Krill motion in the Southern Ocean: quantifying in situ krill movement behaviors and distributions during the late austral autumn and spring

Krill movement behaviors and vertical distributions were measured in spring and autumn using a profiling stereo-camera and environmental sensor system to quantify seasonal changes in the role of krill in Southern Ocean food webs. Krill were observed in May–June 2013 and December 2014 in 3 bays in the Western Antarctic Peninsula. Krill abundances and movement behaviors were determined from in situ image sequences collected for up to 10 min throughout the water column, up to 625 m deep; 3,345 individual krill tracks were collected. Seasonal changes in individual krill behaviors coincided with seasonal shifts in krill vertical distributions. During late spring, net upward swimming direction (0.9 ± 2.1° from horizontal) and vertical velocity (0.3 ± 0.2 body lengths [BL] s−1) resulted in shallower maximum abundances of krill within the water column proximate to near-surface phytoplankton distributions. During late autumn, krill swimming patterns tended downward, including swimming direction (−5.2 ± 0.8° from horizontal) and vertical velocity (−0.1 ± 0.0 BL s−1), leading to deeper distributions proximate to the benthos. Individual krill motility was greater in spring than autumn, as evidenced by an increase in swimming speeds (5.4 ± 0.2 vs. 2.8 ± 0.0 BL s−1) and turning rates (120 ± 5 vs. 107 ± 2° s−1). Remarkably, krill in autumn were capable of swimming as quickly as krill in spring. These results suggest seasonal shifts in krill movement behaviors have direct ramifications for krill distributions, proximity to food sources, and impacts on biogeochemical cycling in coastal Antarctic waters

Is vertical migration in Antarctic krill (Euphausia superba) influenced by an underlying circadian rhythm?

Antarctic krill (Euphausia superba) is a keystone species in the southern ocean ecosystem where it is the main consumer of phytoplankton and constitutes the main food item of many higher predators. Both food and predators are most abundant at the surface, thus krill hide in the depth of the ocean during the day and migrate to the upper layers at night, to feed at a time when the predatory risk is lowest. Although the functional significance of this diel vertical migration (DVM) is clear and its modulation by environmental factors has been described, the involvement of an endogenous circadian clock in this behaviour is as yet not fully resolved. We have analysed the circadian behaviour of Euphausia superba in a laboratory setting and here we present the first description of locomotor activity rhythms for this species. Our results are in agreement with the hypothesis that the circadian clock plays a key role in DVM. They also suggest that the interplay between food availability, social cues and the light:dark cycle acts as the predominant Zeitgeber for DVM in this species