March is National Sleep Awareness month and an annual reminder to check in on our sleeping habits. Whether you can barely make it past 9 pm without crawling into bed or remain wide awake into the early morning hours, sleep schedules between individual people vary immensely. We might choose to burn the midnight oil to binge our favorite Netflix show, but residents of the Aquarium sleep strategically and in a way that would best ensure their survival in their natural habitats.
Most animals have a biological need for sleep and its restorative effects, but little is understood about the origins of sleep or how it evolved in the first place. Researchers have evidence that melatonin, a hormone involved in the sleep cycle, evolved in marine organisms over 700 million years ago. Our idea of what sleep looks like is typically very different than what we might observe in other animals. Whether it's underwater, in the canopy or with one eye open, let's dive into the Aquarium after dark.
While the phrase "sleep with one eye open" might be considered an unfriendly threat to us humans, wildlife does in fact use this method to monitor their environment for danger while sleeping. Dolphins, eared seals, manatees and crocodiles participate in "unihemispheric sleep," a process where one half of the brain, or cerebral hemisphere, is asleep while the other remains awake and alert. Dolphins alternate between the two hemispheres, keeping their open eye facing their pod to maintain the safety of the group while regularly surfacing for air.
Though dolphins sleep during the night, freshwater crocodiles sleep during the day and bask in the sun, waiting for the cover of darkness to exhibit their predatory behaviors. For both of these animals, the benefits of unihemispheric sleep include all the regular regenerative effects of sleep, with the added benefit of remaining vigilant to threats in their environment and predators that might try to sneak up and attack.
Diurnal species, like humans, are those that are typically active during the day and sleep or are inactive at night. Many of the animals in our care are diurnal, including most of the birds and fishes on exhibit. The splash-back poison dart frogs in Amazon River Forest boldly hunt for insects in the leaf litter by day, with their bright orange, yellow or red coloration warning predators of the toxins in their skin. This warning coloration is called aposematism and is characteristically linked with diurnal species. Neither warning coloration nor camouflage coloration of non-toxic species work well at night to deter potential predators, and fishes that are colorful by day often change to muted coloration at night.
Out on the reef, while many fish retreat to cracks and crevices in coral and rocks after dusk, the parrotfish participates in an unusual bedtime routine. For about one hour before it settles to rest for the night, the parrotfish releases mucus from a gland inside the gill cavity, surrounding itself with a mucus cocoon that functions similarly to a mosquito net. Scientists theorize that this practice protects the fish from blood-sucking isopods and other parasites lurking on the reef.
Many fish exhibit sleep-like behaviors; although they're not often completely immobile, they have been recorded having sleep-like brainwaves and reduced reaction to stimuli, such as the predators hunting them at night.
Though many reef fish retreat to safety during the night, nocturnal predators emerge to feed in the darkness. At night, corals extend stinging tentacles out of their polyps to capture plankton drifting through the water column. Many coral species share a symbiotic relationship with algae living in their tissues, which supply them with food produced during the day via photosynthesis.
Sharks circle above the reef, taking advantage of the less-reactive fish resting below. Though shark sleep is not yet understood or adequately researched, many sharks have been observed resting during the day. A common misconception is that sharks would die if they stopped swimming, but most shark species use buccal pumping or spiracles to deliver oxygenated water over the gills without active movement. One buccal pumping species, the nurse shark, has been widely observed resting on the ocean floor during the day in large numbers. The tasselled wobbegong in Blacktip Reef might be observed partially buried in the sand during the day; with its mouth buried, this bottom-dwelling shark breaths by drawing in water through spiracle openings, located behind the eye.
The availability of prey at night is one key indicator of nocturnal behavior. During the daytime, the emerald tree boa sits coiled on a branch, immobile and camouflaged among the foliage. At night, it uses its position to its advantage, hanging its head from the branch and ambushing small rodents and other unsuspecting prey with one quick motion. Other species, like the giant waxy tree frog, use a similar tactic, hunting for nocturnal insects in the darkness with the use of their superior vision. By day, this frog sits motionless on a branch with its eyes completely shut. At night, its large bulging eyes open wide and gather available light to hunt night-active insects.
No Sleep At All?
Sleep is mysterious, complicated and strange. It's still not a well understood process in humans, but we do know that we have a biological need for it. But what about animals that don't seem to sleep at all? We still don't understand the restorative process that occurs in most jellies or ram-ventilating sharks, which don't exhibit any clear signs or measurable aspects of sleep. We might be envious of this trait and wish we too could add more hours to our day, the answers behind these traits will, for now, remain a mystery.