Epitoky: How Marine Worms Release Their Eggs and Sperm

An epitoke, adapted for swimming and carrying eggs or sperm toward the surface. This individual is shown curling its body, a behavior commonly observed during these night time spawning events

Just after sunset, something remarkable can happen in the waters around Grand Cayman. What first appears to be tiny worms simply swimming toward a dive light may actually be one of the ocean’s most unusual reproductive events.

During certain times of the lunar cycle, marine worms called polychaetes can gather near the surface in large numbers. Look closely, and you may see individual worms suddenly break apart, releasing their eggs or sperm into the water.

This fascinating reproductive process is known as epitoky.

What Is Epitoky in Marine Worms?

Reproductive epitoke stage. The enlarged body and prominent swimming bristles help the reproductive section move through the water column

Epitoky is a specialized reproductive process found in some marine polychaete worms. As breeding time approaches, part of the worm's body transforms into a reproductive form called an epitoke.

The epitoke contains the reproductive cells. When conditions are right—often linked to a particular phase of the Moon—it separates from the worm's non-reproductive atoke portion and moves into the water column.

Once at the surface, enormous numbers of epitokes may gather and release eggs and sperm. Fertilization then occurs externally in the surrounding seawater.

What Can Divers See After Sunset?

Epitoke releasing a cloudy trail of gametes from its body

At Divetech, we have observed these worms swarming around dive lights roughly an hour after sunset.

At first glance, it can look as though the worms are simply swimming toward the light. But watch carefully.

Occasionally, you can see an epitoke suddenly burst and release its reproductive material, almost like a string of tiny beads scattering into the darkness.

During stronger spawning events, the effect can be even more dramatic. Cardinalfish and other predators gather to feed on the worms. When a fish swallows a reproductive worm, you may see a sudden cloud-like burst as eggs or sperm are released in the fish's mouth.

Sometimes, all that remains visible is the faint trail left behind by the worm.

Why Does the Moon Matter?

Curled body sinks to bottom while releasing gametes.

For some polychaete species, reproduction is closely synchronized with the lunar cycle. This timing allows huge numbers of worms to reach the reproductive stage and release their eggs and sperm at approximately the same time.

The synchronization creates a spectacular feeding opportunity for fish—and an extraordinary event for divers lucky enough to witness it.

Our observations suggest that the phenomenon can be particularly noticeable around the full moon, although the exact timing varies between species.

A Rare Look at Marine Reproduction

The next time you see tiny worms swimming through your dive light after dark, take a closer look. You may be watching an important moment in the life cycle of a marine animal rather than simply seeing plankton attracted to your light.

At Divetech, these night time observations are a reminder that even familiar dive sites can reveal something completely different after sunset.

The ocean has another world after dark—and sometimes, reproduction happens right in front of your dive mask.