Epitoky

Alitta succinea (common clam worm) in Epitoky stage

Epitoky is a form of reproduction observed in polychaete marine worms. The worms undergo a partial or complete transformation into an epitoke, a pelagic morph capable of sexual reproduction.

Often an epitoke is the posterior part of the worm, which separates from the anterior part (called the atoke); gametes are produced inside the epitoke. The epitoke may also form by asexual budding from the posterior.

There are two methods in which this can occur, epigamy and schizogamy. In epigamy, the whole worm transforms into a swarming epitoke. It rises and sheds its eggs and sperm. After reproduction, it dies. In schizogamy, the sexual forms are budded from the posterior or from the parapodia to swarm. This is effectively asexual reproduction of the epitoke.

The first form is characteristic of Nereidae where the whole worm becomes an epitoke. This form may be accompanied by partial or full degeneracy of digestive organs (causing the animal to die quickly) and enhancement of motor and sensory organs; in particular, whereas in its nonreproductive form the worm is a crawling animal, when it becomes an epitoke the worm develops swimming appendages, such as broad parapodia with paddle-shaped chaetae.

Epitokes swim to the plankton-rich surface region, where the gametes erupt from the body and the larvae hatch. This action is often synchronized with moon cycles, and epitokes swim in swarms.

In the past it was thought that epitokes were separate species from polychaete marine worms.

References

• Brusca, Gary and Brusca, Richard: Invertebrates (2nd edition)