Sea Cucumbers: Holothuroidea

Most sea cucumbers are slow-moving animals. Some rear up and extend their front ends into the water when releasing eggs and sperm. Some twist violently or inflate when they meet a predator. Some deep-sea species can swim. Bottom dwellers wander in an apparently random way as they feed. Many tropical species of sea cucumbers are active at night, staying in crevices or under the sand during the day.

Some sea cucumbers have toxins in their body wall that generate a taste that keeps fishes away. Other sea cucumbers defend themselves by shooting tubes out their anus (AY-nuhs). The tubes become very long and sticky, entangling predators or scaring them away. Some sea cucumbers defend themselves by ejecting their internal organs—some through the head by breaking off the tentacle crown and others through the anus. The sea cucumbers survive the organ ejection, and the organs grow back.

Sea cucumbers use their tentacles for eating. Some sea cucumbers scoop up sand or mud with shovel-shaped tentacles. Others lash the surface with featherlike tentacles. Still others scoop up food particles as they burrow with fingerlike tentacles. Some sea cucumbers have branched tentacles that are lightly coated in mucus and extend into the current to capture algae and plankton. Food sticks to the mucus, and the sea cucumber brings the tentacles into its mouth one at a time to wipe them clean by contracting muscles around its throat.

Some species of sea cucumbers have separate sexes, and others make both sperm and eggs. At least one deep-sea species forms pairs. Some sea cucumbers, mainly those that live in warm water and those that live in the deep sea, release eggs and sperm into the water, where they unite and larvae develop. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults. In other species, mainly those that live near the shore or in cold water, females use their tentacles to gather up the eggs as they are being released and keep them on their bellies or in special pouches for development. In a few species the larvae develop inside the female's body cavity.

Some sea cucumber larvae are non-feeding but have fat stored in them and develop directly into adults. Other sea cucumber larvae have a feeding stage in which they drift in the water. They then transform from a two-sided to a five-armed body plan and settle on the bottom as miniature adults. Adults of some warm-water species of sea cucumbers may also reproduce by splitting in half.