SNOOZING AT SEA

For many animals in the wild, whether they are noctumal or diumal, deep sleep is a luxury they can’t afford if they want to stay alive.

LARGE marine mammals, such as dolphins and whales enter a state called unihemispheric sleep. One side of their brains sleeps while the other is awake. It means one eye is open and the other closed. This enables them to keep a watch for predators as well as come up periodically to the surface to breathe.

Since they need to resurface, marine mammals cannot enter the REM (rapid eye movement) or dream stage of sleep, when the muscles can be moved the least. However, a team of scientists recorded a group of sperm whales resting with their heads just beneath the surface of the ocean. The whales were unusually still which suggested that they may have been in the REM phase.

Sharks breathe not by surfacing but by forcing oxygen-rich water  over their gill slits. Some species like nurse sharks suck the water through their mouths or through openings in their heads called spiracles. Nurse sharks have been seen lying still on the sea bed, leading scientists to believe that they were sleeping. But other species have to swim continuously to force water over their gills.

In spiny dogfish, a species of shark, a study found that swimming was controlled by their spinal cords, allowing them to sleep and rest their brains even when moving.

Sea others use kelp or giant seaweed to wrap themselves in at night. They float on their backs in groups and even hold hands to prevent themselves from dfifting off in the ocean currents! The nest of kelp serves to hide them and anchors them in one place.

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