Investigating the nutritional requirements of larval stages of crabs
The research of Dr. Steve Sulkin focuses on the early life history stages of crabs; specifically, investigating the factors that regulate larval survival and distribution. Given the relatively high fecundity of most crab species and the high potential for loss of larvae from the system, survival and dispersal of larvae can have profound impacts on adult crab population abundance and distribution. In addition, given that at certain times and at certain places larval crabs can dominate the mesozooplankton, they can have significant effects on community interactions, serving as both top down regulators of other plankton and as important prey for other invertebrates and finfish.
Typical of many marine invertebrates, most crab species produce a free-swimming independent larval form, the zoea, that differs considerably from the more familiar juvenile and adult. Zoeae are planktonic and most must feed actively on other planktonic forms to survive and develop normally. As crustaceans, larvae have a hard exoskeleton that they must cast off periodically to grow. The period of zoeal development is thus punctuated by molting events that define a specific number of zoeal stages, the number of which varies among species. These zoeal stages can differ from one another morphologically and behaviorally and can have different nutritional requirements that must be satisfied via the diet. After a period of zoeal development, crabs pass through an intermediate post-larval form, the megalopa. The megalopa can both swim in the water column and crawl about on the substrate, eventually seeking out suitable habitat for settlement and metamorphosis to the benthic juvenile.
Sources of loss of larvae to a population include dispersal away from suitable habitats, predation by other planktonic organisms, mortality imposed by unfavorable water quality conditions and nutritional stress. Sulkin's research at Shannon Point has focused on the nutritional requirements and feeding behavior of the zoeal stages. Past research by Sulkin and others has determined that larvae must feed immediately after hatching and early in each succeeding zoeal stage; that they require a dietary source of long chain omega-3 polyunsaturated fatty acids (PUFA) to develop normally to the megalopa; and that they can ingest a wide variety of prey types in a wide range of sizes. Larvae can ingest microalgae and other protists as well as small animal prey, discriminate among algal cells and reject some cell types. Although they readily ingest many types of protists, they do not appear to gain much nutritionally from doing so. The most successful prey types in laboratory culture tend to be small planktonic adult forms (e.g., rotifers) and the planktonic larval stages of other invertebrates (e.g., nauplii, trochophores).
Present research is focusing on the role played by prey types which themselves must feed in the plankton. This includes both adult micro- and mesoplankton and invertebrate larvae that feed on other planktonic forms. Questions being investigated include whether the diet of such prey affects their nutritional value to larval crabs. A special case of this research involves studying the nutritional value of prey that initially are sustained by stored energy reserves provided in the egg, but who soon deplete the stored reserves and must feed in the plankton. The early non-feeding phase of such prey is known provide a highly nutritional diet to support larval crab development, presumably because the crab larvae can exploit the same stored energy reserves as do the larval prey. It is not known, however, whether the later feeding phase of the same prey provides equal, or satisfactory, nutritional value to the larvae and how the prey's diet influences the process.
Among the algal cells that larvae can ingest are those toxic cell types responsible for red tides. Recent research in Sulkin's lab indicates that larvae will on one hand reject toxic cells of some species of algae, but ingest those of another closely-related species. Research is addressing whether heterotrophic prey that have previously fed on toxic algae vector algal toxin to the larval crabs. Experiments are being conducted using a toxic strain that can be ingested directly by crab larvae and one that is rejected by them, thus not providing a direct source of toxin.
The experimental approach involves hatching crab larvae in the laboratory from egg-bearing females, raising the larvae individually on target prey and determining larval survival and development rates. Prey (both autotrophic and heterotrophic) are maintained in the laboratory in sufficient quantities to be available to feed larvae on a daily basis. Both toxic and non-toxic prey are characterized using appropriate analytical techniques for total lipid, C:N ratios and long chain omega-3 PUFA constituency. Such characterizations assist in comparing nutritional values among prey and in distinguishing between toxic effects and nutritional deficiencies.
Research is carried out primarily by graduate students in WWU's Marine and Estuarine Science Program and undergraduates participating in Shannon Point's National Science Foundation supported Research Experiences for Undergraduates and Multicultural Initiatives in Marine Science: Undergraduate Participation programs.