One aspect of fish behavior that mystifies people is, do fishes sleep, and if so, how can you tell?
The simple answer is, yes, most fishes sleep, as evidenced by their behavior. While their version of shut-eye differs from ours—to begin with, most fishes do not have eyelids—the majority of them do have regular periods of reduced activity and lowered metabolism that, like our slumber, restore them physically and refresh brain memory circuits. You needn’t wish your guppy or goldfish sweet dreams, though. Fish brains do not include a neocortex, where, in mammals, dreams originate.
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“You needn’t wish your guppy or goldfish sweet dreams, though. Fish brains do not include a neocortex, where, in mammals, dreams originate.”
One of the most dramatic sleepers is the spotted wolffish who sometimes rolls onto its side in its sleep, found in Shedd's Oceans gallery.
The green chromis, which nestles among branching corals at night, can be found in Wild Reef.
Yet among the at least 30,000 species of fresh- and saltwater fishes to study, scientists have observed an array of sleep behaviors and adaptations that have them researching answers to how fishes sleep.
Even as they doze, many fishes remain alert for danger, especially those that float motionless, either in the water column or near the bottom.
A number of fishes seek a little more shelter, burrowing in the sand or backing into rock cervices. Some reef species, like the green chromis (pictured above) and three-stripe damselfish that you can find in Wild Reef, nestle at night among elkhorn, staghorn and other branching corals. These fishes and corals have evolved a nocturnal mutualistic relationship. As the fishes nap in the safe shelter of the corals, they energetically swim in place, a unique sleep behavior.
“Even as they doze, many fishes remain alert for danger, especially those that float motionless, either in the water column or near the bottom.”
Their rapid fin motions increase the flow of oxygenated water around the corals, which have higher respiration rates at night, when their zooxanthellae, or resident algae, are not photosynthesizing and producing oxygen. Scientists have demonstrated that, in the absence of the sleep-swimming fishes, low nighttime oxygen levels limit the growth and even survival of the corals.
There’s sleeping under the protective branches of corals, and then there’s hunkering down on the reef floor in your own sleeping bag. Several parrotfish species, including the princess parrotfish in the Caribbean Reef, secrete a clear mucous bubble around themselves at night. Like mosquito netting, this cocoon protects the fish from blood-sucking parasites that are out and about after dark.
Blacktip reef sharks are obligate ram ventilators, meaning that they are in constant motion.
The bluehead wrasse is found in Caribbean Reef and is so unresponsive during sleep that it can be picked up by hand without waking.
Our favorite example of underwater slumber is the spotted wolffish. As she goes into what scientists really do call a “period of slumber,” the bottom-dwelling fish starts to list and completely keels over—because she doesn’t have pelvic fins to help the pectoral fins stabilize her somewhat flattened, top-heavy body. She can remain on her side for half an hour or longer, sometimes alarming concerned aquarium guests.
Other deep sleepers include Spanish hogfish and bluehead wrasse, found in the Caribbean Reef and iguana habitats, and the swell sharks in the Oceans gallery. These fishes are so unresponsive during sleep that they can be picked up by hand and brought to the surface without waking—although that’s not something we do at Shedd.
What about sharks? It’s pretty hard to do a sleep study on these open-ocean fishes. But we know that the obligate ram ventilators—those species that have to swim to breathe, like our blacktip reef sharks and sandbars, shown above—are in constant motion, suggesting that they might not sleep at all.
Recent research on little sharks called spiny dogfish, however, revealed that their swimming motions are not coordinated by the brain but by the spinal cord. It’s possible that sharks can shut down their brains, perhaps one half at a time, and rest while they swim on autopilot.
Fishes have evolved more than a few mechanisms to catch at least 10 or 15 winks at a time—without the ability to close their eyes.
—Karen Furnweger, web editor
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