Cocoons Aren’t Just for Butterflies -

Cocoons Aren’t Just for Butterflies

Article by Alex Brylske

Although most divers are understandably anxious about night diving—especially their first time—it can be one of the most rewarding underwater experiences. In fact, many divers find it their favorite form of diving, which is hardly surprising considering that the reef at night is an entirely different place than during daylight hours. And this is not just a matter of perception. Many creatures, behaviors, and phenomena you’ll witness at night do not—and many cannot—occur in daylight.

At night, diurnal (daytime) species employ various strategies to reduce their chances of being eaten while they sleep. Most wedge themselves into coral reef cracks and crevices. Others, such as some wrasse species, bury themselves in the sand. Parrotfishes and a few other wrasses have a unique strategy: every night, they secrete a mucus cocoon from a gland at the base of their gills. This cocoon surrounds them like a bubble, sometimes called “parrotfish pajamas.” The translucent, jelly-like envelope encases their bodies, sometimes taking up to an hour to fully form. By morning, the cocoon is shed before the fish resumes its daytime activities. This slimy overcoat seals in the wearer’s scent, masking it from the noses of nocturnal predators like moray eels and sharks. Additionally, it serves as a burglar alarm that is activated when the mucus net is disturbed.

Still, there’s more.

Parasites, such as gnathiid isopods, attach themselves to sleeping hosts to feed on their blood, creating a significant nighttime nuisance for reef fish. Studies have shown that removing a parrotfish’s cocoon increases disease-carrying parasites nine-fold. Although this mucus production is energetically costly, consuming about 2.5 percent of the fish’s daily energy budget, the evolutionary success of parrotfish indicates that it must be well worth the effort. However, not all species of parrotfish produce cocoons. Those that don’t possess another defense against parasites: toxic skin. An additional advantage of parrotfish cocoons is that sleeping near the seafloor or coral structures exposes fish to fine sediment and harmful microbes. In this context, the cocoon serves as a protective layer, reducing gill irritation and bacterial infections.

A final fascinating hypothesis regarding the benefit of the mucus cocoon is that it may function as a sunscreen. This makes sense given that, as herbivores, parrotfish are shallow water residents where algae is most prolific, and harmful ultraviolet light can penetrate more than 20 feet in clear tropical water. (Mucus production also occurs in other species, such as wrasses that do not produce cocoons.) In fact, the mucus of many marine fish contains bioactive compounds, including glycoproteins, antioxidants, and sometimes UV-absorbing molecules such as mycosporine-like amino acids (MAAs). These are known to be produced by many reef organisms, including corals, and are effective at absorbing harmful UV radiation.

Another question often arises in night diving: “Do fish really sleep?” Yes, they do sleep, but their sleep patterns and behaviors differ significantly from those of humans and other animals. As fish do not have eyelids, they obviously cannot close their eyes, nor do they experience rapid eye movement (REM) sleep, as humans do. Instead, fish have evolved adaptations to rest and conserve energy while remaining alert to potential threats. One such strategy is to rest, so fish typically reduce their swimming activity and metabolic rate, allowing them to conserve energy. Likewise, many fish seek shelter or sleep in crevices or vegetation, which also helps protect them from predators while they are less active.

Perhaps the most fascinating sleep pattern known among fish is unihemispheric slow-wave sleep. In this type of sleep, one hemisphere of the brain remains active while the other hemisphere rests. (This is a physiologic adaptation they share with whales and dolphins.) This allows fish to stay partially alert and responsive to their surroundings while resting. Overall, many environmental factors, such as light levels, water temperature, and water flow, can influence when and how fish sleep. The take-home message is that the sleep patterns of fish are diverse and vary significantly between species. The exact mechanisms and purpose of sleep in fish health and physiology is still an ongoing subject of research.

The development of the mucus cocoon and sleep patterns among fish exemplify the evolutionary pressures reef fish face. Whether evading predators, avoiding parasites, or protecting themselves from environmental hazards, this adaptation provides a nightly safeguard in the perilous coral reef ecosystem. Further research may reveal even more hidden benefits of this mucus shield, illuminating the intricate survival strategies of marine life.

More fascinating insights into coral reef fishes can be found in the author’s book Beneath the Blue Planet: A Diver’s Guide to the Ocean, available from Amazon at this link. A podcast featuring an extensive review of Dr. Brylske’s book is also available at this link.