SPACING MECHANISMS WITHIN LIGHT-INDUCED COPEPOD SWARMS

Many species of copepods form dense aggregations, known as swarms. In the laboratory, we experimentally induced 5 different species of copep od 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-dim ensional swarm were videotaped. Since images of individual copepods ap pear indistinguishable on the paired 2-dimensional projections, an alg orithm was developed which matched the temporal changes of the vertica l (z) positions of all images from the 2-dimensional projections of th e 3-dimensional copepod movement to produce (x, y, z, t) positions of each individual. With the temporal/spatial positional data of swarm me mbers, we tested the hypothesis that the fluid disturbance surrounding individual moving copepods, rather than the exoskeleton, maintains mi nimum separation distance. As the density of the swarm increased, the average nearest-neighbor distance NND decreased, as did the mean minim um NND (MNND). For 3 of the 5 species, the MNND was significantly grea ter than that predicted from a random distribution, and was greater th an twice the antennule or prosome length. While occasional physical co ntact may occur, resulting in escapes or attempted mating, it appears that most swarm members re main outside the field of self-generated fl uid motion in the boundary layers surrounding their neighbors.