The microdistribution and social behavior of some pelagic mysid shrimps

Certain pelagic mysids occur in well defined layers near the sea floor in bathymetric zones roughly parallel with shore, within which, during the day, they form large shoals composed of smaller swarms and schools. When light is present vision is apparently the primary, but not exclusive, mode of establishing contact between individuals. Aggregation occurs at night in field populations. Experiments showed that parallel swimming orientation occurs in total darkness but is less precise. At night the mysids probably respond to body contact and the currents created by their fellows; the possible role of general chemical cues in promoting aggregation was not established. Copulation occurs only at night. Apparently it involves emission of a specific chemical attractant by molting adult females, to which only adult males of the same species respond. The observations are interpreted in relation to microdistribution and to possible functions, which include: maintenance of position in the habitat, reduction of predation, efficiency of copulation, and population regulation. It is concluded that studies of structure and function of pelagic populations and communities must take sociality into account.