Adaptive changes in size and age at metamorphosis can qualitatively vary with predator type and available defenses

In many taxa the timing of metamorphosis is plastic in response to predation risk during the pre-metamorphic stage, and trends in both age and body size at metamorphosis have been the subject of much study. The responses to cues of predators are predominantly to be larger or equal-sized at the same age or older at metamorphosis. These observations are in direct contrast with existing theoretical treatments of this plasticity, which mostly predict earlier and smaller metamorphosis and never later and larger metamorphosis without invoking indirect effects on growth rate. Here we resolve the discrepancy between theory and observation using a dynamic state-dependent model that incorporates morphological and behavioral responses to predation risk. We allow prey to choose the optimal activity level and/or investment in defense over the growth period. We show that under certain conditions, metamorphosis at a larger size and later time is likely to be optimal. Our analysis allows us to make testable predictions about the changes in activity level of prey as they grow and how the effect of providing refuges will vary with predator type. Several of these predictions are supported by a meta-analysis of metamorphic responses to caged predators by larval amphibians and insects. Our predictions lead to insights about the feedback effects of antipredator responses on growth and subsequent implications for life history.     

Opposite shifts in size at metamorphosis in response to larval and metamorph predators

Predation risk can cause organisms to alter the timing of life history switch points. Theory suggests that increased risk in an early life stage should select for switching earlier and smaller, while increased risk in the subsequent stage should select for switching later and larger. This framework has frequently been applied to metamorphosis in amphibians, with mixed results. Few studies examining the effect of larval predation risk on metamorphosis have observed the predicted pattern, and no studies, to our knowledge, have examined the effect of increased risk during and after metamorphosis on the timing of this switch point. Here we examine the effect of larval and post-metamorphic predation risk on metamorphosis in the red-eyed treefrog, Agalychnis callidryas. We raised tadpoles in the presence or absence of cues from caged water bugs fed larvae and cues from spiders fed emerging metamorphs. Water bugs are effective larval predators, while spiders are poor larval predators but prey on metamorphs. Furthermore, since spiders forage on the water surface it is possible that tadpoles could assess future risk from this predator. Predators induced opposite shifts in life history. Tadpoles emerged smaller and less developed in response to water bugs, but later and larger in response to spiders. Interestingly, predator effects on larval duration were not independent; tadpoles delayed emerging in response to spiders, but only in the absence of water bugs.     

Predator-induced morphological defenses in marine zooplankton: a larval case study

While there are numerous reports of predator-induced morphological defenses for freshwater zooplankton, freshwater larvae, and benthic marine animals, a literature search revealed no reports of predator-induced morphological defenses for marine zooplankton. Rarity of predator-induced morphological defenses in marine zooplankton would imply a difference in predation risks compared to those experienced by freshwater organisms and benthic marine adults, whereas the presence of such plasticity in defenses would imply that risks are modified by developmental responses. This study reports a predator-induced change in defenses and vulnerability of a marine planktonic larva. Specifically, when reared in the presence of zoea larvae of Cancer spp., veliger larvae of the intertidal snail Littorina scutulata developed significantly smaller shell apertures and rounder shells than did cohort veligers reared in the absence of predator cues. Pairwise predation trials demonstrated that veligers reared with caged zoeas throughout development had greater survival than predator-naive veligers during short-term exposure to zoeas. The development of predator-induced morphological defenses by some marine larvae introduces a range of testable hypotheses on developmental plasticity that reduces vulnerability of planktonic larvae and other marine zooplankton to predators.     

Multiple predators indirectly alter community assembly across ecological boundaries

Models of habitat selection often assume that organisms choose habitats based on their intrinsic quality, regardless of the position of these habitats relative to low-quality habitats in the landscape. We created a habitat matrix in which high-quality (predator-free) aquatic habitat patches were positioned adjacent to (predator-associated) or isolated from (control) patches with single or two species of caged predators. After 16 days of colonization, larval insect abundance was reduced by 50% on average in both the predator and predator-associated treatments relative to isolated controls. Effects were largely similar among predator treatments despite variation in number of predator species, predator biomass, and whether predators were native or nonnative. Importantly, the strength of effects did not depend on whether predators were physically present. These results demonstrate that predator cues can cascade with equal strength across ecological boundaries, indirectly altering community assembly via habitat selection in intrinsically high-quality habitats.     

Threat-sensitive foraging by larval threespine sticklebacks (Gasterosteus aculeatus)

Summary The threat-sensitive predator avoidance hypothesis predicts that prey can assess the relative threat posed by a predator and adjust their behaviour to reflect the magnitude of the threat. We tested the ability of larval threespine sticklebacks to adjust their foraging in the presence of predators by exposing them to conspecific predators of various sizes and recording their foraging and predator avoidance behaviours. Larvae (<30 days post-hatch) displayed predator escape behaviours only towards attacking predators. At 3 weeks post-hatch larvae approached the predator after fleeing, a behaviour which may be the precursor to predator inspection. Larvae reduced foraging and spent less time in the proximity of large and medium-sized predators compared to small predators. The reduction in foraging was negatively correlated to the predator/larva size ratio, indicating that larvae increased their foraging as they increased in size relative to the predator. We conclude that larval sticklebacks can assess the threat of predation early in their ontogeny and adjust their behaviour accordingly.Correspondence to: J.A. Brown     

Differential timing of larval starvation effects on filtration rate and growth in juvenile Crepidula onyx

This study demonstrates that the timing of larval starvation did not only determine the larval quality (shell length, lipid content, and RNA:DNA ratio) and the juvenile performance (growth and filtration rates), but also determine how the latent effects of larval starvation were mediated in Crepidula onyx. The juveniles developed from larvae that had experienced starvation in the first two days of larval life had reduced growth and lower filtration rates than those developed from larvae that had not been starved. Lower filtration rates explained the observed latent effects of early larval starvation on reduced juvenile growth. Starvation late in larval life caused a reduction in shell length, lipid content, and RNA:DNA ratio of larvae at metamorphosis; juveniles developed from these larvae performed poorly in terms of growth in shell length and total organic carbon content because of “depletion of energy reserves” at metamorphosis. Results of this study indicate that even exposure to the same kind of larval stress (starvation) for the same period of time (2 days) can cause different juvenile responses through different mechanisms if larvae are exposed to the stress at different stages of the larval life.     

Larval alarm pheromones as a potential control for invasive cane toads (Bufo marinus) in tropical Australia

Novel approaches to control invasive species are urgently needed. Cane toads (Bufo marinus) are large, highly toxic anurans that are spreading rapidly through tropical Australia. Injured toad larvae produce an alarm pheromone that elicits rapid avoidance by conspecifics but not by frog larvae. Experiments in outdoor ponds show that repeated exposure to the pheromone reduced toad tadpole survival rates (by >50%) and body mass at metamorphosis (by 20%). The alarm pheromone did not induce tadpoles to seek shelter, but accelerated ontogenetic differentiation. Perhaps reflecting mortality of weaker individuals during larval life, growth rates post-metamorphosis were higher in animals emerging from the pheromone exposure treatment than from the control treatment. Nonetheless, body size differentials established at metamorphosis persisted through the first 8 days of post-metamorphic life. We will need substantial additional research before evaluating whether the alarm pheromone provides a way to reduce cane toad recruitment in nature, but our field trials are encouraging in this respect.     

Effects of embryonic exposure to conspecific chemical cues on hatching and larval traits in the common frog <Emphasis Type="Italic">(Rana temporaria)</Emphasis>

Summary. Recent studies indicate that amphibian eggs are capable of hatching plasticity in response to chemical cues released by predators feeding on conspecific eggs or larvae. However, information is scarce on the relative importance of predator and conspecific cues in such a process. In particular, no attempt has been made to compare the effects of embryonic exposures to chemical cues indicative of a predation risk for eggs and larvae, although both life stages can co-occur in natural habitats. In this context, common frog embryos (Rana temporaria) were raised until hatching in the presence of crushed conspecific extracts from eggs and tadpoles to assess their respective influences on some hatching and larval traits. While a significant delay in hatching time was observed in embryos exposed to chemical cues from tadpole extract, this life-history shift appeared unaffected by embryonic exposure to egg extract. Hatchlings derived from eggs incubated in the presence of both conspecific extracts showed a significantly greater weight than unexposed controls. However, such an effect was no longer apparent 15, 30 and 50 days after hatching, suggesting that embryonic exposure to chemical cues from damaged conspecific eggs and tadpoles has no influence on larval growth. Lastly, morphological measurements performed on hatchlings and older tadpoles (15, 30 and 50 days old) revealed no significant effect of embryonic treatments on the shape of body and tail.     

Palatability of invertebrate larvae to corals and sea anemones

Risk of larval mortality is an underlying theme in debates and models concerning the ecology and evolution of the differing reproductive characteristics among marine benthic invertebrates. In these discussions, predation is often assumed to be a major source of larval mortality. Previous studies, focused primarily on planktotrophic larvae, suggested that marine larvae generally were susceptible to, and poorly defended against, planktivorous fishes and invertebrates. Larval-planktivore interactions involving larger and more conspicuous lecithotrophic larvae that are typical of many brooding sessile invertebrates have not been well studied. This lack of data for diverse larval types has hindered testing broad generalities about marine larvae and planktivore prey-preferences. This study demonstrates that lecithotrophic larvae of many Caribbean and temperate western Atlantic invertebrates are distasteful to co-occurring corals and anemones. These larval predators frequently rejected larvae of sponges (6 of 9 species), gorgonians (7 of 9 species), corals (3 of 3 species), hydroids (2 of 2 species) and a bryozoan. Larvae of three temperate colonial ascidians were readily consumed. Frequencies of survivorship for larvae captured but rejected by corals and anemones were generally high and, in 20 of 24 assays, were not statistically different from those of unattacked control larvae. Levels of metamorphosis (when it occurred) of rejected larvae also rarely differed significantly from those of unattacked controls. These results provide further evidence that larval palatability to predators may not be as high as once thought, particularly for brooded larvae of sessile colonial invertebrates. The means by which larvae may avoid or deter predators, and the demographic consequences for marine invertebrates and for the evolution of invertebrate life-history patterns, need to be assessed.     

The anti-predator behaviour of wild white-handed gibbons (<Emphasis Type="Italic">Hylobates lar</Emphasis>)

Predation on gibbons is rarely observed in the wild. However, the gibbons' moderate body size and relatively small social groups suggest high vulnerability to predation. To assess the role of predation and to study their anti-predator behaviour, we presented visual predator models to nine groups of wild white-handed gibbons at Khao Yai National Park, Thailand. We measured subjects' immediate and delayed responses to four potential predators: tiger, clouded leopard, crested serpent eagle and reticulated python. Subjects reliably approached all four predators. In response to tigers and leopards, they additionally produced predator-specific songs and defecated copiously. In terms of delayed responses, distance between mated adults decreased, but only after exposure to the tiger model. In response to eagles and pythons, gibbons consistently vocalised, but this did not always include predator singing, and we found no long-term effects in overall activity or strata use. However, during 6 of 26 predator encounters, the gibbons produced songs with a structure that was intermediate between a duet song and a predator song more than 20 min after the predator encounter, indicating a long-term effect on their vocal behaviour. This study demonstrates that gibbons discriminate between different potential predators and respond to them with adaptive anti-predator behaviour, which include predator-specific vocal responses. We conclude that gibbons are not immune to predation and that terrestrial predators elicit consistent immediate and delayed anti-predation responses.     

The influence of size-specific indirect interactions in predator-prey systems

Nonlethal indirect interactions between predators often lead to nonadditive effects of predator number on prey survival and growth. Previous studies have focused on systems with at least two different predator species and one prey species. However, most predators undergo extreme ontological changes in phenotype such that interactions between different-sized cohorts of a predator and its prey could lead to nonadditive effects in systems with only two species. This may be important since different-sized individuals of the same species can differ more in their ecology than similar-sized individuals of different species. This study examined trait-mediated indirect effects in a two-species system including a cannibalistic predator with different-sized cohorts and its prey. I tested for these effects using larvae of two stream salamanders, Gyrinophilus porphyriticus (predator) and Eurycea cirrigera (prey), by altering the densities and combinations of predator size classes in experimental streams. Results showed that the presence of large individuals can significantly reduce the impact of density changes of smaller conspecifics on prey survival through nonlethal means. In the absence of large conspecifics, an increase in the relative frequency of small predators significantly increased predation rates, thereby reducing prey survival. However, with large conspecifics present, increasing the density of small predators did not decrease prey survival, resulting in a 14.3% lower prey mortality than predicted from the independent effects of both predator size classes. Small predators changed their microhabitat use in the presence of larger conspecifics. Prey individuals reduced activity in response to large predators but did not respond to small predators. Both predators reduced prey growth. These results demonstrate that the impact of a predator can be significantly altered by two different types of trait-mediated indirect effects in two-species systems: between different-sized cohorts and between different cohorts and prey. This study demonstrates that predictions based on simple numerical changes that assume independent effects of different size classes or ignore size structure can be strongly misleading. We need to account for the size structure within predator populations in order to predict how changes in predator abundance will affect predator-prey dynamics.     

Mechanisms of nonlethal predator effect on cohort size variation: ecological and evolutionary implications

Understanding the factors responsible for generating size variation in cohorts of organisms is important for predicting their population and evolutionary dynamics. We group these factors into two broad classes: those due to scaling relationships between growth and size (size-dependent factors), and those due to individual trait differences other than size (size-independent factors; e.g., morphology, behavior, etc.). We develop a framework predicting that the nonlethal presence of predators can have a strong effect on size variation, the magnitude and sign of which depend on the relative influence of both factors. We present experimental results showing that size-independent factors can strongly contribute to size variation in anuran larvae, and that the presence of a larval dragonfly predator reduced expression of these size-independent factors. Further, a review of a number of experiments shows that the effect of this predator on relative size variation of a cohort ranged from negative at low growth rates to positive at high growth rates. At high growth rates, effects of size-dependent factors predominate, and predator presence causes an increase in the scaling of growth rate with size (larger individuals respond less strongly to predator presence than small individuals). Thus predator presence led to an increase in size variation. In contrast, at low growth rates, size-independent factors were relatively more important, and predator presence reduced expression of these size-independent factors. Consequently, predator presence led to a decrease in size variation. Our results therefore indicate a further mechanism whereby nonlethal predator effects can be manifest on prey species performance. These results have strong implications for both ecological and evolutionary processes. Theoretical studies indicate that changes in cohort size variation can have profound effects on population dynamics and stability, and therefore the mere presence of a predator could have important ecological consequences. Further, changes in cohort size variation can have important evolutionary implications through changes in trait heritability.     

Tidal regime dictates the cascading consumptive and nonconsumptive effects of multiple predators on a marsh plant

Prey perception of predators can dictate how prey behaviorally balance the need to avoid being eaten with the need to consume resources, and this perception and consequent behavior can be strongly influenced by physical processes. Physical factors, however, can also alter the density and diversity of predators that pursue prey. Thus, it remains uncertain to what extent variable risk perception and antipredator behavior vs. variation in predator consumption of prey underlie prey-resource dynamics and give rise to large-scale patterns in natural systems. In an experimental food web where tidal inundation of marsh controls which predators access prey, crab and conch (predators) influenced the survivorship and antipredator behavior of snails (prey) irrespective of whether tidal inundation occurred on a diurnal or mixed semidiurnal schedule. Specifically, cues of either predator caused snails to ascend marsh leaves; snail survivorship was reduced more by unrestrained crabs than by unrestrained conchs; and snail survivorship was lowest with multiple predators than with any single predator despite interference. In contrast to these tidally consistent direct consumptive and nonconsumptive effects, indirect predator effects differed with tidal regime: snail grazing of marsh leaves in the presence of predators increased in the diurnal tide but decreased in the mixed semidiurnal tidal schedule, overwhelming the differences in snail density that resulted from direct predation. In addition, results suggest that snails may increase their foraging to compensate for stress-induced metabolic demand in the presence of predator cues. Patterns from natural marshes spanning a tidal inundation gradient (from diurnal to mixed semidiurnal tides) across 400 km of coastline were consistent with experimental results: despite minimal spatial variation in densities of predators, snails, abiotic stressors, and marsh productivity, snail grazing on marsh plants increased and plant biomass decreased on shorelines exposed to a diurnal tide. Because both the field and experimental results can be explained by tidal-induced variation in risk perception and snail behavior rather than by changes in snail density, this study reinforces the importance of nonconsumptive predator effects in complex natural systems and at large spatial scales.     

Predator hunting mode and habitat domain alter nonconsumptive effects in predator-prey interactions

Predators can affect prey populations through changes in traits that reduce predation risk. These trait changes (nonconsumptive effects, NCEs) can be energetically costly and cause reduced prey activity, growth, fecundity, and survival. The strength of nonconsumptive effects may vary with two functional characteristics of predators: hunting mode (actively hunting, sit-and-pursue, sit-and-wait) and habitat domain (the ability to pursue prey via relocation in space; can be narrow or broad). Specifically, cues from fairly stationary sit-and-wait and sit-and-pursue predators should be more indicative of imminent predation risk, and thereby evoke stronger NCEs, compared to cues from widely ranging actively hunting predators. Using a meta-analysis of 193 published papers, we found that cues from sit-and-pursue predators evoked stronger NCEs than cues from actively hunting predators. Predator habitat domain was less indicative of NCE strength, perhaps because habitat domain provides less reliable information regarding imminent risk to prey than does predator hunting mode. Given the importance of NCEs in determining the dynamics of prey communities, our findings suggest that predator characteristics may be used to predict how changing predator communities translate into changes in prey. Such knowledge may prove particularly useful given rates of local predator change due to habitat fragmentation and the introduction of novel predators.     

Ontogeny of predator-sensitive foraging and routine metabolism in larval shorthorn sculpin, <Emphasis Type="Italic">Myoxocephalus scorpius</Emphasis>

Most animals will reduce foraging activity in the presence of a predatory threat. However, little is known about the onset of this decision-making ability during the early life stages of fishes, and how the trade-off between foraging and predator-avoidance may be affected by changes in metabolic demand during ontogeny. To examine these issues, the foraging behaviour of larval shorthorn sculpin Myoxocephalus scorpius was monitored during visual exposure to a predatory threat (juvenile Atlantic cod, Gadus morhua) throughout development at 3°C (March–April, 2004). Larvae did not respond to predatory exposure during the first week post-hatch, but thereafter showed drastic reductions in foraging activity when exposed to predators. During early development, the mass-specific routine metabolism of shorthorn sculpin larvae displayed a triphasic ontogeny and peaked during metamorphosis. This high mass-specific metabolic demand could make reduced foraging under predation threat very costly during this stage of development. To further investigate this possibility, additional experiments were performed (March–April, 2005) where larvae were reared with visual exposure to predators for 6 h day⁻¹ during the feeding period. At 7-week post-hatch, larvae exposed to predators were smaller (wet mass and SL), showed decreased levels of whole-body lipids and certain fatty acids, and experienced higher rates of mortality as compared to control larvae. In environments where abundant predators cause larval fish to reduce their foraging rate, growth and survival of larvae may be negatively affected. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Interactions of an Insecticide with Larval Density and Predation in Experimental Amphibian Communities

Abstract: This study examines the effects of the short-lived insecticide carbaryl, a neurotoxin, on amphibian communities experiencing natural stresses of competition and predation. Tadpoles of three species (  Woodhouse's toad [ Bufo woodhousii ], gray treefrog [ Hyla versicolor ], and green frog [ Rana clamitans ]), representing a commonly encountered assemblage in Missouri, were reared in outdoor polyethylene pond mesocosms. We determined the effects of initial tadpole density ( low or high), predation (newts [  Notophthalmus viridescens ] absent or present), chemical exposure (0, 3.5, or 7.0 mg /L carbaryl), and their interactions on body mass, larval period, and survival to metamorphosis. Green frogs in high-density ponds did not reach metamorphosis, but metamorphs in low-density ponds and tadpoles in high-density ponds were not significantly affected by treatments or their interaction. Carbaryl reduced survival to metamorphosis in toads and treefrogs and increased mass at metamorphosis in treefrogs. Effects of carbaryl varied with predator environment and initial larval density. Interactions of carbaryl with predator and with density may result in an indirect effect of carbaryl causing increased food resources through the elimination of zooplankton populations that may compete for similar resources. Our results indicate that differences in biotic conditions influenced the potency of carbaryl and that even low concentrations induce changes that may alter community dynamics in ways not predicted from single-factor, laboratory-based studies.     

A trade-off between prey- and predator-induced polyphenisms in larvae of the salamander Hynobius retardatus

Organisms in natural habitats participate in complex ecological interactions that include competition, predation, and foraging. Under natural aquatic environmental conditions, amphibian larvae can simultaneously receive multiple signals from conspecifics, predators, and prey, implying that predator-induced morphological defenses can occur in prey and that prey-induced offensive morphological traits may develop in predators. Although multiple adaptive plasticity, such as inducible defenses and inducible offensive traits, can be expected to have not only ecological but also evolutionary implications, few empirical studies report on species having such plasticity. The broad-headed larval morph of Hynobius retardatus, which is induced by crowding with heterospecific anuran (Rana pirica) larvae, is a representative example of prey-induced polyphenism. The morph is one of two distinct morphs that have been identified in this species; the other is the typical morph. In this paper, we report that typical larval morphs of Hynobius can respond rapidly to a predatory environment and show conspicuous predator-induced plasticity of larval tail depth, but that broad-headed morphs cannot respond similarly to a predation threat. Our findings support the hypothesis that induction or maintenance of adaptive plasticity (e.g., predator-induced polyphenism) trades off against other adaptive plastic responses (e.g., prey-induced polyphenism). For a species to retain both an ability to forage for larger prey and an ability to more effectively resist predation makes sense in light of the range of environments that many salamander larvae experience in nature. Our results suggest that the salamander larvae clearly discriminate between cues from prey and those from predators and accurately respond to each cue; that is, they adjust their phenotype to the current environment.     

Correlating gene expression with larval competence,and the effect of age and parentage on metamorphosis in the tropical abalone<Emphasis Type="Italic"> Haliotis asinina</Emphasis>

The non-geniculate crustose coralline alga (CCA) Mastophora pacifica can induce the metamorphosis of competent Haliotis asinina (Vetigastropoda) larvae. The ability to respond to this natural cue varies considerably with larval age, with a higher proportion of older larvae (e.g. 90 h) able to metamorphose in response to M. pacifica than younger larvae (e.g. 66 h). Here we document the variation in time to acquisition of competence within a larval age class. For example, after 18 h of exposure to M. pacifica, approximately 15 and 36% of 84 and 90-h-old H. asinina larvae had initiated metamorphosis, respectively. This age-dependent response to M. pacifica is also observed when different aged larvae are exposed to CCA for varying periods. A higher proportion of older larvae require shorter periods of exposure to CCA than younger larvae in order to initiate metamorphosis. In this experiment, as in the previous, a small proportion of young larvae were able to respond to brief periods of CCA exposure, suggesting that they had developed the same state of competency as the majority of their older counterparts. Comparisons of the proportions of larvae undergoing metamorphosis between families reveals that parentage also has a significant (P<0.05) affect on whether an individual will initiate metamorphosis at a given age. These familial differences are more pronounced when younger, largely pre-competent larvae (i.e. 66 h old) are exposed to M. pacifica, with proportions of larvae undergoing metamorphosis differing by as much as 10 fold between families. As these data suggest that variation in the rate of development of the competent state has a genetic basis, and as a first step towards identifying the molecular basis to this variation, we have identified numerous genes that are differentially expressed later in larval development using a differential display approach. Spatial expression analysis of these genes suggests that they may be directly involved in the acquisition of competence, or may play a functional role in the postlarva following metamorphosis.Communicated by M.S. Johnson, Crawley     

Improving escape responses of hatchery-reared scallops <Emphasis Type="Italic">Argopecten purpuratus</Emphasis>

Hatchery rearing of the scallop Argopecten purpuratus has resulted in successive generations of scallops not exposed to predators that are less sensitive to and escape more slowly from predators than wild scallops. The present study examined whether conditioning hatchery-reared A. purpuratus to its natural predator, the sea star Meyenaster gelatinosus, improved its escape responses. Both juvenile and adult A. purpuratus from Tongoy Bay, Chile, were exposed for 7 days to different conditions: (1) continuous predator odor, (2) predator contact for 30 min three times a day, (3) a combination of the two previous conditions, and (4) no exposure to the predator (control). After conditioning, we evaluated scallop’s escape responses: reaction time, total clap number, duration of the clapping response, clapping rate, and the time scallops spent closed when exhausted. Conditioning with contact and odor plus contact (i.e., high predation risk) resulted in 25 and 50% shorter reaction times of juveniles and adults, respectively. Further, these stimuli caused juveniles to increase the number of claps and clapping rate. For adults, the time spent closed after exhaustion decreased by 50 and 63% after conditioning with contact and odor plus contact, respectively. Therefore, it is shown for the first time that exposure of scallops to increasing predator stimuli enhances escape responses, evidence of threat-sensitive predator avoidance.     

Feeding selectivity in larvae of the European hake (<Emphasis Type="Italic">Merluccius merluccius</Emphasis>) in relation to ontogeny and visual capabilities

Feeding ecology was analysed for the first time in the larvae of the European hake (Merluccius merluccius) to determine whether their diet and selectivity were constrained by environmental conditions and how these feeding characteristics were related to ontogeny, prey availability and visual capabilities. Larvae collected during both day and night were analysed, and it was found that feeding incidence was high, regardless of the time of day. Examination of the visual system corroborated the hypothesis that hake larvae should be able to cope with a wide range of photic conditions and to forage even at low light intensity. A clear preference for adult calanoid copepods and, especially, for Clausocalanus spp. was observed in all sizes analysed. Prey number increased with larval size, but prey size did not. This finding indicates that hake larvae behave as selective and specialist predators that consume an increasing number of prey rather than larger prey during larval growth.