Featured Image Credit: Pam and Richard Winegar
By Eva Gruber
Today, March 13th is… World Napping Day! While we urge you to participate and treat yourself to at least one power nap (there is evidence that napping is healthy and can increase productivity), napping, and sleeping in general, can look very different in the marine realm.
Lets start with fish, the most visibly dominant life form beneath the surface. Fish diversity is incredible but most of them do get their rest in similar ways.
Many tropical coral reef fish like parrotfish, wrasse, and triggerfish, when settling in for the night, swim into a burrow in the reef and extrude a jelly cocoon around themselves. It is thought that this jelly sleeping bag serves at least two purposes: it dampens their electrochemical field, which makes them less detectable to predators (and parasites), as well as makes it harder to attack in case a predator does find them. When they wake up, they consume the cocoon and assimilate the proteins and do it all over again the following evening. If this seems like a lot of work, one study found that the mucus cocoon took up only about 2.5% of the fishes’ energy – that’s a small price to pay for a good night’s rest!
Sharks extract oxygen from the water with their gills – but in order to be able to do so, water must be constantly flowing over the gills (“ram ventilation”). The sleeping habits of sharks puzzled people for a long time before careful study was devoted to this issue. Many species evolved spiracles – nostril-like openings on the snout through which the sharks can pump water into and over the gills, to breathe while they are not moving.
However, white sharks never evolved spiracles and must constantly move to get the water flowing over their gills in order to breathe. So how in the world do they rest if the water around them can’t be still? Marine biologists have seen white sharks parked in small gullies and areas with strong current flow, and determined that this must be when they are resting, or sleeping. In this way, they can stay stationary for hours and still be able to breathe.
Whales and dolphins have only ever been observed to rest with, quite literally, one eye open. They achieve this by shutting off half of their brain, keeping the other half aware and active. This is done, presumably, because they need to perform conscious activities such as taking a breath occasionally (humans breathe unconsciously). Cetaceans – at least those studied in captivity where brain waves can be closely monitored – never seemed to fully enter sleep.
This was the prevailing assumption until recently, when a group of marine biologists on a sailboat found themselves in the middle of a pod of sperm whales who were hanging vertically at the surface. They realized the sperm whales were asleep – fully. The enormous animals were completely oblivious to the presence of the sailboat, whose motor was not running.
Data loggers attached to sperm whales also revealed an interesting pattern: about 7% of the time the animals seemed to be drifting inactive in areas of shallow water – what they were doing was unclear until scientists could connect the dots through multiple observation reports such as the one above. Divers around these floating giants were also able to observe minute rapid eye movement which proves that they behemoths are indeed sleeping.
A synthesis of these encounters and reports suggests that the sperm whales will float at rest for periods of up to 15 minutes at a time – a true power nap.