The Sea Cucumber shares the five-part symmetrical body plan. They have an unusual method of respiration: they take in water through their anus to breathe. When disturbed or frightened, some sea cucumbers pour out a mass of sticky white threads to confuse or trap their enemies. Others are capable of releasing toxins which in aquaria have been known to kill all the animals and including the sea cucumbers themselves.
Sea cucumber / Stichopus variegatus / Family Stichopodidae
Sea cucumbers are cucumber shaped with no arms.
The holothurians or sea cucumbers live in sandy and muddy areas.
Some sea cucumbers have a commensal fish known as the pearl fish (Carpus sp.) which lives inside their anus during the day and emerges at night to feed. When morning comes, it simply waits for the anus to open as the sea cucumber breathes, and swims back inside.
Pearlfish are highly modified eel-like fishes which lack scales and pelvic fins. Most live in the body cavities of sea cucumbers, giant clams and pearl oysters during the day, leaving at night to feed on small fishes and shrimp. Pearlfish enter the body cavities of sea cucumbers through their anus. When it tries to enter the body cavities of the sea cucumber it closes its anus, but as sea cucumbers breathe through their anus it must eventually open and in swims the pearlfish. Juvenile pearlfish are parasitic, feeding upon the gonads of sea cucumbers.
A spawning sea cucumber
Most sea cucumbers feed on detritus (dead plant and animal material) in the sand. The sand is taken in through the mouth, the detritus digested and the clean sand expelled through the anus. Others, like the beach-ball sea cucumber, use feather-like arms to filter food from the surrounding seawater.
The holothurians were once the basis for an important international fishery. Macassans from the island now called Sulawesi, used to come to northern Australia to collect them. After processing, they would be sold as béche-de-mer.