Daytime surface swarms of the euphausiid Thysanoessa spinifera were observed and sampled in spring 1985 in the Gulf of the Farallones off San Francisco, California. Samples were dipnetted at the surface from nine discrete swarm patches; and depth-stratified plankton tows were
made before, during, and after swarm activity. Analyses of sex and maturity stage of surface patch samples showed that 94% of all euphausiids were in breeding condition. Sex ratios differed significantly in six of the nine patches, with males dominating four and females dominating two patches.
This sexual segregation is thought to be related to differences in size between sexually mature males and females. Biological and oceanographic observations at the swarm site, and examinations of food baskets of swarm individuals, gave little indication that the animals were passively transported
to the surface, or that they were there to feed or avoid predators. T. spinifera eggs and Stage 1 nauplii were numerous in plankton collections taken the night following, and day after surface swarming was observed. It is concluded that swarming in this species is related to reproduction.
Its adaptive value in regard to different breeding-related activities is still unclear, but probably involves a combination of factors that relate to the timing of sexual maturation, egg release, and larval dispersal in the changeable upwelling environment off central California.
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