Greenland sharks conform to expectations of neutral buoyancy but exhibit inter-individual variation.

Buoyancy is an omnipresent force that governs how aquatic animals move through three-dimensional space, leading to physiological and behavioural adaptions across species. Swimming effort during vertical movements is increasingly being used to infer buoyancy in animals that exhibit oscillatory diving behaviours and for which collecting direct measurements of buoyancy is infeasible. Here, we estimated the relative buoyancy of 29 Greenland sharks (Somniosus microcephalus) tagged with biologgers by comparing swimming effort [overall dynamic body acceleration (ODBA)/speed] during ascending and descending dives, and accounting for dive geometry. We found that buoyancy in Greenland sharks varied on both sides of the spectrum, with effort in the counter-buoyancy direction reaching as high as 225% of the effort exerted in the buoyancy-aided direction in negatively buoyant individuals, and up to 177% for positively buoyant individuals. Despite this, there was no overall bias towards positive or negative buoyancy for this species, supporting the expectation that slow-moving deep-sea species face selective pressure towards neutral buoyancy. Given that gliding behaviour was rare and that small deviations from neutral buoyancy could drive large differences in effort for a minimally active species, more research is needed to determine the exact magnitude of the buoyancy variation observed here. Even so, its impact on swimming efficiency probably regulates movement and foraging strategies, as sharks are forced to either cope with, or exploit, their tendency to sink or float. These findings also highlight the need for studies directly linking biologger-inferred buoyancy to true buoyancy, with implications for the study of body condition in sharks.