Directional Swimming Variance in Salp Colonies May Underpin Diel Vertical Migration Behavior

Abstract

52 pages.Salps are pelagic tunicates that swim with jet propulsion. Their life cycle involves a colonial stage where multiple zooids pulse asynchronously to swim. Across the 48 described salp species, there is a diversity of colony forms depending on the positioning of zooids, referred to as colony architecture. Recent work shows that swimming speed differs across architecture types, suggesting that colony architecture influences locomotion. To test whether salp species vary in their directional swimming behavior and orientation, we used in situ SCUBA-based videography to analyze swimming tortuosity (crookedness of path) and directionality (swimming angle) across colony architectures. We found that in many cases streamlined colony architectures (linear) swam straighter than less streamlined architectures (whorl). Because some salp species are strong diel vertical migrators, we also tested how swimming behavior varies with light level and found significant differences in swimming directionality between night and day. In the daytime, non-streamlined colony architectures that are considered non-migrators or shallow migrators were more common and swim angle was predominantly upward towards the surface. At night, streamlined colony architectures that are deep migrators dominated, and swim angle was predominantly downward. Our findings therefore indicate that colony architecture plays a role in diel vertical migration. Overall, we find evidence for an association between colonial morphology, diel vertical migration, and swimming behavior