Sea Urchin Larvae Research Articles

The effect of copper ions on behavior of larvae of the sea urchin Strongylocentrotus intermedius at different temperatures of sea water

We investigated the behavior of larvae of the sea urchin Strongylocentrotus intermedius at the blastula and early pluteus stages in the water column at a temperature of 20, 22, and 23.5°C and in the presence of copper in concentrations of 0.01 and 0.02 mg/l. In clean sea water, at all tested temperatures, sea urchin larvae preferred the subsurface layer to where they rose from the bottom of the vessel to form dense aggregations. In water supplemented with copper, the behavior of larvae at 20 and 22°C was not different from that in clean water. An increase in temperature to 23.5°C and the addition of copper altered the behavior of the larvae; they did not leave near-bottom water, did not rise to the surface, although they continued moving.

The major yolk protein is synthesized in the digestive tract and secreted into the body cavities in sea urchin larvae

Major yolk protein (MYP), a transferrin superfamily protein contained in yolk granules of sea urchin eggs, also occurs in the coelomic fluid of male and female adult sea urchins regardless of their reproductive cycle. MYP in the coelomic fluid (CFMYP; 180 kDa) has a zinc-binding capacity and has a higher molecular mass than MYP in eggs (EGMYP; 170 kDa). CFMYP is thought to be synthesized in the digestive tract and secreted into the coelomic fluid where it is involved in the transport of zinc derived from food. To clarify when and where MYP synthesis starts, we investigated the expression of MYP during larval development and growth in Pseudocentrotus depressus. MYP mRNA was detected using RT-PCR in the early 8-arm pluteus stage and its expression persisted until after metamorphosis. Real-time RT-PCR revealed that MYP mRNA increased exponentially from the early 8-arm stage to metamorphosis. Western blotting showed that maternal EGMYP disappeared by the 4-arm stage and that newly synthesized CFMYP was present at and after the mid 8-arm stage. In the late 8-arm larvae, MYP mRNA was detected in the digestive tract using in situ hybridization, and the protein was found in the somatocoel and the blastocoel-derived space between the somatocoel and epidermis using immunohistochemistry. These results suggest that CFMYP is synthesized in the digestive tract and secreted into the body cavities at and after the early 8-arm stage. We assume that in larvae, CFMYP transports zinc derived from food via the body cavities to various tissues, as suggested for adults.

A conserved role for the nodal signaling pathway in the establishment of dorso‐ventral and left–right axes in deuterostomes

Nodal factors play crucial roles during embryogenesis of chordates. They have been implicated in a number of developmental processes, including mesoderm and endoderm formation and patterning of the embryo along the anterior-posterior and left-right axes. We have analyzed the function of the Nodal signaling pathway during the embryogenesis of the sea urchin, a non-chordate organism. We found that Nodal signaling plays a central role in axis specification in the sea urchin, but surprisingly, its first main role appears to be in ectoderm patterning and not in specification of the endoderm and mesoderm germ layers as in vertebrates. Starting at the early blastula stage, sea urchin nodal is expressed in the presumptive oral ectoderm where it controls the formation of the oral-aboral axis. A second conserved role for nodal signaling during vertebrate evolution is its involvement in the establishment of left-right asymmetries. Sea urchin larvae exhibit profound left-right asymmetry with the formation of the adult rudiment occurring only on the left side. We found that a nodal/lefty/pitx2 gene cassette regulates left-right asymmetry in the sea urchin but that intriguingly, the expression of these genes is reversed compared to vertebrates. We have shown that Nodal signals emitted from the right ectoderm of the larva regulate the asymmetrical morphogenesis of the coelomic pouches by inhibiting rudiment formation on the right side of the larva. This result shows that the mechanisms responsible for patterning the left-right axis are conserved in echinoderms and that this role for nodal is conserved among the deuterostomes. We will discuss the implications regarding the reference axes of the sea urchin and the ancestral function of the nodal gene in the last section of this review.

Humic Acids Increase Dissolved Lead Bioavailability for Marine Invertebrates

The presence of dissolved organic matter (DOM), as humic acids (HA), in natural waters is assumed to decrease dissolved metal bioavailability by binding metal ions and, therefore, decreasing the free ion concentration in solution. In this study, Pb complexation by HA in artificial seawater was checked by means of square wave anodic stripping voltammetry (SWASV). Uptake and toxicity of this metal in the absence and presence of HA was tested using excised gills of Mytilus edulis and the Paracentrotus lividus embryo-larval bioassay respectively. Both Pb uptake by mussel gills and Pb toxicity to sea urchin larvae increased in the presence of HA, and this increase was higher at higher HA concentrations. Since it is shown that the presence of DOM can enhance the uptake and toxicity of lead in some important marine invertebrate species, these results challenge the general applicability of the free ion activity and related models used for deriving environmental water quality criteria for metals.

Evolution of direct‐developing larvae: selection vs loss

Observations of a sea urchin larvae show that most species adopt one of two life history strategies. One strategy is to make numerous small eggs, which develop into a larva with a required feeding period in the water column before metamorphosis. In contrast, the second strategy is to make fewer large eggs with a larva that does not feed, which reduces the time to metamorphosis and thus the time spent in the water column. The larvae associated with each strategy have distinct morphologies and developmental processes that reflect their feeding requirements, so that those that feed exhibit indirect development with a complex larva, and those that do not feed form a morphologically simplified larva and exhibit direct development. Phylogenetic studies show that, in sea urchins, a feeding larva, the pluteus, is the ancestral form and the morphologically simplified direct-developing larva is derived. The current hypothesis for evolution of the direct-developing larval form in sea urchins suggests that major developmental changes occur by neutral loss of larval features after the crucial transition to a nonfeeding life history strategy. We present evidence from Clypeaster rosaceus, a sea urchin with a life history intermediate to the two strategies, which indicates that major developmental changes for accelerated development have been selected for in a larva that can still feed and maintains an outward, pluteus morphology. We suggest that transformation of larval form has resulted from strong selection on early initiation and acceleration of adult development.

A switch in the cellular basis of skeletogenesis in late-stage sea urchin larvae

Primary mesenchyme cells (PMCs) are solely responsible for the skeletogenesis during early larval development of the sea urchin, but the cells responsible for late larval and adult skeletal formation are not clear. To investigate the origin of larval and adult skeletogenic cells, I first performed transplantation experiments in Pseudocentrotus depressus and Hemicentrotus pulcherrimus, which have different skeletal phenotypes. When P. depressus PMCs were transplanted into H. pulcherrimus embryos, the donor phenotype was observed only in the early larval stage, whereas when secondary mesenchyme cells (SMCs) were transplanted, the donor phenotype was observed in late and metamorphic larvae. Second, a reporter construct driven by the spicule matrix protein 50 ( SM50) promoter was introduced into fertilized eggs and their PMCs/SMCs were transplanted. In the resultant 6-armed pluteus, green fluorescent protein ( GFP) expression was observed in both PMC and SMC transplantations, suggesting SMC participation in late skeletogenesis. Third, transplanted PMCs or SMCs tagged with GFP were analyzed by PCR in the transgenic chimeras. As a result, SMCs were detected in both larval and adult stages, but GFP from PMCs was undetectable after metamorphosis. Thus, it appears that SMCs participate in skeletogenesis in late development and that PMCs disappear in the adult sea urchin, suggesting that the skeletogenesis may pass from PMCs to SMCs during the late larval stage.

Food availability and physiological state of sea urchin larvae (Strongylocentrotus purpuratus)

Food availability is highly variable in the ocean. Many species of marine invertebrates have a larval form that depends upon exogenous nutrients for growth, yet there are few biochemical and physiological indices for determining changes in the nutritional status of larvae. In this study, the effects of food availability on biochemical compositions and metabolic processes of larvae of the sea urchin, Strongylocentrotus purpuratus, were determined. Larvae were cultured under different food concentrations (fed-to-excess and unfed) and a suite of biological processes assayed, ranging from measurements at the level of the whole organism to that of specific molecules. These data were normalized to DNA content (an index of cell number) to allow comparisons of physiological rates in larvae of different sizes. Changes in the following were measured during larval growth: free amino acid pool, protein, lipid classes (cholesterol, free fatty acids, hydrocarbons, phospholipids, triacylglycerol), enzyme activities (Na+, K+-ATPase and citrate synthase), and respiration rates. In growing larvae, the two key components that showed differential cell-specific content relative to unfed larvae were glycine in the free amino acid pool and phospholipids. Additionally, several lipid classes were detectable only in fed larvae (cholesterols, free fatty acids, and hydrocarbons). While triacylglycerols were present in eggs and utilized during pre-feeding development, they were not re-accumulated at detectable levels in feeding larvae. Respiration rates, protein content, and enzyme activities were all similar on a cell-specific basis, showing that these variables did not provide useful indices of differences in physiological state between fed and unfed larvae. In contrast, measurements of the cell-specific content of glycine and certain lipid classes did provide useful indices of physiological state of larvae. Application of these indices could potentially allow for determinations of nutritional state of larvae in the ocean.

Time and Extent of Ciliary Response to Particles in a Non-Filtering Feeding Mechanism

Mechanisms of suspension feeding are usually described by the physics of inanimate filters. High-speed videorecordings in this study demonstrated that sea urchin larvae concentrate particles without filtration. They actively captured individual particles. At most times and places, the effective strokes of the swimming/feeding ciliary band were away from the circumoral field. Cilia of this band responded to particles by a reversal of beat that redirected the particle toward the circumoral field. A change of beat occurred along approximately 80 micro m of ciliary band during particle capture. Cilia responded 0.02 to 0.06 s after the particle was within reach of effective strokes and reversed beat, usually for about 0.1 to 0.2 s. The whole event (disruption of forward beat) generally lasted between 0.13 and 0.5 s. These observations imply reversed movement of a parcel of water much larger than the included captured particle, but particles are nevertheless greatly concentrated because water is directed toward the circumoral field only when and where a particle is sensed. Thus most of the concentration of particles occurs by a temporarily and locally redirected current, without filtration, and size and quality of particles captured depends on sensory capabilities, not the mechanics of filtration.

Ars insulator protects transgenes from long-term silencing in sea urchin larva

Reporter genes have been used as a powerful tool to analyze cis-regulatory elements responsible for temporal and spatial expression in the early development of sea urchin. However, here we show that the transgenes introduced into the sea urchin embryos undergo suppression in larval stage. The transgene silencing could be one of major obstacle in the analysis of regulatory regions in the late stages of development. We previously demonstrated that a DNA fragment (ArsI) located in the upstream region of sea urchin (Hemicentrotus pulcherrimus) arylsulfatase gene has the property of an insulator. We show that tandem ArsI prevents silencing of a transgene in sea urchin larvae when the ArsI is fused to the 5' end of the reporter gene. Furthermore, we demonstrate that DNA of the reporter gene introduced into the sea urchin eggs is methylated during development and that the ArsI protects the transgene from the DNA methylation.

One Genome, One Piece of the Puzzle

The publication of species genomes is becoming almost commonplace--almost. In the last few months, the genomes of the honeybee (Wilson, 2006), the black cottonwood tree, a poplar (Stokstad, 2006), and the bacteria living in the gut of a parasitic worm (Stahl & Davidson, 2006) have been published. I must admit that I don't attend to this deluge of information too carefully. I do read summary essays that often accompany these publications, but that's about it. With so many other areas of biology to explore, I can't spend too much time on genomics. However when the genome of the purple sea urchin, Stronglyocentrotus purpuratus, was published in November (Sea Urchin Genome Sequencing Consortium et al., 2006), I did pay a little more attention to the results for a number of reasons. First, my interest was whetted by the Science cover photo of sea urchin larvae which caught my eye with their colors and delicacy. Also, when I was in high school I had a teacher who was involved in research on an anti-cancer drug derived from sea urchins. I don't think anything came of this work, but she woke me up to the beauty and mystery of these creatures. Finally, the summary of the findings on this genome made me want further information; this publication was more than just another run-of-the-mill genome-if such a thing exists. It is only a half dozen years since the publication of the human genome, and the number of species that have sequenced genomes is still minuscule relative to the diversity of life. The black cottonwood, Populus trichocarpa, was only the third plant to be sequenced, after Arabidopsis and rice. And though there are hundreds of thousands of named insect species, the honeybee is only the third insect to have its genome sequenced, along with fruit flies and mosquitoes: With such rarity, genome sequencing is still news, and as in any field where there is a scarcity of information, there is a wealth of surprises. Every article describing a new genome sequence highlights the unexpected in the findings: Honeybees have fewer genes for immunity than do other insects, even though bees live in close quarters, and the cottonwood sequence has been duplicated at least three times. This element of surprise is what makes genome exploration fun: there are wonders lurking around every genetic corner. Humans love to make generalizations from limited data, for example extrapolating all insects' genetic characteristics based on a couple of genomes. The only good thing about this dangerous habit is that it makes for a lot of wonderful surprises as assumptions are ripped to shreds by each new genome publication. The sea urchin genome is no exception, but it also has special importance for other reasons. Though u might seem surprising, this work provides crucial insights into the human genome. This is because, like the chordates--the taxon that humans belong to--the sea urchin is a deuterosome. And as its genome sequence verifies, this makes the sea urchin and other echinoderms more closely related to humans and other chordates than to other invertebrates such as insects, mollusks, and crustaceans (Pennisi, 2006). Finding out about sea urchin genes is another step in making sense of the human genome because it provides information about the genetic makeup of early chordates. and about where these genes might have originated. The sea urchin serves as a valuable outgroup for evolutionary comparisons. While the publication of the chimpanzee genome shed great light on the human genetic sequence, it is also important to have information about more distantly related species as points of comparison (Bottjer, Davidson, Peterson & Cameron. 2006? This made the mouse genome particularly useful, relative to that of the primates, but still more distant relatives, like the sea urchin, are also crucial for filling in the picture. History Yet another reason why the sea urchin genome merited a special section in the November 10. …

The effect of diet type on growth and fatty-acid composition of sea urchin larvae, I. Paracentrotus lividus (Lamarck, 1816) (Echinodermata)

This study investigated the growth and fatty acid composition of the sea urchin larvae Paracentrotus lividus fed on four types of diets: the microalgae Dunaliella tertiolecta, two types of microencapsulated formulated feeds and a concentrated algal paste. The larvae were successfully raised to metamorphosis on three diets, including the microalgae and the two formulated feeds. No significant difference was found in survival rate and metamorphosis rate for larvae fed with microalgae, formulated feeds, or microalgae plus formulated feeds. The fastest growth rate was obtained for larvae fed with microalgae, while normal growth and survival were only achieved for larvae fed with formulated feeds at a high frequency feeding rate. Normal larval development was not supported by either of the formulated feeds at a low feeding rate. A concentrated algal paste was also used for the low ration experiment, but failed to support any larval growth. Post-larval development was better for larvae fed on the formulated feeds, presumably reflecting the nutritional value of these feeds. P. lividus larvae grew well on D. tertiolecta which had a relatively low content of protein (37% dry weight) and lipid (7% dry weight). The larvae could assimilate highly unsaturated fatty acids (HUFAs) such as DHA, EPA and ARA from the diet, or synthesize them from linoleic acid 18:2 n-6 and linolenic acid 18:3 n-3. They could also biosynthesize non-methylene-interrupted-dienoic fatty acid NMID 20:2 de novo when feed on D. tertiolecta and the formulated feeds. An accumulation of PUFA n-3 may improve larval growth. P. lividus larvae showed the tendency of food selection towards D. tertiolecta, and a better formulated feed with additives or attractants will give a better performance in larval growth.

The effect of diet type on growth and fatty acid composition of the sea urchin larvae, II. Psammechinus miliaris (Gmelin)

This study investigated the growth and fatty acid composition of the sea urchin larvae Psammechinus miliaris fed on four types of diets: the microalgae Dunaliella tertiolecta, two types of microencapsulated formulated feed (L and F) and a concentrated algal paste. The fastest growth rate was obtained from D. tertiolecta, but there was no significant difference in larval survivorship between treatments. Larvae fed the formulated feeds showed good growth and survival, although those fed on feed L performed better than those fed on feed F. The metamorphosis rate of competent larvae fed on D. tertiolecta and feed L was not significantly different, but higher than those in the other treatments. Similar post-settlement survival was obtained for larvae fed on microalgae and both formulated feeds, yet 10 d post-settlement juveniles from larvae fed on the formulated feeds were significantly larger. The concentrated algal paste did not support normal growth of the larvae. P. miliaris larvae could grow well on relatively low protein (37% dry weight) and lipid (7% dry weight) D. tertiolecta diet, but high levels of dietary protein and lipid of the formulated feeds may benefit the post-settlement growth of newly metamorphosed juveniles. Dietary fatty acid had a strong influence on lipid composition of the larval tissue. The larvae could incorporate dietary highly unsaturated fatty acids (HUFAs) directly from the formulated feeds, or obtain ARA, EPA and DHA through the elongation of linoleic acid 18:2 n-6 and linolenic acid 18:3 n-3 when fed the HUFA-deficient D. tertiolecta. Higher levels of HUFAs did not seem to act as a growth promoter for the larvae, but a higher concentration of DHA in the formulated feeds may have improved post-settlement growth. An accumulation of PUFA n-3, especially 18:3 n-3, 18:4 n-3, 20:3 n-3 and 20:4 n-3 may promote larval growth. The shortage of PUFA n-3 and PUFA n-6, and especially the lack of non-methylene-interrupted-dienoic fatty acid NMID 20:2 may have impeded the growth of larvae fed on the algal paste. It is suggested that a better formulated feed with improved buoyancy and a more even particle size distribution would improve larval growth.

Study of Larval and Adult Skeletogenic Cells in Developing Sea Urchin Larvae

The larval skeleton of sea urchin embryos is formed by primary mesenchyme cells (PMCs). Thereafter, the larvae start feeding and additional arms develop. An adult rudiment that contains spines, tube feet, tests, and other parts of the adult body is formed in the eight-armed larva. The cellular mechanism of the later skeletogenesis and the lineage of the adult skeletogenic cells are not known. In this study, the morphogenesis of larval and adult skeletons during larval development of the sea urchin Hemicentrotus pulcherrimus was investigated by immunostaining cells with PMC-specific monoclonal antibodies, which are useful markers of skeletogenic cells. All spicules and the associated cells in the later larvae were stained with the antibodies. We could observe the initiation of skeletal morphogenesis at each developmental stage and visualize the cellular basis of skeleton formation in whole-mount embryos that possessed an intact morphology. There were some similarities between PMCs and the later skeletogenic cells. Both had a rounded shape with some filopodia, and the antigen expression started just before overt spicule formation. In the later-stage embryos, cells with filopodia and faint antigen expression were observed migrating in the blastocoel or aggregating in the presumptive location of new skeletogenesis.

Good eaters, poor swimmers: compromises in larval form

Compromises between swimming and feeding affect larval form and behavior. Two hypotheses, with supporting examples, illustrate these feeding-swimming trade-offs. (1) Extension of ciliated bands into long loops increases maximum clearance rates in feeding but can decrease stability of swimming in shear flows. A hydromechanical model of swimming by ciliated bands on arms indicates that morphologies with high performance in swimming speed and weight-carrying ability in still water differ from morphologies conferring high stability to external disturbances such as shear flows. Instability includes movement across flow lines from upwelling to downwelling water in vertical shear. Thus a hypothesis for the high arm elevation angles of sea urchin larvae, which reduce speed in still water, is that they reduce a downward bias imposed by the vertical shear in turbulence. Observations of sea urchin larvae in vertical shear and comparisons among brittle star larvae are consistent with the performance trade-offs predicted by the model. (2) Structures and behaviors that reduce swimming speed can enhance filtering for feeding. In the opposed-band feeding mechanisms of veligers and many trochophores, cilia push water to swim but movement of cilia relative to the water occurs when cilia overtake and capture particles. Features that may increase clearance rates at the expense of speed and weight capacity include structures that increase drag or body weight and a ciliary band that beats in opposition to the feeding-swimming current. Larval feeding mechanisms inherited from distant ancestors result in different swimming-feeding trade-offs. The different trade-offs further diversify larval form and behavior.

Ion flow regulates left–right asymmetry in sea urchin development

The degree of conservation among phyla of early mechanisms that pattern the left-right (LR) axis is poorly understood. Larvae of sea urchins exhibit consistently oriented LR asymmetry. The main part of the adult rudiment is formed from the left coelomic sac of larvae, the left hydrocoel. Although this left preference is conserved among all echinoderm larvae, its mechanism is largely not understood. Using two marker genes, HpNot and HpFoxFQ-like, which are asymmetrically expressed during larval development of the sea urchin Hemicentrotus pulcherrimus, we examined in this study the possibility that the recently discovered ion flux mechanism controls asymmetry in sea urchins as it does in several vertebrate species. Several ion-transporter inhibitors were screened for the ability to alter the expression of the asymmetric marker genes. Blockers of the H(+)/K(+)-ATPase (omeprazole, lansoprazole and SCH28080), as well as a calcium ionophore (A23187), significantly altered the normal sidedness of asymmetric gene expression. Exposure to omeprazole disrupted the consistent asymmetry of adult rudiment formation in larvae. Immuno-detection revealed that H(+)/K(+)-ATPase-like antigens in sea urchin embryos were present through blastula stage and exhibited a striking asymmetry being present in a single blastomere in 32-cell embryos. These results suggest that, as in vertebrates, endogenous spatially-regulated early transport of H(+) and/or K(+), and also of Ca(2+), functions in the establishment of LR asymmetry in sea urchin development.

Correction: Fixed metabolic costs for highly variable rates of protein synthesis in sea urchin embryos and larvae

Correction: Fixed metabolic costs for highly variable rates of protein synthesis in sea urchin embryos and larvae

Anodic stripping voltammetry measures copper bioavailability for sea urchin larvae in the presence of fulvic acids

Copper speciation in the presence of fulvic acids (FAs) was studied in chemically defined seawater by square wave anodic stripping voltammetry (SQWASV). A simple complexation model assuming a single type of ligand and a 1:1 reaction stoichiometry successfully explained the measured data. A conditional stability constant of 5.80 +/- 0.07 and a complexing capacity of 610 +/- 80 micromol Cu/g FAs were obtained. This conditional stability constant would correspond to 7.23 on a free copper ion basis. The complexation kinetics was easily followed by anodic stripping voltammetry (ASV) measurements, and complexation equilibria were not reached until approximately 12 h. Coupled to speciation studies, the toxicity of copper in the presence of FAs was studied with the sea urchin Paracentrotus lividus embryogenesis bioassay. A clear protective effect of FAs was observed on Cu toxicity, and this effect could be accurately explained by SQWASV Cu speciation measurements. Additional experiments in the presence of high Cu and FA concentrations were performed, and no extra toxicity of Cu-FA complexes was observed. These results are in agreement with metal bioavailability models and also underline the usefulness of ASV for determining copper bioavailability in the presence of natural organic matter.

From larval bodies to adult body plans: patterning the development of the presumptive adult ectoderm in the sea urchin larva

Echinoderms are unique among bilaterians for their derived, nonbilateral adult body plan. Their radial symmetry emerges from the bilateral larval body plan by the establishment of a new axis, the adult oral-aboral axis, involving local mesoderm-ectoderm interactions. We examine the mechanisms underlying this transition in the direct-developing sea urchin Heliocidaris erythrogramma. Adult ectoderm arises from vestibular ectoderm in the left vegetal quadrant. Inductive signals from the left coelom are required for adult ectodermal development but not for initial vestibule formation. We surgically removed gastrula archenteron, making whole-ectoderm explants, left-, right-, and animal-half ectoderm explants, and recombinants of these explants with left coelom. Vestibule formation was analyzed morphologically and with radioactive in situ hybridization with HeET-1, an ectodermal marker. Whole ectodermal explants in the absence of coelom developed vestibules on the left side or ventrally but not on the right side, indicating that left-right polarity is ectoderm autonomous by the gastrula stage. However, right-half ectodermal explants robustly formed vestibules that went on to form adult structures when recombined with the left coelom, indicating that the right side retains vestibule-forming potential that is normally suppressed by signals from the left-side ectoderm. Animal-half explants formed vestibules only about half the time, demonstrating that animal-vegetal axis determination occurs earlier. However, when combined with the left coelom, animal-half ectoderm always formed a vestibule, indicating that the left coelom can induce vestibule formation. This suggests that although coelomic signals are not required for vestibule formation, they may play a role in coordinating the coelom-vestibule interaction that establishes the adult oral-aboral axis.

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).

Complete larval development of the sea urchin Lytechinus variegatus fed an artificial feed

Complete development of sea-urchin larvae fed an artificial feed has been carried out for the first time. Naturally spawned gametes were obtained from adult Lytechinus variegatus fed an artificial feed in the laboratory for 9 months. Larvae from these adults, cultured in artificial seawater and fed microencapsulated capsules, reached the rudiment stage between 9 and 12 days, the usual time required. Larvae produced by artificial spawning and cultured in natural seawater and fed microencapsulated capsules or phytoplankton ( Dunaliella tertiolecta) had similar chronology of development and sizes of newly metamorphosed juveniles. The larvae fed capsules were significantly smaller than those fed D. tertiolecta. Larval survival was 72±6% for those fed capsules and 85±4% for those fed D. tertiolecta. Artificial feed for larvae eliminates the necessity of culturing phytoplankton and provides the potential of establishing nutritional requirements of larvae and of standardizing and manipulating the diet. This has application in mass aquaculture of sea urchins as well as using larvae as models in developmental studies.