The animal kingdom abounds in behavioural peculiarities. Honeybees, as Karl von Frisch famously discovered, dance to communicate to each other the direction and distance of food from their hive. The male wood spider, a tenth the size of the female, will ply her with ‘nuptial gifts’ (of dead insects, nicely wrapped in silk) so that he is not eaten while trying to mate with her. Then there are tap-dancing birds of paradise and sex-changing fish.
Of course, all these were adaptations for good reason — and ultimately contributed to the survival of that species. But the phylogenetic tree is full of such oddities, and the parrotfish is a perfect example of an animal that has found a smart but amusing way to get around a common problem.
The free-swimming larvae of gnathiidae (or isopod crustaceans) are a common parasite on coral reefs, feeding on the blood of many reef fish. They burrow into fish gills and skin and can cause serious infection and disease if not removed. Cleaner wrasses are a family of small, ribbon-like fish that have benefited from this parasitic association. They are able to digest these crustaceans, and fish regularly seek them out at cleaning stations manned by one or two cleaner wrasses, usually located conspicuously on the top of big coral boulders. But cleaners only provide their services during the day. What happens at night, when fish are sleeping, and the threat from parasitic crustaceans looms large?
Reef fish do a variety of things to ward off gnathiidae at night. Some, like wrasses, bury themselves in sand. Gobies have toxic skin cells on certain body areas to ward off parasites. Some fish sleep in areas away from the reef and a few may even use nocturnal cleaner organisms. However, the survival costs associated with these strategies would be high — both in terms of the energy required to continuously seek out cleaners at night, for instance, and the risk associated with swimming around and being exposed to night-time predators as a result. Which is why the parrotfish’s solution seems especially elegant: mucous cocoons. Every night, many parrotfish blow bubbles of mucus around themselves and spend the night inside it. For a long time since the discovery of this phenomenon in the 1950s, it was thought that they were doing this to avoid being eaten by larger animals, like eels. But a study done recently on the bullethead parrotfish on the Great Barrier Reef has found that these cocoons are actually working against gnathiidae, acting as a kind of mosquito net against night-time pests.
The researchers manipulated mucous cocoons of several bullethead parrotfish (Chlorurus sordidus) in a controlled laboratory experiment, removing some cocoons from sleeping parrotfish and leaving others intact. They then introduced an equal number of gnathiid crustaceans into their separate tanks, and left them in there for the rest of the night. Perhaps unsurprisingly, they found that parrotfish without cocoons got attacked almost nine times more than ones with cocoons. The fullest, most satisfied gnathiidae were also the ones that had been inside the tanks of unprotected fish, happily feeding away on their flesh. This confirmed that the cocoons were indeed acting as a sort of force field against parasitic attack, keeping fish safe inside a blanket of their mucus.
The mechanisms used by mucous cocoons to prevent gnathiid attack are probably a combination of several things. The mucus could mask smells from the fish that parasites use to locate them with. They also act as a physical barrier against attack, repelling the gnathiidae that do reach its surface. Another study done on mucus across the animal kingdom found that mucus is itself loaded with microbe-killing viruses that have evolved in a unique symbiotic partnership with animals. It could be that the mucus of parrotfish cocoons is also full of viruses that have evolved over time to fight off gnathiidae.
Cocoon-producing reef fish are the only animals known to secrete a structure that not only protects the whole fish but also allows it to perform the function of sleep — a combination of features that hasn’t been observed in another animal before. Most of them will also eat their cocoon the next day as morning snack, a behaviour I observed on an early dive. Not only do these fish sleep in capsules of their own mucus, they also minimise the energy lost in production by consuming it the next day. Peculiar? The parrotfish is too clever to care.
( Shreya Yadavis a research affiliate with the Nature Conservation Foundation, Mysore)