A mechanism for swarming in the tunicate Salpa thompsoni (Foxton, 1961)

Two populations of Salpa thompsoni, collected from the Weddell-Scotia confluencc area at the end of two summers (1994 and 1995) with contrasting densities, are compared. The present study was aimed at corroborating whether fluctuations in abundance could be related to sonic life history parameters considered as indicators of growth rate in salp populations, such as the body size of solitary reproductive individuals, number of buds per block or chain in the stolon, and the offspring to parent ratio. No differences were observed in the sizes reached by aggregate individuals, or in the size at which oocyte maturity was attained (moment at which the development of the embryo can be detected). There were no differences in the size reached by the embryos during their residence inside the mother's blastozooid. In 1995, the percentage of non-fecundated blastozooids (which included even the largest individuals) was higher than in 1994. The mean size of the solitary individuals was significantly higher in 1994 than in 1995. The number of buds per block was also significantly higher in most of the stages, confirming that this is the mechanism used by these organisms to produce swarms. This species eventually attains high population densities under favourable environmental conditions such as poor sea-ice cover in winter. Under optimal conditions, the number of potential descendants produced by a single solitary individual would exceed 800.