Cocoons are not exclusive to insects. Other groups of animals also make cocoons. For example, the fresh water mussels (bivalves or clams).
Fresh water mussels, as their name implies, live in ponds, lakes, or streams. They are bottom dwellers crawling very slowly with their muscular foot. They feed on small organisms. Their life history has four stages, egg, larva, cocoon, and adult.
The female mussel lays number of eggs. The eggs are not released into water, but retained in a cavity within. From the eggs hatch out tiny larvae. The larva is known as Glochidium (pronounced as, glow-kid-e-yum). The larva stays in the motherís cavity for sometime.
The glochidium has a special hook, which helps to anchor itself on a fish. It then builds a cocoon around itself on the body of fish. The glochidium cocoon is also called glochidium cyst.
For the glochidium to cling on to a fish, the mother mussel employs a lure technique. It protrudes a portion of mantle from her shell.. The mantle is a soft skin that lines the inside of the shell. The protruded part of mantle acts like a fish lure.
Fish get attracted by this mantle lure and converge on the mother mussel. As they attempt to bite the mantle lure, the mother mussel ejects the glochidia larva. The larvae anchor themselves on the fish skin, by their special hooks. Once lodged on the fish, the glochidium builds a cyst or cocoon. In the making of cocoon, the glochidium uses the fish tissue. The glochidium cocoon is different from insect cocoons in its construction.
The female shin rayed pocket book (a fresh water mussel), is known for its peculiar fishing lure. The lure looks like a small fish. It is a bag containing glochidia larva. The lure is connected by a slender line with the mother mussel. Actually, this lure is an extension of the mantle. With the water currents, the lure sways up and down. It appears like a small fish swimming.
Fish are attracted by this lure. When they attempt to swallow the lure, glochidia are released into their mouth. The fish may try to spit out, but before long, the glochidia attach themselves on the body of the fish .Soon they build cocoons.
Inside the cocoon, the glochidium derives nutrition from the fish skin and develops into a tiny mussel. The tiny mussel breaks open the cocoon and drops down on to the bottom, where it grows to repeat the cycle.
The Glichidium Cocoon is a parasite on the fish, as it derives material, nutrition and protection. It takes the help of fish for dispersal. This is important as the adult mussels are sedentary and are confined to one place.