During a late afternoon dive several years ago in Fiji, I came across a sea cucumber (Bohadschia graeffei) that was upright and swaying like a cobra dancing to a snake charmer’s tune. That was very strange and I didn’t know what to make of it. I watched for a while, swam away, then came back, intending to photograph the sea cuke’s mouth and the tentacles surrounding it, which were black with flat, leafy white tips. 

That’s when things got stranger. Rising in the water from behind the mouth was a smoky looking substance that, when viewed later on the 35mm slides I took, turned out to be tiny golden eggs. The sea cucumber had been spawning.
Sexes are usually separate in sea cucumbers. Spawning often occurs at night and tends to be seasonal. The organisms involved aren’t always right next to each other, they may be as much as 16 feet or more apart. A male (or males) starts the process by releasing sperm, which cues a female (or females) to release eggs. Fertilized in mid-water, the eggs of most sea cukes develop into free-swimming plankton-feeding larvae. The larvae of others, however, including Cucumaria miniata subsist on the yolk found in their eggs. Also, fertilization is internal in some species and the female sea cukes may brood the eggs until they hatch into larvae or the larvae morph into juveniles that are able to fend for themselves.
Sea cucumbers are classified as Echinodermata, which means, “spiny skinned.” According to the New Oxford American Dictionary, “They have fivefold radial symmetry, a calcareous skeleton, and tube feet operated by fluid [i.e., sea water] pressure.” The 6,000-member phylum also includes crinoids, sea stars, brittle stars, sea lilies and sea urchins. Sea cukes also belong to the class Holothuroidea, which numbers about 1,700 species, most of which live in the Asia Pacific region. About 80 are found off the West Coast, where eight to ten species are common. Two of the largest, Parastichopus californicus (also known as Apostichopus californicus) and Parastichopus parvimensis, have been harvested commercially for human consumption.
Other than in body shape, many sea cukes bear little resemblance to their terrestrial namesakes. They can be black, brown, red, orange or white. They are also the only echinoderms that have a circle of tube feet around their mouths. Also called tentacles, these can be digitate (finger-like), pinnate (feather-like) or peltate (shield-like). The latter two types of tentacles resemble tiny soft corals. Sea cukes also have rows of small, flexible tube feet on their bodies. The ends of the hydraulically operated tube feet are used for a variety of tasks, such as gathering food, gripping the substrate or providing motion. 
Interestingly, the respiratory trees, which are also the excretory organs, are in the posteriors of sea cukes and water is pumped into and out of the anus for their functions. (In nonscientific terms, they breathe out of their butts.) Sea cucumbers serve a useful role in the marine ecosystem by recycling what they eat into substances that can be further degraded by bacteria. 
When threatened, some sea cucumbers discharge sticky threads to ensnare their enemies. Others can forcefully eject some of their internal organs (including the respiratory trees) from their anus. The missing body parts will be quickly regenerated. 
There are a lot of sea cucumbers, which is good because they, their eggs and their larvae are preyed upon by sea stars, fishes, birds, mammals and other marine animals, such as crabs and snails. Some are also consumed by humans.
As you have probably realized by now there is a lot of diversification in holothurians and they come in a variety of forms. Adults range in length from less than one-half inch to 16 feet. Some, like our warty sea cucumber, mostly lie or crawl very, very slowly across the bottom, often out in the open. Those with soft bodies tuck them in crevices so only the sticky oral tentacles are visible. Said tentacles constantly sweep the water for food, are inserted into the mouth where they are cleaned of what they gathered, then thrust back out to continue searching the water.
Pharmaceutical firms are researching some sea cukes because they produce toxins that are antimicrobial or anti-inflammatory agents and anticoagulants.
The following are four sea cucumbers divers may encounter in SoCal waters. Most do not have common names.
Parastichopus parvimensis (warty sea cucumber) has numerous papillae tipped with black interspersed with larger conical spines on its back. Nine to 12 inches long, it is orange-brown. There are tube feet on the animal’s underside and around the mouth. Unlike the other sea cukes shown here the mouth tentacles are digitate and, in the photo of the head, look a lot like human fingertips with fingernails. This is a very common sea cuke, found from Central California to Baja, on rocks and sandy bottoms down to 100 feet.
Pachythyone rubra has tube feet scattered randomly over its body. The animal is orange to orange-red on top, white below. It can be three inches long and is found from Monterey Bay to SoCal on rocky and sandy bottoms from the low intertidal to about 60 feet. Much like the proliferating anemone, female P. rubra brood their young inside their bodies. In the laboratory, babies have been observed leaving from both the anterior and posterior ends of their mothers and no one is sure which way is normal. Once released, the babies don’t stray far from their mothers so P. rubra of all ages and sizes are found living together. 
Cucumaria miniata has bright orange peltate tentacles. This sea cuke can grow to lengths of nearly ten inches. The ten-branched tentacles encircle the mouth and are usually the only part of the animal that is visible because the soft white body is hidden in a crevice. According to A Living Bay, “The [suckered] tube feet occur in five rows, consistent with the pentamerous symmetry of adult modern echinoderms.” The same source points out that, “This sea cucumber’s bright color comes from very high concentrations of carotenoid pigments in its skin.” C. miniata is found from Alaska to Central California as deep as 80 feet. 
Cucumaria piperata is white or cream colored with black speckles. Its retractile tube feet form five double rows on the length of body, which can be three inches long. There are ten equal-sized tentacles around the mouth. C. piperata lives in crevices or under algae on rocky substrates from shallow subtidal waters down to 275 feet. Found from the Queen Charlotte Islands to Baja. 
The author wishes to thank Genny and Shane Anderson for their help in the preparation of this article. 
Sea Cucumber Stats
Phylum: Echinodermata
Subphylum: Echinozoa
Class: Holothuroidea
Order:  Aspidochirotida  (Parastichopus parvimensis)
Dendrochirotida  (Pachythyone rubra, Cucumaria miniata,  Cucumaria piperata)
Family:  Sclerodactylidae  (Pachythyone rubra)
Stichopodidae  (Parastichopus parvimensis)
Cucumariidae  (Cucumaria piperata, Cucumaria miniata)