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).     

Prefeeding larval development time is not correlated with egg size in regular echinoids (Strongylocentrotus species)

Summary

We compared prefeeding development times, from fertilized egg to prism larva, for Strongylocentrotus embryos from four clutches of eggs (each from a different species) differing in size. Development times did not vary consistently with egg diameter, and trends among eggs of different sizes varied with stage of development. In some cases, development times for eggs of intermediate diameter (S. franciscanus) were longer than those for larger or smaller eggs. Although mean egg diameters in clutches ranged from 84 μm (S. purpuratus) to 162 μm (S. pallidus), differences in development time to the last embryonic stage (prism) were very small. We conclude that the inverse relationship between parental investment in offspring and premetamorphic development time in echinoids depends only on the functional consequences of reduced size of feeding larval stages: effects of egg size on prefeeding development time are not evident.     

DEVELOPMENTAL CONSEQUENCES OF AN EVOLUTIONARY CHANGE IN EGG SIZE: AN EXPERIMENTAL TEST

Larvae of two species of sea urchins (Strongylocentrotus droebachiensis and S. purpuratus) differ in initial form and in the rate of development. To determine whether these differences are attributable to the large interspecific difference in egg size, we experimentally reduced egg size by isolating blastomeres from embryos. The rate of development of feeding larvae derived from isolated blastomeres was quantified using a novel morphometric method. If the differences early in the life histories of these two species are due strictly to differences in egg size, then experimental reduction of the size of S. droebachiensis eggs should yield an initial larval form and rate of development similar to that of S. purpuratus. Our experimental manipulations of egg size produced three clear results: 1) smaller eggs yielded larvae that were smaller and had simpler body forms, 2) smaller eggs resulted in slower development through the early feeding larval stages, and 3) effects of egg size were restricted to early larval stages. Larvae from experimentally reduced eggs of the larger species had rates of development similar to those of the smaller species. Thus, cytoplasmic volumes of the eggs, not genetic differences expressed during development, account for differences in larval form and the rate of form change. This is the first definitive demonstration of the causal relationship between egg size (parental investment per offspring) and life-history characteristics in marine benthic invertebrates. Because larval form influences feeding capability, the epigenetic effects of egg size on larval form are likely to have important functional consequences. Adaptive evolution of egg size may be constrained by the developmental relationships between egg size and larval form: evolutionary changes in egg size alone can result in concerted changes in larval form and function; likewise evolutionary changes in larval form and function can be achieved through changes in egg size. These findings may have broader implications for other taxa in which larval morphology and, consequently, performance may be influenced by changes in egg size.     

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.     

Bioenergetics of early life-history stages of the brachyuran crab Cancer setosus in response to changes in temperature

In many marine invertebrates, a latitudinal cline in egg size is considered an adaptive response to a decrease in temperature, and enhances the energetic fitness of their larvae at hatching. However, the amount of energy carried over from the egg to the larval stage depends on the metabolic efficiency of egg development. In the present study, eggs of the brachyuran crab Cancer setosus were sampled for their dry mass (DM), carbon (C), nitrogen (N), and fatty acid (FA) content throughout development from blastula stage until hatching of zoea 1-larvae at Antofagasta (23°S) and Puerto Montt 41°S (Chile) under different temperature treatments (12, 16 and 19 °C). Hatching zoea 1 larvae contained 60 ± 3% of the initial blastula egg C content, regardless of site or temperature. However, the ontogenetic decrease in egg C content was to a significantly higher extend based on the utilization of energy-rich FA at 12 °C (− 1.16 µg/egg) compared to the 19 °C treatments in Antofagasta and Puerto Montt (− 0.63 to − 0.73 µg FA per egg). At 19 °C egg-metabolism was based to a substantial extend on protein, which allowed for the saving of energy-richer lipids. We conclude that the production of larger eggs with high FA content appears to be adaptive not only to fuel the larval development, but is also a response to the prolonged egg developmental times at lower temperatures.     

Feeding and growth kinetics of the planktotrophic larvae of the spionid polychaete Polydora ciliata (Johnston)

We studied the effect of food concentration on the feeding and growth rates of different larval developmental stages of the spionid polychaete Polydora ciliata. We estimated larval feeding rates as a function of food abundance by incubation experiments with two different preys, presented separately, the cryptophyte Rhodomonas salina (ESD = 9.7 µm) and the diatom T.weissflogii (ESD = 12.9 µm). Additionally, we determined larval growth rates and gross growth efficiencies (GGE) as a function of R. salina concentration.P.ciliata larvae exhibited a type II functional response. Clearance rates decreased continuously with increasing food concentration, and ingestion rates increased up to a food saturation concentration above which ingestion remained fairly constant. The food concentration at which feeding became saturated varied depending on the food type, from ca. 2 µg C mL^− 1 when feeding on T. weissflogii to ca. 5 µg C mL^− 1 when feeding on R. salina. The maximum carbon specific ingestion rates were very similar for both prey types and decreased with increasing larval size/age, from 0.67 d^− 1 for early larvae to 0.45 d^− 1 for late stage larvae. Growth rates as a function of food concentration (R. salina) followed a saturation curve; the maximum specific growth rate decreased slightly during larval development from 0.22 to 0.17 d^− 1. Maximum growth rates were reached at food concentrations ranging from 2.5 to 1.4 µg C mL^− 1 depending on larval size. The GGE, estimated as the slope of the regression equations relating specific growth rates versus specific ingestion rates, were 0.29 and 0.16 for early and intermediate larvae, respectively. The GGE, calculated specifically for each food level, decreased as the food concentration increased, from 0.53 to 0.33 for early larvae and from 0.27 to 0.20 for intermediate larval stages.From an ecological perspective, we suggest that there is a trade-off between larval feeding/growth kinetics and larval dispersal. Natural selection may favor that some meroplanktonic larvae, such as P.ciliata, present low filtration efficiency and low growth rates despite inhabiting environments with high food availability. This larval performance allows a planktonic development sufficiently long to ensure efficient larval dispersion.     

Sexual size dimorphism in the ontogeny of the solitary predatory wasp Symmorphus allobrogus (Hymenoptera: Vespidae)

Sex-specific patterns of individual growth, resulting in sexual size dimorphism (SSD), are a little studied aspect of the ontogeny related to the evolutionary history and affected by the ecology of a species. We used empirical data on the development of the predatory wasp Symmorphus allobrogus (Hymenoptera, Vespidae) to test the hypotheses that sexual differences of growth resulting in the female-biased SSD embrace the difference in (1) the egg size and the starting size of larva, (2) the larval development duration, and (3) the larval growth rate. We found that eggs developing into males and females have significant differences in size. There was no significant difference between the sexes in the duration of larval development. The relative growth rate and the food assimilation efficiency of female larvae were significantly higher than compared to those of male larvae. Thus, the SSD of S. allobrogus is mediated mainly by sexual differences in egg size and larval growth rate.     

Studies on the interactions of sperm with the surface of the sea urchin egg

We have examined the relationship between sperm adhesion and fertilization in the cross species insemination of Arbacia punctulata eggs by Strongylocentrotus purpuratus sperm. As previously reported (Kinsey et al., 1980) the addition of S. purpuratus egg jelly results in induction of the acrosome reaction in sperm and significant numbers of S. purpuratus sperm adhere to A. punctulata eggs. However, in the absence of S. purpuratus egg jelly, S. purpuratus sperm fail to bind to A. punctulata eggs. Although at least 200 S. purpuratus sperm bind to an A. punctulata egg in the presence of S. purpuratus jelly, less than 8% of the eggs are fertilized. The adhesion of S. purpuratus sperm meets the same functional criteria as homologous A. punctulata sperm-egg adhesion. Electron microscopy shows that S. purpuratus sperm that have undergone the acrosome reaction adhere to A. punctulata eggs by their bindin-coated acrosomal process in a manner that is morphologically identical to that observed with homologous A. punctulata sperm. We have also compared the ability of S. purpuratus and A. punctulata sperm to fuse and fertilize with A. punctulata eggs after removal of the vitelline layer. Using high levels of sperm of either species, heterologous as well as homologous fertilization is readily detectable. Under these conditions, where stable binding is not demonstrable, there is no difference in the ability of S. purpuratus and A. punctulata sperm to fertilize A. punctulata eggs. These observations suggest that the failure of S. purpuratus sperm to fertilize A. punctulata eggs under normal conditions may be due to their inability to penetrate the vitelline layer so that they can fuse with the egg plasma membrane. In relation to the possible mechanism of vitelline layer penetration, we have also investigated the mode of action of chymostatin, an inhibitor of chymotrypsin that has been reported to inhibit fertilization of sea urchin eggs (Hoshi et al., 1979). Our findings suggest that the fertilization inhibitory activity of chymostatin is not related to its antichymotrypsin activity. Rather, it appears that this inhibition is due to the induction of an abnormal acrosome reaction in sperm that precludes formation of the acrosome process.     

Fatty acid content of eggs determines antipredator performance of fish larvae

Recent work has suggested that provisioning of eggs with certain critical nutrients could be a more meaningful measure of maternal investment and correlate of offspring fitness than traditional measures of egg size. The aim of our study was to assess variability in egg quality and larva quality and to identify connections between them and the implications for larval survival. Egg size, proximate composition, and fatty acid composition were measured for 40 batches of eggs from 8 captive pairs of red drum (Osteichthyes: Sciaenops ocellatus). We reared larvae from these batches of eggs to a common size (10 mm total length, 2-3 weeks posthatching) and assessed routine activity and escape response performance of 671 individuals. Egg fatty acid composition varied more than egg size or proximate composition. Concentrations of certain long chain, highly unsaturated essential fatty acids (e.g., arachidonic acid and docosahexaenoic acid) were the only egg traits that were significantly related to larva quality (measured as escape performance). Reduced escape performance of larvae from eggs with low fatty acid concentrations was not compensated by 3 weeks of feeding on a diet enriched with fatty acids, suggesting irreversible developmental effects. Since fatty acids in eggs originate from the maternal diet, offspring survival may be determined in part by availability of nutrient-rich prey to pre-spawning adults. Migrations, regime shifts, and exploitation of marine communities could operate through this mechanism to influence recruitment in fish populations. Our findings underscore the importance of non-genetic maternal contributions to egg quality and the linkage between environmental conditions experienced by adult females and offspring fitness.     

Effect of Acanthocephala infection on the reproductive potential of crustacean intermediate hosts

The effect of a naturally acquired infection by three acanthocephalan parasites Dentitruncus truttae, Echinorhynchus truttae, and Polymorphus minutus on the reproductive potential of their intermediate host, Echinogammarus tibaldii (Amphipoda) from Lake Piediluco (Centre of Italy) was assessed. During May 2007, 1135 amphipods were collected from two different samplings and examined for larval helminths. Forty-five amphipods were infected and of those, 16 were infected with D. truttae (intensity = 1-3 larvae), 15 with E. truttae (intensity = 1-2 larvae), and 14 with P. minutus (intensity = 1 larva). The sex ratio was nearly 1:1 in all examined amphipods. One female infected with D. truttae contained six eggs in the brood pouch and another female infected with E. truttae contained five eggs. However, none of the eight female amphipods harbouring P. minutus larva contained eggs in their brood pouch. Uninfected females of the same size and body length as that of the infected females contained between 20 and 32 eggs. No acanthocephalan species were found to co-occur.     

Survival and behavioral characteristics of amphidromous goby larvae of Sicyopterus japonicus (Tanaka, 1909) during their downstream migration

To understand the ecology and environmental tolerances of newly hatched larvae of the amphidromous fish Sicyopterus japonicus during their downstream migration, the salinity tolerance of eggs, 0-15 day old larvae, and adults, and the temperature tolerance, specific gravity and phototaxis of hatched larvae were examined. Tolerances of adults were measured as survival after a 24 h challenge in freshwater (FW), brackish water (1/3 SW) and seawater (SW). The survival rate of adult S. japonicus was 100% in FW and 1/3 SW, while none survived in SW. Hatching success of eggs (30 eggs each) was significantly higher in FW (mean: 73%) and 1/3 SW (73%) than in SW (19%). Tolerance of newly hatched larvae to salinity and temperature was investigated in different combinations of salinities (FW, 1/3 SW and SW) and temperatures (18, 23 and 28 °C). Larval survival was significantly different in each salinity and temperature. Survival rate was significantly higher in 1/3 SW than in FW and higher in SW than in FW at 23 °C and 28 °C. At the latter part of the experiment, there was no survival in FW and at 28 °C. Survival was higher in lower temperatures, but larval development did not occur in FW. Specific gravity of newly hatched larvae was 1.036 at 28 °C and 1.034 at 23 °C. When exposed to a light source on one side of an aquarium, larval distribution was not affected. Our results indicated larval S. japonicus are more adapted to brackish water and seawater than freshwater, while the adults and eggs are more adapted to freshwater and brackish water than seawater. This is consistent with their amphidromous life history with growth and spawning occurring in freshwater and the larval stage utilizing marine habitats.     

EVOLUTIONARY LOSS OF LARVAL FEEDING: DEVELOPMENT,FORM AND FUNCTION IN A FACULTATIVELY FEEDING LARVA,BRISASTER LATIFRONS

Species with large eggs and nonfeeding larvae have evolved many times from ancestors with smaller eggs and feeding larvae in numerous groups of aquatic invertebrates and amphibians. This change in reproductive allocation and larval form is often accompanied by dramatic changes in development. Little is known of this transformation because the intermediate form (a facultatively feeding larva) is rare. Knowledge of facultatively feeding larvae may help explain the conditions under which nonfeeding larvae evolve. Two hypotheses concerning the evolutionary loss of larval feeding are as follows: (1) large eggs evolve before modifications in larval development, and (2) the intermediate form (facultatively feeding larva) is evolutionarily short-lived. I show that larvae of a heart urchin, Brisaster latifrons, are capable of feeding but do not require food to complete larval development. Food for larvae appears to have little effect on larval growth and development. The development, form, and suspension feeding mechanism of these larvae are similar to those of obligate-feeding larvae of other echinoids. Feeding rates of Brisaster larvae are similar to cooccurring, obligate-feeding echinoid larvae but are low relative to the large size of Brisaster larvae. The comparison shows that in Brisaster large egg size, independence from larval food, and relatively low feeding rate have evolved before the heterochronies and modified developmental mechanisms common in nonfeeding echinoid larvae. If it is general, the result suggests that hypotheses concerning the origin of nonfeeding larval development should be based on ecological factors that affect natural selection for large eggs, rather than on the evolution of heterochronies and developmental novelties in particular clades. I also discuss alternative hypotheses concerning the evolutionary persistence of facultative larval feeding as a reproductive strategy. These hypotheses could be tested against a phylogenetic hypothesis.     

Effect of ellagic acid on the larvae of Spodoptera litura (Lepidoptera:Noctuidae) and its parasitoid Bracon hebetor (Hymenoptera:Braconidae)

The present study was aimed at investigating the effect of plant based compound, ellagic acid on parasitoid Bracon hebetor (Say) through its host, the common cutworm, Spodoptera litura (Fabricius). The effect on S. litura was ascertained by feeding six days old larvae on artificial diet incorporated with different concentrations (5 ppm, 25 ppm, 125 ppm, 625 ppm, 3125 ppm) of ellagic acid and water as control. Its effect on B. hebetor was determined by allowing the adult B. hebetor to parasitize the treated host larvae. The mortality of S. litura larvae was increased whereas adult emergence declined with increasing concentration of ellagic acid. The developmental period was delayed significantly and all the nutritional indices were reduced with treatment. Ellagic acid at LC₃₀ (7.70 ppm) had not much influence on the growth of parasitoid B. hebetor but LC₅₀ (43.45 ppm) adversely influenced the development of the parasitoid, B. hebetor when reared on treated larvae of S. litura. This was evident from reduced parasitization, fecundity, egg laying, egg hatching, emergence, increased larval mortality, reduced pupation and prolonged development of the immature stages at LC₅₀. However, parasitization, egg hatching and larval mortality of the parasitoid were not significantly impacted at LC₃₀ indicating the possibility of its use in integrated pest management programmes.     

Differentiated egg size of the cannibalistic salamander <Emphasis Type="Italic">Hynobius retardatus</Emphasis>

Larvae of the salamander, Hynobius retardatus, are carnivorous, and even though there are two morphs, a typical morph and a broad-headed or “cannibal” morph, both are cannibalistic. They also sometimes eat other large prey, for example larvae of the frog, Rana pirica. In natural habitats, use of both conspecific and R. pirica larvae as food may contribute more strongly to high survival and substantially to fitness when larval densities are higher, because early-stage H. retardatus larvae sometimes experience scarcity of their typical prey. In cannibalistic oviparous amphibians, larger individuals that developed from larger eggs can more efficiently catch and consume larger prey and thus their survival may be better than that of smaller individuals developed from smaller eggs. Populations might therefore diverge in respect of egg size in response to variation in the density of conspecific and R. pirica larvae in natural ponds, with eggs being larger when larval density is higher. I examined how variance in hatchling size correlated with the incidence of cannibalism, and whether increasing larval density in natural ponds correlated with increasing egg size. Variance in initial larval body size facilitated cannibalism, and egg size increased as larval density in the ponds increased. In ponds with high larval density, where cannibalism and large prey consumption is a critical factor in offspring fitness, the production of fewer clutches with larger eggs, and thus of fewer and larger offspring, results in greater maternal fitness. Variation among the mean egg size in populations is likely to represent a shift in optimum egg size across larval density gradients.     

Egg size, first instar behaviour and the ecology of Lepidoptera

There is striking variation in egg size among Lepidoptera. Part of the explanation could be a link between egg size and larval feeding ecology.
The relationship between absolute egg size and aspects of feeding ecology for different Lepidoptera families from different temperate regions is examined. Species that overwinter in the egg stage have larger eggs. There are significant differences in egg size with respect to feeding specificity but different families show different patterns. Woody plant feeders have larger eggs than herb feeders. There is little effect of proximity of the egg to the plant part that is eaten.
Patterns in the behaviour and survival of newly hatched larvae of 42 spp. of British Lepidoptera and their relationship to egg and larval size and to food plant characteristics are examined. Patterns in egg size with respect to feeding ecology are similar to those described above. There is a strong correlation between egg size and the size of newly hatched larvae. Newly hatched larvae survived for a mean of 1–20 days without food. Survival is not correlated with larval weight. Grass feeders survive longer than herb and woody plant feeders; the species surviving the longest feeds on lichens. Newly hatched larvae moved at a mean speed of 0.7-267.8 cm h^-1. Speed is not correlated with larval weight or survival time. Grass feeders move faster than woody plant feeders which, in turn, move faster than herb feeders. Woody plant feeders tend to move upwards, grass feeders downwards and herb feeders both upwards and downwards. The proportion of larvae silking is negatively correlated with larval weight.
The strong links between egg size and larval feeding ecology and between feeding ecology and larval behaviour are discussed. It is surprising that larval body size does not appear to constrain the speed of movement, nor tolerance to starvation.     

Laboratory evaluation of the chemosterilant lufenuron against the fruit flies Ceratitis capitata, Bactrocera dorsalis, B. cucurbitae, and B. latifrons

Four species of tephritid fruit flies, Ceratitis capitata, Bactrocera dorsalis, B. cucurbitae, and B. latifrons were evaluated for toxic, developmental, and physiological responses to the chemosterilant lufenuron. No significant mortality of laboratory strains of the first three species was observed after their exposure up to 50 μg/mL of lufenuron in agar adult diet, whereas B. latifrons adults fed with 50 μg/mL of lufenuron in the diet caused significant mortality compared to the control. Fertility of C. capitata adults fed on 50 μg/mL lufenuron-fortified diet between 7 and 12 days of age was approximately 46% of the no lufenuron control. Fertility of B. dorsalis and B. latifrons adults fed on 50 μg/mL lufenuron-incorporated diet was about 45% and 62% of the control, respectively. Lufenuron did not significantly affect fertility of B. cucurbitae adults. Lufenuron did not affect fecundity of C. capitata and B. dorsalis. Fecundity of B. cucurbitae and B. latifrons was not evaluated due to difficulty to count the eggs laid deep in the agar diet. Larvae fed on a liquid larval diet with ≤ 0.1 μg/mL of lufenuron were also evaluated. Pupal recovery, adult emergence, adult fliers, mating, egg hatch, and egg production of C. capitata were significantly decreased, while for B. dorsalis, pupal recovery, larval duration and adult emergence were affected. No effect of lufenuron on B. cucurbitae larvae was observed. B. latifrons was not performed because shortage of eggs at the time of this research. Lufenuron is a potential agent for management and control of C. capitata and B. dorsalis.     

Natural variation of carotenoids in the eggs and gonads of the echinoid genus, Strongylocentrotus: implications for their role in ultraviolet radiation photoprotection

We examined variability in carotenoid concentration in the gonads and eggs of four sea urchin species (Strongylocentrotus purpuratus, Strongylocentrotus franciscanus, Strongylocentrotus pallidus and Strongylocentrotus droebachiensis) to explore the possible role of carotenes as photoprotectants. Carotene concentrations were measured in gonads and gametes of each species, while in eggs the ultraviolet radiation (UV-R) sensitivity and self-shading capacity by carotenes were calculated. Mean concentrations of carotenes in gonads ranged from 0.13±0.017 mg g⁻¹ dw (S. purpuratus), 0.14±0.019 mg g⁻¹ dw (S. franciscanus), 0.29±0.079 mg g⁻¹ dw (S. pallidus) to 0.36±0.06 mg g⁻¹ dw (S. droebachiensis). In eggs, concentrations ranged from 0.026±0.003 to 0.09±0.034 mg g⁻¹ dw. UV-R sensitivity in eggs was quantified by measuring UV-R induced first-cleavage delay. Intra-specifically, cleavage delay varied significantly between individuals, and could be correlated with carotene concentration. Interspecific differences in cleavage delay and carotene concentrations were not correlated. Using the observed concentration of β, β-echinenone (which makes up between 82.4% and 94.9% of the total carotene concentration in the eggs) and a molar extinction coefficient of ε=13.7×10³ mol⁻¹ cm⁻¹ at 334 nm, we calculated self-shading efficiency in the eggs. Self-shading capacity (J₃₃₄) indicated that the eggs could only screen from 4.6% (J₃₃₄=0.046) down to 1.5% (J₃₃₄=0.015) of UV-R at 334 nm. While not sunscreens, we suggest that carotenes can photoprotective in echinoid eggs, probably by mitigating the effects of reactive oxygen species.     

Size-specific predation on marine invertebrate larvae

Predation on planktonic larval stages is frequently a major source of mortality for the offspring of benthic marine invertebrates. Mortality rate likely varies with larval size and developmental stage, but few experiments have measured how these factors affect predation rates. I used experimental reductions in egg size to test how variation in larval size affects the likelihood of predation during planktonic development. Blastomeres of the sand dollar Dendraster excentricus were separated at the two-cell stage to produce half-sized zygotes. Larvae resulting from this manipulation were tested for their susceptibility to predation relative to whole-sized siblings at four ages. Individuals from each size class were simultaneously presented as prey items to five predators (crab zoeae, crab megalopae, chaetognaths, solitary tunicates, and postlarval fish) in the laboratory. Four predators consumed significantly more half-sized larvae than whole-sized larvae, but one predator type (postlarval fish) consumed more whole-sized larvae. Predators that consumed more half-sized larvae also preferentially consumed younger larvae. In contrast, postlarval fish showed no significant prey preference based on larval age. These results suggest that assumptions of constant mortality rates during development should be modified to account for the effects of larval size and age.