Swimming in formation in krill (Euphausiacea), a hypothesis: Dynamics of the flow field, properties of antennular sensor systems and a sensory-motor link

Abstract

The act of swimming in formation by species such as Euphausia superba, Antarctic krill, is assumed to be regulated by a sensitivity to the characteristic and spatially elaborate flow field produced by this species of shrimp. We used a related species, Meganyctiphanes, North Atlantic krill, to visualize the flow field produced by tethered shrimps in an aquarium. In this situation, the propulsion jet flow some centimeters behind the shrimp is surrounded by a vortex ring of recoiling water motion from which, if the vortex is also produced by unrestrained swimming shrimp, a following shrimp hypothetically can draw forces of lift and propulsion to decrease energy expense in long-distance migration. Two antennular sensitivities to water vibration in frequency ranges 5-40 and 40-150 Hz were calibrated, and the activity of connected interneurons was traced into the abdominal pleopod-carrying segments. Water oscillation of 3-10 Hz frequency, applied to the antennules, was shown to entrain a closely synchronous pleopod beat in the stimulated specimens.