Heart rate as a measure of stress 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 effect of stress on solitary living we tethered krill to wooden skewers and measured heart rate both when they were held isolated from conspecifics and when they were held at normal schooling distances (∼1 body length). Heart rate did not differ significantly with sex or body size. However, intermoult krill had a significantly 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 significantly (106-98 beats min-1), 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.