Some sharks can stay motionless on the sand (white tips reef sharks, nurse sharks, etc.).
These sharks don't have to swim to bring oxygen to their gills like other sharks (grey, hammerheads tc.)
Mammals have a horizontal tail.
Fishes have a vertical fin.
Parrotfish create a protective envelope around them when they are sleeping.
So, don't put the light on them during night dive to don't to wake them up and break this envelope.
