Benthic rays and skates have smaller livers with lower concentrations of lipids; they are therefore denser than water and they do not swim continuously, intermittently resting on the seabed. Some fish have no buoyancy aids but use their pectoral fins which are so angled as to give lift as they swim.
Benthic rays and skates have smaller livers with lower fat levels; this makes them heavier than water, causing them to rest occasionally on the ocean floor. Some fish do not need buoyancy aids but instead use their pectoral fins, which are angled in such a way that they generate lift as the fish swim.