudibranch mollusk Phestilla sibogae is stimulated by scattering of encapsulated offspring, as by a predator feeding on the gelatinous egg ribbon. When egg ribbons are undisturbed, hatching is at or near metamorphic competence. Disturbance of an unguarded benthic egg mass can insert 4 or more days of obligate larval dispersal into the life history. As another example, the spionid annelid Boccardia proboscidea broods capsules, each with both cannibalistic and developmentally arrested planktivorous siblings plus nurse eggs. Early hatching produces mainly planktivorous larvae with a planktonic duration of 15 days. Late hatching produces mainly adelphophages who have eaten their planktivorous siblings and metamorphose with little or no period of swimming. Mothers actively hatch their offspring by tearing the capsules, and appeared to time hatching in response to their environment and not to the stage of development of their offspring. Higher temperature increased the variance of brooding time. Females appeared to hatch capsules at an earlier developmental stage at lower temperatures. Species that release gametes or zygotes directly into the plankton have less scope for plasticity in stage at hatching. Their embryos develop singly with little protection and hatch at early stages, often as blastulae or gastrulae. Time of hatching cannot be greatly advanced, and sensory capabilities of blastulae may be limited.     

Chemical defense of toad tadpoles under risk by four predator species

Many organisms use inducible defenses as protection against predators. In animals, inducible defenses may manifest as changes in behavior, morphology, physiology, or life history, and prey species can adjust their defensive responses based on the dangerousness of predators. Analogously, prey may also change the composition and quantity of defensive chemicals when they coexist with different predators, but such predator‐induced plasticity in chemical defenses remains elusive in vertebrates. In this study, we investigated whether tadpoles of the common toad (Bufo bufo) adjust their chemical defenses to predation risk in general and specifically to the presence of different predator species; furthermore, we assessed the adaptive value of the induced defense. We reared tadpoles in the presence or absence of one of four caged predator species in a mesocosm experiment, analyzed the composition and quantity of their bufadienolide toxins, and exposed them to free‐ranging predators. We found that toad tadpoles did not respond to predation risk by upregulating their bufadienolide synthesis. Fishes and newts consumed only a small percentage of toad tadpoles, suggesting that bufadienolides provided protection against vertebrate predators, irrespective of the rearing environment. Backswimmers consumed toad tadpoles regardless of treatment. Dragonfly larvae were the most voracious predators and consumed more predator‐naïve toad tadpoles than tadpoles raised in the presence of dragonfly cues. These results suggest that tadpoles in our experiment had high enough toxin levels for an effective defense against vertebrate predators even in the absence of predator cues. The lack of predator‐induced phenotypic plasticity in bufadienolide synthesis may be due to local adaptation for constantly high chemical defense against fishes in the study population and/or due to the high density of conspecifics.     

A comparative analysis of predator-induced plasticity in larval Triturus newts

Species that occupy similar habitats are expected to show convergent phenotypes. If habitats are defined by the presence of predators, then traits that modify vulnerability to predation, including predator-induced phenotypic plasticity, should be similar within habitats. We tested this idea using larvae of six syntopic newt species belonging to the two Triturus clades. Behavioural plasticity induced by odonate predators was strongly dissimilar between the two main clades but similar within them. Morphological plasticity was variable among species, even between one pair of closely related species. A predation experiment tested whether differences between clades could be caused by differences in body size. Size-specific vulnerability differed between newts in the small-bodied and large-bodied clades, indicating that similar predators may affect the two clades differently. The results showed both similarity and dissimilarity in predator-induced phenotypic plasticity in syntopic larval newts although theory suggests that divergence is unlikely in such ecologically similar species.