Determining the Nutritional Role of Algae in the Diet of First Stage Brachyuran Crab Larvae

The survival and distribution of benthic invertebrate larvae have important implications for adult population dynamics. Nutrition and food availability have been considered among the primary determinants of larval survival. The opportunistic feeding strategy of brachyuran larval crabs, a combination of omnivory and selectivity, is complex and not yet completely understood. Such an opportunistic feeding strategy, beneficial in a sparse prey environment, may not always convey advantages. Although gut analyses of zoeae collected from the field have identified microalgae in the diet, and laboratory studies have confirmed that brachyuran zoeae will ingest a variety of microalgae, diets consisting solely of microalgae are generally not sufficient to sustain development under laboratory conditions. This study examined the role ingesting phytoplankton plays in the nutrition of first stage larval crabs by simulating conditions in which ingestion of algal prey might increase survival or accelerate development. Experiments included initial starvation treatments designed to assess the effect of algae when larval crabs do not encounter zooplankton immediately after hatching, as well as patchy prey diet experiments designed to assess the effect of algae when larval crabs encounter algae between encounters with zooplankton. Larvae of black-clawed crabs (Lophopanopeus bellus) were exposed to combinations of control zooplankton treatments consisting of rotifers, an experimental brown microalga Isochrysis treatment, and starvation. Stage survival and stage duration were compared. Results showed that as initial starvation duration was extended, larval survival decreased and development was delayed; however, no difference in survival and development was detected between unfed and algal diet treatments. Crabs in patchy prey experiments showed decreased survival and delayed development in comparison to the fed control; however, no difference in survival and development was detected between unfed and algal diet treatments. In both experiments results suggest that algae do not function to provision larvae with nutrients or energy during periods of no zooplankton prey, and do not increase survival or accelerate development of first-stage larval black-clawed crabs. Ingesting algae may be a neutral side effect of an opportunistic feeding strategy in an uncertain prey environment.