Differences in relative heart mass among deep-sea and coastal sharks with increasing depth

Cardiac morphology and mass have been associated with activity levels for bony fishes and elasmobranchs; however, there is little information on cardiac morphology of deep-sea sharks (living primarily below 200 m) and how that morphology compares to cardiac morphology of shallow coastal species. We examined relative heart mass and relative ventricle dry mass in 10 species of sharks from coastal to bathydemersal habitats (23–870 m) to test our hypothesis that relative heart mass decreases with depth. Relative heart mass and relative ventricle dry mass decreased with increasing depth of capture for sharks. The coastal, obligate ram ventilator Carcharhinus limbatus had the highest relative heart mass (1.17 g/kg) and relative ventricle dry mass (0.18 g/kg), whereas the deep-sea species Centrophorus uyato had the lowest (relative heart mass, 0.34 g/kg; relative ventricle dry mass, 0.057 g/kg). Our findings of decreasing relative heart and ventricle size with increasing depth support the visual interaction hypothesis as an explanation for reduced metabolic capacity in deep-sea sharks.