Spacing mechanisms within light-induced copepod swarms
Leising, Andrew W.
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Many species of copepods form dense aggregations, known as swarms. In the laboratory, we experimentally induced 5 different species of copepod to swarm in response to a point source of light. To map out the (x, y, z, t) positions of swarm members, 2 right-angle views of the 3-dimensional swarm were videotaped. Since images of individual copepods appear indistinguishable on the paired 2-dimensional projections, an algorithm was developed which matched the temporal changes of the vertical (z) positions of all images from the 2-dimensional projections of the 3-dimensional copepod movement to produce (x, y, z, t) positions of each individual. With the temporal/spatial positional data of swarm members, we tested the hypothesis that the fluid disturbance surrounding individual moving copepods, rather than the exoskeleton, maintains minimum separation distance. As the density of the swarm increased, the average nearest-neighbor distance NND decreased, as did the mean minimum NND (MNND). For 3 of the 5 species, the MNND was significantly greater than that predicted from a random distribution, and was greater than twice the antennule or prosome length. While occasional physical contact may occur, resulting in escapes or attempted mating, it appears that most swarm members remain outside the field of self-generated fluid motion in the boundary layers surrounding their neighbors.