Parrotfish are a colorful group of marine fish, with a gaudy appearance, unusual reproductive habits and bill-like mouths. Depending on the number of parrotfish in a particular area, males may remain quite small, or they can grow larger and become more territorial. In the absence of males, female parrotfish can change gender and become reproductively-active in just a few days.
Fish generally have a protective layer of mucus covering their bodies which helps to protect them from infection. This is usually called a slime coat.
Various species of parrotfish, such as the rainbow parrotfish (Scarus guacamaia), also produce a different, more profuse type of mucus. This originates from special glands present on the surface of their bodies every day at dusk. The mucus creates a thick cocoon enveloping the parrotfish, as it settles down to rest at night. It takes about an hour for the parrotfish to complete this process.
Each morning, the parrotfish frees itself from its nocturnal covering, and resumes swimming normally. Unlike some other reef inhabitants, parrotfish are strictly diurnal, being only active during the day. They spend the night in a torpid state, with their rate of respiration noticeably slowed.
This additional slimy casing ensures that the parrotfish can still continue breathing, with holes in the mucus allowing its mouth and gills to function. Researchers originally showed two reasons why this mucus was produced. The first was to prevent the parrotfish’s gills from becoming badly silted up with sand while it was resting on the seabed, and the second was the foul smell of the mucus served to deter predatory species that hunt by smell.
Recent research, however, suggests a third reason as to why these fish may behave in this manner, based on findings made by scientists at the University of Queensland in Australia. These researchers took a group of parrotfish and housed them in plastic tubs. At midnight, the researchers carefully stripped away the cocoons from half of the parrotfish in the study, and introduced isopods, which are common, blood-sucking fish parasites, into all the tubs.
Researchers then surveyed the parrotfishfish the following morning. This showed that nearly all those (94 percent of the sample) in the tubs deprived of the protection of their cocoons had suffered bites from these parasitic crustaceans. In contrast, just 10 percent of the parrotfish left with their cocoons had been bitten. The new research clearly shows that this thick barrier would provide protection for the fish in the wild while they are resting overnight.