Developmental plasticity in Macrophiothrix brittlestars: are morphologically convergent larvae also convergently plastic?

The pluteus larval forms of sea urchins (echinoids) and brittlestars (ophiuroids) use an internal skeleton to project arms that bear a long ciliated band used in swimming and feeding. The length of this ciliated band influences rates of maximum food clearance for larvae of both echinoderm classes and affects rates of growth and development in the plankton. Phylogenetic and morphological evidence, however, tend to support the view that the pluteus morphologies of the two classes are independently derived. Studies with echinoplutei have shown that investment in skeletal growth and ciliated band length changes in response to food conditions, with poorly fed larvae investing more in growth of the larval skeleton and arms either absolutely or in relation to other larval or developing postlarval structures. We present evidence for similar plasticity of skeletal growth in ophioplutei. We examined four species in the brittlestar genus Macrophiothrix that spanned a 3.8-fold range in egg size. Sibling larvae in 14 male-female crosses were reared with high (H) or low (L) food rations, and measurements were recorded for five skeletal arm rods and three non-arm body dimensions. The expression of adaptive plasticity (significantly longer arms in L versus H cultures on a given day) was apparent for most crosses in M. koehleri, the species with the smallest egg size. In the single cross for M. longipeda, larvae from L cultures had longer arms for their body length or stomach width than did larvae from H cultures. In these cases, plasticity was similar in timing, persistence, and magnitude to previously published results from echinoplutei. If internal skeletons are independently derived in the two classes, then plasticity in the expression of this homoplastic trait may itself be homoplastic.     

The effect of the quality of food patches on larval vertical distribution of the sea urchins Lytechinus variegatus (Lamarck) and Strongylocentrotus droebachiensis (Mueller)

Although most invertebrate larvae are weak swimmers and act as passive particles on horizontal scales, they may be able to regulate their vertical position in response to different factors, including increased food concentration. We examined the effect of the quality of food patches on larval vertical distribution for the sea urchins Lytechinus variegatus and Strongylocentrotus droebachiensis, and determined the effect of dietary conditioning on that response in the laboratory. We reared larvae on a mixed algal diet of Dunaliella tertiolecta and Isochrysis galbana under low (500 cells ml⁻¹) and high (5000 cells ml⁻¹) rations. Food patches were maintained in Plexiglas rectangular columns (30×10×10 cm) using a density gradient, where practical salinity in the bottom layer was 33, in the middle layer 30, and in the top layer 27. We examined the magnitude and mechanism of a behavioural response of larvae of L. variegatus in the four-arm stage, and on two developmental stages of S. droebachiensis (four- and six-arm), by manipulating patch quality. In the absence of a patch, larvae of both species and developmental stages swam through to the surface of the experimental columns. In the presence of algae, fewer larvae were present above the patch and more were at the patch than in control columns. More larvae swam through patches of “unflavoured” algal mimics than algal patches, and aggregated at the surface. Larval distribution relative to patches of algal filtrate without algal cells or of “flavoured” algal mimics in algal filtrate was not consistently different from that in either control or algal patches; thus, the magnitude of larval response to filtrate (with or without particles) was intermediate between that to control and algal patches. For L. variegatus, more larvae crossed the patches when reared on low than high rations, indicating that poorly conditioned larvae may be less responsive to environmental cues. Our results suggest that larvae can actively aggregate and maintain a vertical position in response to a food patch that depends on the quality and quantity of food. The response appears to be based mostly on a chemosensory rather than a mechanosensory mechanism.     

Larval feeding structure plasticity during pre-feeding stages of echinoids: Not all species respond to the same cues

Much of the work on phenotypic plasticity has focused on inducible defenses. As a result, little is known about induced phenotypes that improve the acquisition of resources (i.e. inducible offenses). Feeding larvae of several marine invertebrate species, gastropods and echinoderms, have inducible offenses, and produce larger feeding structures when given less food. To better understand inducible offenses, I investigated when in development sea urchin and sand dollar larvae can first alter their feeding morphology in response to different concentrations of food. Food induced feeding structure changes in both sea urchin and sand dollar larvae before larvae were able to ingest food. This suggests that the nervous system and a regulator gene, orthopedia, play a mechanistic role. In addition, larvae of the two species, Strongylocentrotus purpuratus and Dendraster excentricus, responded to different cues. Pre-feeding larvae of both species developed relatively shorter arms when given algal cells (i.e. chemical and physical stimuli), whereas only pre-feeding larvae of D. excentricus developed shorter arms when exposed to algal exudates (i.e. chemical stimuli). Larvae of neither species responded morphologically to the presence of polystyrene beads (i.e. physical stimuli).     

Evolution of feeding structure plasticity in marine invertebrate larvae: a possible trade-off between arm length and stomach size

The ability of an organism to alter its morphology in response to environmental conditions (phenotypic plasticity) occurs in several species of marine invertebrates. Examples are sea urchin and sand dollar larvae (plutei). When food is scarce, plutei produce longer food-gathering structures (larval arms and a ciliary band) and smaller stomachs than when food is abundant. However, it is unclear whether stomach size is actually induced through changes in morphogenesis or simply by food distending the stomach. Distinguishing between these two hypotheses is possible because plutei morphologically respond to food concentrations and change the length of their food-gathering structures before they are capable of feeding. More importantly, these two hypotheses provide insights to whether a trade-off exists between the response in food-gathering structures and the response in stomach size—a possible explanation for the evolution of feeding-structure plasticity in marine invertebrate larvae. In this study, I investigated whether sea urchin larvae (Strongylocentrotus purpuratus and S. franciscanus) reared in different amounts of food produced stomachs of different sizes before they were capable of feeding. Prior to having the ability to ingest food, larvae produced larger stomachs and shorter arms when food was abundant than when food was scarce, consistent with the hypothesis that food induced changes in morphogenesis. In addition, there was a strong negative correlation between the magnitude of plasticity in larval arm length and the magnitude of plasticity in stomach size. These results are consistent with the idea that a trade-off exists between the response in arm length and the response in stomach size, and at least in part, explains the evolution of feeding structure plasticity in plutei. This may also explain why feeding-structure plasticity has evolved in larvae of other taxa (e.g. other echinoderms and gastropods).     

Egg size and the evolution of phenotypic plasticity in larvae of the echinoid genus Strongylocentrotus

Planktotrophic larvae grow by utilizing energy obtained from food gathered in the plankton. Morphological plasticity of feeding structures has been demonstrated in multiple phyla, in which food-limited larvae increase feeding structure size to increase feeding rates. However, before larvae can feed exogenously they depend largely on material contained within the egg to build larval structures and to fuel larval metabolism. Thus, the capacity for plasticity of feeding structures early in development may depend on egg size. Using the congeneric sea urchins Strongylocentrotus franciscanus and S. purpuratus, which differ in egg volume by 5-fold, I tested whether the degree of expression of feeding structure (larval arm length) plasticity is correlated with differences in the size of the egg. I experimentally manipulated egg size of S. franciscanus (the larger-egged species) by separating blastomeres at the 2-cell stage to produce half-sized larvae. I reared half-size and normal-size larvae under high and low food treatments for 20 days. I measured arm and body lengths at multiple ages during development and calculated the degree of plasticity expressed by larvae from all treatments. Control and unmanipulated S. franciscanus larvae (from ∼ 1.0 nl eggs) had significantly longer arms relative to body size and a significantly greater degree of plasticity than half-sized S. franciscanus larvae (from < 0.18 nl eggs), which in turn expressed a significantly greater degree of plasticity than S. purpuratus larvae (from ∼ 0.3 nl eggs). These results indicate that egg size affects larval arm length plasticity in the genus Strongylocentrotus; larger eggs produce more-plastic larvae both in an experimental and a comparative context. However, changes in egg size alone are not sufficient to account for evolved differences in the pattern of plasticity expressed by each species over time and may not be sufficient for the evolutionary transition from feeding to non-feeding.     

Effects of fine grain environmental variability on morphological plasticity

Virtually all studies investigating morphological plasticity have focused on how organisms change in response to treatments that are constant throughout the experiment but which have different means. In this study, we investigated the possibility that organisms can morphologically respond to other environmental parameters like the amount of environmental variability or environmental maximum. Sea urchin larvae adjust the length of their feeding structure, a band of cilia, in response to different mean food concentrations. We investigated whether sea urchin larvae are also capable of responding to environmental variability or maxima by rearing larvae on four fluctuating diets, where all treatments had the same mean concentration of food. Larvae reared on a low variable diet produced longer larval arms (i.e. a longer ciliary band) than larvae reared on more variable diets. This response is similar to the morphological change that occurs when the mean food concentration is reduced (small mean = long arms), and indicates that organisms can morphologically respond to environmental parameters other than the environmental mean – such as the amount of environmental variability or the environmental maxima. We also quantified the shape of the relationship between larval arm length and fixed food concentrations to determine whether our results might be explained by nonlinearity in this relationship (i.e. Jensen's inequality). The shape of the relationship was inconsistent with a Jensen's inequality explanation. In addition, sea urchin larvae were unable to track fluctuations in food concentrations. This inability to track our imposed environmental fluctuations indicates that there was a time delay greater than 2 days in the response of larvae to changes in food concentrations. Since plutei likely experience fluctuations in food concentrations at least once a day, it is possible that larvae cannot track natural fluctuations in food concentration. We discuss the importance of our results in light of adaptive interpretations of plasticity and predictions of morphological response.     

Short-term fluctuation in salinity promotes rapid larval development and metamorphosis in Dendraster excentricus

The effect of constant and fluctuating salinity on larval development and metamorphosis of the sand dollar Dendraster excentricus was investigated in the laboratory. Sand dollar larvae at different stages of development were kept either at 32‰ (controls), exposed to constant low salinity (22‰) throughout development, or exposed to fluctuating salinity (i.e. transferring larvae from 32‰ to 22‰ for 7 days then back to 32‰ for the rest of their development). Larvae exposed to constant low salinity were significantly smaller but developed all larval arms at a slower rate than larvae in all other treatments. Larvae exposed to fluctuating salinity recovered and developed significantly longer larval arms and bigger rudiments than larvae kept at constant low salinity. Larvae exposed to fluctuating salinity produced more juveniles than larvae at constant high salinity (32‰), while those at constant low salinity produced few or no juveniles. Four-arm larvae exposed to fluctuating salinity produced significantly more juveniles than six-arm larvae exposed to the same treatment. Transferring competent 8-arm larvae from 31‰ to 15‰ for 2 days then back to 31‰, induced metamorphosis with juvenile production being significantly higher than for those kept at a constant salinity of 20, 25 and 31‰. This study indicates that a short-term decrease in salinity might induce metamorphosis for this species.     

HETEROCHRONIC DEVELOPMENTAL PLASTICITY IN LARVAL SEA URCHINS AND ITS IMPLICATIONS FOR EVOLUTION OF NONFEEDING LARVAE

Preexisting developmental plasticity in feeding larvae may contribute to the evolutionary transition from development with a feeding larva to nonfeeding larval development. Differences in timing of development of larval and juvenile structures (heterochronic shifts) and differences in the size of the larval body (shifts in allocation) were produced in sea urchin larvae exposed to different amounts of food in the laboratory and in the field. The changes in larval form in response to food appear to be adaptive, with increased allocation of growth to the larval apparatus for catching food when food is scarce and earlier allocation to juvenile structures when food is abundant. This phenotypic plasticity among full siblings is similar in direction to the heterochronic evolutionary changes in species that have greater nutrient reserves within the ova and do not depend on particulate planktonic food. This similarity suggests that developmental plasticity that is adaptive for feeding larvae also contributes to correlated and adaptive evolutionary changes in the transition to nonfeeding larval development. If endogenous food supplies have the same effect on morphogenesis as exogenous food supplies, then changes in genes that act during oogenesis to affect nutrient stores may be sufficient to produce correlated adaptive changes in larval development.     

Effects of egg size reduction and larval feeding on juvenile quality for a species with facultative-feeding development

In free-spawning marine invertebrates, larval development typically proceeds by one of two modes: planktotrophy (obligate larval feeding) from small eggs or lecithotrophy (obligate non-feeding) from relatively large eggs. In a rare third developmental mode, facultative planktotrophy, larvae can feed, but do not require particulate food to complete metamorphosis. Facultative planktotrophy is thought to be an intermediate condition that results from an evolutionary increase in energy content in the small eggs of a planktotrophic ancestor. We tested whether an experimental reduction in egg size is sufficient to restore obligate planktotrophy from facultative planktotrophy and whether the two sources of larval nutrition (feeding and energy in the egg) differentially influence larval survival and juvenile quality. We predicted, based on its large egg size, that a reduction in egg size in the echinoid echinoderm Clypeaster rosaceus would affect juvenile size but not time to metamorphosis. We reduced the effective size of whole (W) zygotes by separating blastomeres at the two- or four-cell stages to create half- (H) or quarter-size (Q) “zygotes” and reared larvae to metamorphosis, both with and without particulate food. Larvae metamorphosed at approximately the same time regardless of food or egg size treatment. In contrast, juveniles that developed from W zygotes were significantly larger, had higher organic content and had longer and more numerous spines than juveniles from H or Q zygotes. Larvae from W, H and Q zygotes were able to reach metamorphosis without feeding, suggesting that the evolution of facultative planktotrophy in C. rosaceus was accompanied by more than a simple increase in egg size. In addition, our results suggest that resources lost by halving egg size have a larger effect on larval survival and juvenile quality than those lost by withholding particulate food.     

Efficiencies and costs of larval growth in different food environments (Asteroidea: Asterina miniata)

The ocean is a nutritionally heterogeneous environment. For feeding larval forms, food variability has significant consequences for growth and later recruitment success. In this study, the physiological and biochemical responses to a range of different food concentrations (unfed, 4, 20, and 40 algal cells μl^− 1) were examined in larvae of the asteroid, Asterina miniata. Measurements of growth, protein synthesis rates, and the energetic cost of protein synthesis were made. Under conditions of rapid growth, protein comprised a larger percent (66%) of a larva's organic biomass compared to similar-aged, slower-growing larvae (26%). Larvae fed at the highest food concentration tested (40 algal cells μl^− 1) had a protein depositional efficiency of 80% (± 16%), a value 3-fold higher than larvae fed 20 algal cells μl^− 1 (28% ± 11%). Also, faster-growing larvae required 3-fold less energy per unit mass of protein growth. Larvae fed 40 algal cells μl^− 1 deposited protein at a respiratory cost of 65 ± 11 pmol O₂ h^− 1 (μg protein)^− 1; larvae fed 20 algal cells μl^− 1 had a cost of 192 ± 47 pmol O₂ h^− 1 (μg protein)^− 1. While there were differences in the cost to deposit protein (i.e., protein growth, the balance of synthesis and degradation), there were no differences in the energetic cost of protein synthesis for all food concentrations tested. The energetic cost of protein synthesis was fixed at 13.8 (± 0.92) Joules (mg protein synthesized)^− 1 and was independent of developmental stage, growth rates, and large changes (58-fold) in protein synthesis rates. A major conclusion from this study is that larvae grown in high-food environments not only grew faster, but did so for considerably less energy. Defining the complex relationships of food availability and metabolic efficiency will provide more accurate predictions of larval growth under variable food conditions in the ocean.     

Quantitative short-day photoperiodic response in larval development and its adaptive significance in an adult-overwintering cerambycid beetle, Phytoecia rufiventris

The chrysanthemum longicorn beetle, Phytoecia rufiventris, overwinters in the adult stage and reproduces in spring. Larvae of this beetle develop during summer inside a host stem or root. In the present study, photoperiodic control of larval development and its adaptive significance were examined in this beetle using an artificial diet. Larvae showed a short-day photoperiodic response at 25 °C with a critical day length of around 14 h; larvae reared under short-day conditions pupated, whereas those reared under long-day conditions entered summer diapause with some supernumerary molts and did not pupate. A similar response was found at 30 °C, but with a shorter critical day length. Below the critical day length, a shorter day length corresponded to a shorter larval period. Larvae transferred from long-day conditions to various photoperiods showed a similar quantitative response. Field rearing of larvae starting at various times of year showed that pupation occurs within a relatively short period in early autumn. Field rearing of pupae and adults at various times indicated that only pupation in early autumn results in a high survival rate until winter. Earlier or later pupation led to a low survival rate due to death before overwintering in the adult and pupal stages, respectively. Thus, in P. rufiventris, timing of pupation regulated by the quantitative short-day photoperiodic response is vital for survival. Relatively lower developmental threshold in the pupal stage supports this hypothesis.     

Correlated evolution of phenotypic plasticity in metamorphic timing

Phenotypic plasticity has long been a focus of research, but the mechanisms of its evolution remain controversial. Many amphibian species exhibit a similar plastic response in metamorphic timing in response to multiple environmental factors; therefore, more than one environmental factor has likely influenced the evolution of plasticity. However, it is unclear whether the plastic responses to different factors have evolved independently. In this study, we examined the relationship between the plastic responses to two experimental factors (water level and food type) in larvae of the salamander Hynobius retardatus, using a cause-specific Cox proportional hazards model on the time to completion of metamorphosis. Larvae from ephemeral ponds metamorphosed earlier than those from permanent ponds when kept at a low water level or fed conspecific larvae instead of larval Chironomidae. This acceleration of metamorphosis depended only on the permanency of the larvae's pond of origin, but not on the conspecific larval density (an indicator of the frequency of cannibalism) in the ponds. The two plastic responses were significantly correlated, indicating that they may evolve correlatively. Once plasticity evolved as an adaptation to habitat desiccation, it might have relatively easily become a response to other ecological factors, such as food type via the pre-existing developmental pathway.     

Effects of food concentration and availability on the incidence of cloning in planktotrophic larvae of the sea star Pisaster ochraceus

A decade ago, cloning was first observed in the planktotrophic larvae of sea stars obtained from plankton tows. However, no controlled experimental studies have investigated what factors may regulate this remarkable phenomenon. In the present study we offer the first documentation of cloning in the planktotrophic larvae of Pisaster ochraceus from the northern Pacific coast. This species was used as a model system to investigate three factors that may influence the incidence of asexual reproduction (cloning) in planktotrophic sea star larvae. In an initial experiment, larvae were reared under nine combinations of three temperatures and three food (phytoplankton) concentrations. Larvae reared at 12-15 degrees C and fed the highest food concentrations grew larger than the other larvae and produced significantly more clones. In a second experiment, qualitatively different algal diets were fed to larvae reared under the conditions found to be optimal in the initial experiment. Up to 24% of the larvae consuming a mixed phytoplankton diet of Isochrysis galbana, Chaetocerous calcitrans, and Dunaliella tertiolecta cloned, and significantly more clones were produced by these larvae than by those fed monospecific diets. Our experiments indicate that cloning generally occurs after larvae have attained asymptotic body length and only when food is abundant and of high quality. Since larval mortality is considered to be extremely high for marine invertebrates with planktotrophic larvae, production of clones under optimal conditions of temperature and food may serve to increase larval populations when the environment is most conducive to larval growth.     

Relative importance of parental and larval nutrition on larval development and metamorphosis of the sea urchin Strongylocentrotus droebachiensis

We examined the relative importance of parental nutritional condition and larval food ration on the rates of development, growth and metamorphosis of larvae of Strongylocentrotus droebachiensis (Müller) in a laboratory experiment. Parents were reared for 22 months on either a high ration of kelp (Laminaria spp., 6 days week⁻¹) supplemented with mussel flesh (Mytilus spp., 1 day week⁻¹) (KM), or a low ration of kelp (1 day week⁻¹) (KL). Larvae were fed either a high ration (5000 cells ml⁻¹) or a low ration (500 cells ml⁻¹) of microalgae (Dunaliella tertiolecta). Larval food ration had a strong effect on the rates of development, growth, and metamorphosis, which were all significantly greater in larvae fed the high ration. Test diameter of settlers also was significantly greater in the high than the low ration. Parental nutritional condition had little or no effect on the rates of development and growth, and no effect on settler size. The rate of metamorphosis was significantly higher in larvae from the KM than the KL treatment in the high but not the low ration (where rates of metamorphosis were similar). Although parental condition generally had a small effect on larval development, our results suggest that when planktonic food is abundant, larvae of adults from nutritionally rich habitats (such as kelp beds) may metamorphose sooner than those of adults from nutritionally poor habitats (such as barrens).     

Can salinity-induced mortality explain larval vertical distribution with respect to a halocline?

For the larvae of two echinoderm species that coexist in Atlantic Canada (bipinnaria of the sea star Asterias rubens and 4- and 6-arm echinoplutei of the sea urchin Strongylocentrotus droebachiensis), we examined the effect of short- and long-term exposure to salinity (ranging from 18 to 35) on the probability of larval survival in laboratory experiments. We also related larval vertical distributions in response to sharp haloclines generated in the laboratory to survival probability in the salinity of different layers in the water column. For both species and developmental stages, survival probability decreased with decreasing salinity, and a salinity range of 24-27 emerged as the critical threshold for larval tolerance. The relationship between the proportion of larvae that crossed a halocline into the top water layer and the survival probability of larvae in the salinity of that layer was significant for both species. Interestingly, the shape of this response was species-specific but not stage-specific for S. droebachiensis. Our findings suggest that larval avoidance of low-salinity water layers may be an adaptive behavior that increases survival and indirectly influences larval distribution.     

Effects of predators and food availability on activity and growth of Chironomus tentans (Chironomidae: Diptera)

1. Larvae of Chironomus tentans Fab, decreased the amount of time they spent outside their tubes as the presence of predatory pumpkinseed sunfish (Lepomis gibbosus L.) increased. Greatest reductions in activity occurred at low levels of fish presence; above a certain level further increases in fish presence had little effect on activity. 2. Whether the pattern of predator presence was ordered or random had no effect on larval behaviour. Larvae did not habituate to short- or long-term predator presence. 3. Larvae were less active when more food was available and predator-induced reductions in activity were negatively related to food availability. Larval activity was much higher in the dark than it was in the light. 4. Over 7 days, presence of fish reduced the proportion of third-instar larvae that moulted but did not affect head width or dry mass; low food availability reduced the number of larvae that moulted as well as head width and dry mass of larvae in the fourth instar. 5. Results indicate that the behavioural response of larval chironomids to predator presence depends strongly on environmental conditions and that estimating the developmental costs of these behavioural responses under field conditions will be complicated.     

Interactive effects of haloclines and food patches on the vertical distribution of 3 species of temperate invertebrate larvae

In laboratory experiments, we examined the effect of haloclines and determined whether the presence of food patches overrides this effect on larval vertical distribution of the sea star Asterias rubens, the sea urchin Strongylocentrotus droebachiensis and the mussel Mytilus edulis. We experimentally constructed haloclines in which the salinity of the bottom water layer was 35 and that of the top layer was 21, 24, 27, and 30 (21/35, 24/35, 27/35, and 30/35) for A. rubens and S. droebachiensis, and 24, 27, 30 and 32 (24/35, 27/35, 30/35, and 32/35) for M. edulis. For each species and stage, additional halocline treatments (A. rubens: 24/32 and 27/32; 4-arm S. droebachiensis: 21/29 and 24/32; 6-arm S. droebachiensis: 24/29 and 24/32; M. edulis: 27/32 and 30/32) were used to determine whether the larval response to inhibitory salinity gradients was due to the absolute salinity of the top layer or the relative salinity difference between the two layers. Also, we measured the density of A. rubens and M. edulis to determine whether the specific gravity of larvae can explain the observed vertical distributions. Larvae aggregated at and below the halocline and these aggregations were more pronounced with increasing strength of the vertical salinity gradient. Threshold salinities in the top layer which inhibited ~ 100% of the larvae from crossing the halocline were 24 for A. rubens and M. edulis, and 21 for S. droebachiensis. These distributional patterns were not the result of larval density, which was greater than all treatment water densities for M. edulis and S. droebachiensis and lower for A. rubens. The effect of the presence of a food patch at inhibitory haloclines (A. rubens: 24/35 and 27/35; 4-arm S. droebachiensis: 21/34 and 24/34; M. edulis: 27/35) was determined by using three algal densities: 0, 5000 or 10 000 cells ml^- 1Thalassiosira pseudonana in either the top or the bottom water layer. For both A. rubens and M. edulis, the number of larvae at the halocline increased in the presence of a food patch, but this effect did not depend on algal density in the patch. For 4-arm S. droebachiensis, there was no effect of the presence of a food patch on larval vertical distribution. Our results suggest that low salinity may act as a barrier to vertical movement and that the presence of food patches above the halocline may strengthen the larval aggregation response to inhibitory haloclines.     

UV wavelengths experienced during development affect larval newt visual sensitivity and predation efficiency

We experimentally investigated the influence of developmental plasticity of ultraviolet (UV) visual sensitivity on predation efficiency of the larval smooth newt, Lissotriton vulgaris. We quantified expression of SWS1 opsin gene (UV-sensitive protein of photoreceptor cells) in the retinas of individuals who had developed in the presence (UV+) or absence (UV−) of UV light (developmental treatments), and tested their predation efficiency under UV+ and UV− light (testing treatments). We found that both SWS1 opsin expression and predation efficiency were significantly reduced in the UV− developmental group. Larvae in the UV− testing environment displayed consistently lower predation efficiency regardless of their developmental treatment. These results prove for the first time, we believe, functional UV vision and developmental plasticity of UV sensitivity in an amphibian at the larval stage. They also demonstrate that UV wavelengths enhance predation efficiency and suggest that the magnitude of the behavioural response depends on retinal properties induced by the developmental lighting environment.     

Effect of changes in resource level on age and size at metamorphosis in Hyla squirella

Recent experiments suggest that timing of metamorphosis is fixed during development in some anurans, insects, and freshwater invertebrates. Yet, these experiments do not exclude a growth rate optimization model for the timing of metamorphosis. I manipulated food resources available to larvae of squirrel treefrogs (Hyla squirella) to determine if there is a loss of plasticity in duration of larval period during development and to critically test growth rate models for the timing of metamorphosis. Size-specific resource levels for individual tadpoles were switched from low to high or high to low at three developmental stages spaced throughout larval development. The effects of changes in resource availability on larval period and mass at metamorphosis were measured. Switching food levels after late limb bud development did not significantly affect larval period in comparison to constant food level treatments. Therefore, developmental rate in H. squirella is better described by a fixed developmental rate model, rather than a growth rate optimization model. The timing of fixation of developmental rate in H. squirella is similar to that found in other anuran species, suggesting a taxonomically widespread developmental constraint on the plasticity of larval period duration. Mass at metamorphosis was not significantly affected by the timing of changes in food levels; the amount of food available later in development determined the size at metamorphosis. Larval period and mass at metamorphosis were negatively correlated in only one of two experiments, which contrasts with the common assumption of a phenotypic trade-off between decreased larval period and increased mass at metamorphosis. Received: 19 August 1996 / Accepted: 20 June 1997     

Microhabitat selection by larval Chironomus tentans (Diptera: Chironomidae): effects of predators, food, cover and light

1. As part of a study designed to estimate the developmental costs of antipredator behaviour of larval chironomids, we used laboratory experiments to study effects of food and factors that could influence predation risk [presence of fish, cover from fish (simulated debris) and light level], on microhabitat selection by Chironomus tentans larvae in the third and fourth instar. 2. Larvae were more likely to build tubes where there was more food although their ability to move far to find food appeared limited. 3. Larvae did not avoid areas with fish and the presence of fish did not alter larval response to food. 4. Larvae avoided areas of cover (simulated debris) but cover did not alter larval response to food. 5. When provided with a choice between light and dark areas, larvae initially without tubes were found more often in the dark areas. Light level had no effect on location of larvae that had begun the experiment with tubes. 6. Results suggest the tubicolous life-style of larval Chironomus tentans limits their ability to select microhabitats that could alter their risk of predation.