What creature eats rocks, breathes water, changes gender at will, and makes its own pajamas? No, it's not some creepy insect from your worst nightmare; it's the well-known resident of the reef, the parrotfish.

These colorful fish are common in these waters, but they have some very uncommon adaptations. Named for their unusual mouth, they have large fused front teeth that resemble a bird's beak, thus the name parrotfish. The fish graze on stony corals, biting off pieces of the coral skeletons along with the delicate coral polyps, seeking out the nourishment provided by the algae that live inside the coral polyps, the zooxanthellae. Any of the stony material that is swallowed by the fish is finely ground by another set of specially adapted teeth in the parrotfish's throat. This sand passes through the fish's system and is expelled as white coral sand. It is estimated that these creatures create one ton of sand for each acre of coral every year.

If you think the act of crunching stony corals in order to extract tiny single-cell algae is odd, you haven't heard anything yet. When the sun goes down , these daytime grazers are ready for bed. Many species just rest on the bottom at night. Others bury themselves in the sand for protection. Certain types of parrotfish, however, have developed the ability to enclose themselves in a protective bubble at night. This mucus cocoon keeps nighttime predators like moray eels from detecting the scent of a delectable parrotfish. It takes the fish about half an hour to secrete their protective pj's.

But wait, there's more. Parrotfish can and will undergo sex changes when necessary. There is a multitude of options in their world. They are born both male and female, but males can become females and vice versa when population densities are low and breeding males or females are in short supply. Some species live in a harem structure, with one male to a few females. If something happens to the male, oftentimes the dominant female will become a male and breed with the rest of the females. If another male moves in, that fish may again become female.

With at least one species -- the redband parrotfish -- all males are derived from born females through sex reversal. When a female of a spawning group becomes a male, it will wander until it finds an unprotected group of females to create its own harem.

Added to this androgyny are the complicated pattern and color changes that each species undergoes at different stages in growth and in conjunction with its gender changes.

So next time you run across a parrotfish while you're swimming, you may not dismiss the sighting as just another pretty parrotfish. You may find yourself wondering whether you're seeing a princess supermale, a female rainbow, or a juvenile stoplight parrotfish. And if you figure it out, you can wonder what it will be tomorrow, after it sheds it nighttime cocoon.

- Alyson Matley
Florida Keys National Marine Sanctuary
The Sounding Line