in Antarctic krill, Euphausia superba

Antarctic krill, Euphausia superba, normally live in social aggregations (schools) but rarely aggregate in laboratory tanks. In order to study the e¡ect of stress on solitary living we tethered krill to wooden skewers and measured heart rate both when they were held isolated from conspeci¢cs and when they were held at normal schooling distances (1 body length). Heart rate did not di¡er signi¢cantly with sex or body size. However, intermoult krill had a signi¢cantly lower heart rate than postmoult animals. When two individuals were held at schooling distance, with one slightly higher in the water column than the other, the heart rate of the higher individual slowed signi¢cantly (106^98 beats min71), while that of the lower individual remained the same.We interpret these results to mean that krill living solitarily are stressed but will respond to neighbouring individuals by decreasing their metabolic rate and saving energy. Only two authors have reported that Antarctic krill, Euphausia superba Dana, exhibited schooling behaviour when held in laboratory tanks (O’Brien, 1987; Strand & Hamner, 1990). This is surprising given that krill in the ¢eld invariably occur in schools from furcilia stage IV (Hamner et al., 1989). We have tried without success to induce schooling in this species. Since we were unable to achieve this, we resorted to tethering krill s