Epitoky is a process found in many types of polychaete marine worms. During this process, a sexually immature worm called an atoke changes into a sexually mature worm called an epitoke. In some species, the reproductive part of the worm separates from the main body to form a new individual called a stolon. Stolons are free-swimming forms that can reproduce sexually. Unlike the immature atoke, which usually lives on the ocean floor, epitokes are adapted for swimming and reproduction. The main advantage of epitoky is that it helps the worm find other members of its species to reproduce.

There are two ways epitoky can happen: schizogamy and epigamy.

Schizogamy

Many species use a process called schizogamy. There are two types of schizogamy: scissiparity and gemmiparity. In worms that use scissiparity, the tail end segments change shape before separating from the parent body as a single stolon. In species that reproduce by gemmiparity, several stolons may be produced at the same time, either as a long chain of connected stolons that separate when fully mature, or as separate stolons growing from the tail end of the body. These new parts can survive to create more stolons in future reproduction cycles.

Epigamy

Epigamy is a common method used to create epitokes. In species that use this process, the atoke changes its body structure and function as it becomes an epitoke. Usually, male worms experience more noticeable changes during this transformation. These changes may include larger parapodia, paddle-shaped chaetae for better swimming, a smaller gut, the body filling with gametes (eggs or sperm), the development of larger eyes, and muscles that change to help with swimming instead of eating. Most species that go through epigamy cannot return to the atoke form and die after reproducing.

Male and female epitokes are released into the water at specific times of the year and often swim to the surface together in a behavior called swarming. Swarming helps group members of the same species together, which increases the chance of successful reproduction. Some polychaete species use bioluminescence, likely to help keep swarms together. Both schizogamous and epigamous epitokes do not eat and die after releasing their gametes into the water.

In the past, epitokes were thought to be a separate group of polychaete worms because they look very different from atokes. For example, the atokes of Platynereis dumerilii are yellowish-brown, while female epitokes are yellow due to the eggs they carry, and male epitokes have a white front part from sperm and a red back part from blood vessels.