FEEDING ASSOCIATION OF THE COPEPOD RHINCALANUS-GIGAS WITH THE TUNICATE SALP SALPA-THOMPSONI IN THE SOUTHERN-OCEAN

During a repeat grid survey and drogue study carried out in the Lazare v Sea in the austral summer of 1994 to 1995, a sudden collapse of a ri ch population of the tunicate Salpa thompsoni was observed at the onse t of a phytoplankton bloom. This may have been related to the inabilit y of salps to regulate their filtration rate and avoid clogging of the ir filtering apparatus at particle concentrations greater than or equa l to 1 mg (chlorophyll a) m(-3). It was at this stage that large numbe rs of salp individuals had their branchial cavities invaded by the cop epod 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 efficienc y. This is likely to represent a typical form of opportunistic parasit ism. However, the timing of the invasion, and the observation that mos t 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 filte ring apparatus when clogging due to high particle concentrations occur s.