Reproduction

Mollusk reproduction is most commonly done externally, with the release of eggs and sperm into the environment resulting in the fertilization of the eggs. The larvae formed from this type of reproduction are usually free swimming, and either settle of the ocean floor or drift to mature. Some members of the phylum, such as tentacled mollusks and some types of snails, fertilize their eggs internally. Some mollusks are hermaphrodites. They can alternate between the production of male and female gametes, which gives the animal another potential mate... itself.

Mollusks produce a special kind of larvae known as a trochophore, which is translucent and free-swimming. They are round or pear-shaped, and are surrounded by a ring of cilia called the prototroch which propels the larvae through its environment. Also found on the trochophore, above the prototroch, is an ocellus (a simple eye) and a tuft of cilia. Below the belt of cilia are the mouth, stomach, anus, and solenocyte, which regulates internal salt-water balance. In some mollusks, marine snails and bivalves, the trochophore enters a second stage called the veliger before the animal if fully matured. In this stage, the larvae begins to develeop a foot and shell. Trochophores also appear in segmented worms, in the phylum annelida. This fact has led biologists to believe that the two phyla are related, and probably branched off from a common ancestor during the precambrian period, about 600 million years ago.