In estuarine systems, the brachyuran crabs exhibit a diversity of larval development and dispersal strategies. There is a gradient of dispersal ranging from strong retention within hatching habitats to rapid export of larvae from the estuary to the continental shelf. This dissertation examined the mechanisms by which physical transport and nutritional stress influence recruitment success in species with different larval dispersal strategies.
The first portion of the study examined how larval behaviors influence advective transport. Larval vertical migration patterns were examined within the York River, VA. Larvae from different families exhibited tidally and light driven vertical migration behaviors that promote different patterns of advective transport. The degree to which vertical migration is selected for by large scale processes is likely associated with the constraint on larval recruitment success imposed by factors within the estuary.
The transport patterns of vertically migrating larvae were examined using a three-dimensional hydrodynamic model of the York River. This study compared the distribution and dispersal of vertically migrating larvae to those of non-migrating particles. Differences in the phase of tidal vertical migration and light limited behaviors influenced the rate and direction of dispersal. Tidal shear fronts appear to strongly effect the distribution of larvae and may play an important role in determining the strength of biotic interactions such as predation and nutritional stress.
The role of nutritional stress was examined in a combined field and laboratory study. The laboratory study examined the impact of starvation and food quality on the survival and biochemical composition of larvae with different dispersal strategies. Larvae that develop on the continental shelf generally had greater resistance to starvation than those that develop in the estuary. This study examined the biochemical mechanisms underlying differences in nutritional resilience. Larvae that develop on the continental shelf showed more efficient utilization of essential biochemical constituents during nutritional stress. In particular, phospholipid metabolism appeared to be an important aspect of the starvation response of brachyuran zoeae.
The biochemical composition of larvae in the natural habitat was examined to assess variability in nutritional condition over short time scales associated with the tidal cycle. Larvae collected from the field were larger and had higher contents of important energetic lipids than those in laboratory studies. These data suggest that nutritional stress is relatively rare in the estuary and feeding conditions in the natural habitat promote the accumulation of energetic reserves. There was a large degree of short term variability in phospholipid composition of larvae in the plankton. These changes are likely associated with small scale physical factors that determine the contact rates between zoeae and their prey.
The results from this dissertation highlight the importance of small scale and ephemeral physical features in the recruitment success of crab zoeae. While large scale gradients play an important role in selecting for dispersal strategies, small scale physical processes on scales of hours strongly influence the variability in recruitment success of crab zoeae.