Feeding association of the copepod Rhincalanus gigas with the tunicate salp Salpa thompsoni in the southern ocean

Abstract

During a repeat grid survey and drogue study carried out in the Lazarev Sea in the austral summer of 1994 to 1995, a sudden collapse of a rich population of the tunicate Salpa thompsoni was observed at the onset of a phytoplankton bloom. This may have been related to the inability of salps to regulate their filtration rate and avoid clogging of their filtering apparatus at particle concentrations ≥1 mg (chlorophyll a) m⁻³. It was at this stage that large numbers of salp individuals had their branchial cavities invaded by the copepod Rhincalanus gigas. Incubations, to compare the feeding rates of R.␣gigas in the presence and absence of salps, showed that copepods are able to utilize the high concentrations of microplankton accumulated in the food strand of the salp, thus enhancing their grazing efficiency. This is likely to represent a typical form of opportunistic parasitism. However, the timing of the invasion, and the observation that most salps could survive prolonged exposure to R. gigas invasion, suggest that the association may also constitute a novel type of symbiosis. S.␣thompsoni could potentially benefit from R. gigas cleaning its filtering apparatus when clogging due to high particle concentrations occurs.