The importance of horizontal swimming behavior in maintaining patches of larval crabs.

摘要

To test the hypothesis that crab larvae alter their swimming behavior, horizontally, in response to biochemical cues in the water, fiddler crab ( Uca pugnax) and blue crab (Callinectes sapidus ) megalopae were video-recorded in a laminar-flow flume while putative cues were introduced and allowed to establish a gradient in the flow field. Specifically, changes in their horizontal orientation, distribution, and swimming speed were analyzed and compared across six different cue treatments---control (filtered offshore water) and five water-soluble exudates from biological sources: (1) conspecific adult, (2) conspecific megalopae, (3) interspecific megalopae, (4) predator, and (5) prey. The results showed that crab larvae can detect chemical cues in the ambient water column, and are sometimes able to discriminate between the cues and determine the direction of the cue source, adjusting aspects of their swimming behavior accordingly.; In complement to this laboratory-based observation of larval swimming behavior, a field investigation was conducted to test the idea that maintenance of patches, across time and space, is influenced by swarming behavior of the larvae. Fiddler crab and blue crab patches were tagged with a satellite-tracked drifter and followed across days while taking both physical and biological samples in a 4.0 km x 4.5 km grid (alongshelf and across-shelf, respectively). This allowed observation of changes in the spatial dynamics of the patches (i.e., larval density, size, and shape) and physical conditions in and around them (e.g., salinity, temperature, flow). There was some overlap of the two species, but the movement, abundance, and coherence of their aggregations differed. A two-dimensional trajectory of the patches supported previous conceptual models and yielded the first demonstration that patches can actually remain intact while moving through a buoyant plume, which further supports the idea that patches are maintained throughout development. In addition, related numerical modeling conducted by other members of our research group revealed that typical larval patches are not characterized by equal across- and alongshelf dimensions, but instead are slender shapes that are aligned with salinity contours. The bio-physical data collected here failed, however, to completely explain how patches are maintained, specifically the possible role of larval swimming behavior in offsetting their physical dispersion. The laboratory study suggests some possible behaviors (e.g., increase in larval swimming speed) that could be involved, though physical components in the field seem to be dominant. (Abstract shortened by UMI.)