Fish fishes underwater ocean sea sealife nature 274. Wind waves 2 ft mixed swell w 1 to 2 ft at 8 secand sw 2 ft at 15 seconds. Mantle length ml is the standard size measure for coleoid cephalopods shell diameter being more common for nautiluses and is almost universally reported in the scientific literaturethe mantle is the cephalopods body lying posterior to the head and enclosing the visceral mass and mantle cavity the latter being used for locomotion by jet propulsion.