Sea Cucumbers (Class Holothuroidea)
A person need wade out only knee-deep around many of the Florida keys to encounter, lying con- spicuously exposed on the muddy bottom, large sausage-shaped creatures 1 foot or more in length and better than 2 inches in diameter. Against the pale gray surface on which they lie, the contrast may be striking: dark brown with light spots, or brick red with raised lumps of black or dark brown. They are sea cucumbers, with a name given them (Cucumis marinus) in the first century a.d. by Pliny, the Roman elder and encyclopedist.
Upon closer examination, none of the usual clues is evident to show which is the head end of the animal. As it lies there quietly, both ends of the cucumber appear to be doing something. At one end, an opening appears, sometimes as much as 1 inch in diameter. If the water is shallow, a current may be noted pouring out of the opening. Or the sea cucumber may be taking in water just as rapidly. The movements of the opening and the slow enlargements and contractions of the whole body suggest a sort of underwater whistling. Actually, they are breathing movements and, in this unusual animal, occur at its rear end.
As though further to astonish the beachcomber on tropical and subtropical shores, a sea cucumber may be found from which a fish's head projects. The fish is very much alive, and the cucumber's breathing movements simply take in water or expel it around the fish. If nudged, the fish may swim out, exposing a slender tapering body as much as six inches long.
Usually a minor drama follows at once. The blenny-like fish turns back immediately to the side of the sea cucumber and moves about along the surface, evidently searching for the respiratory opening again. Often the sea cucumber closes the aperture tightly, as though to keep the fish from returning to its refuge. But eventually the need for oxygen becomes too great. The cucumber opens again, the fish slips in either tail first or head first (and turns around immediately).
The cavity into which the fish goes is the cloaca of the sea cucumber, a chamber serving not only respiration but also as a common exit for wastes from the digestive tract and sex cells from the reproductive system. Sea cucumbers are unique in having a pair of generously branched "respiratory trees" extending blindly from the cloaca far forward in the body cavity. Through their walls oxygen and water pass, keeping the other internal organs aerated and maintaining the plumpness of the cucumber's body.
At the opposite end of the animal a set of tentacles moves slowly, obtaining food. In most sea cucumbers, including the large kinds found near shore in tropical and subtropical waters, these soft organs around the mouth shovel the surface mud into the digestive tract, letting the animal get the nourishment from a great assortment of microscopic life, especially diatoms. The gritty residue is expelled from the cloaca, and sometimes accumulates into conspicuous heaps. The late Professor W. J. Crozier estimated from measurements of the cones of debris that the sea cucumbers on each acre of bottom in one region off Bermuda would pass between 100 and 200 pounds of sand through their bodies annually.
Substantial amounts of the nourishment obtained by a sea cucumber are stored in its body wall. There the food reserve usually gains protection from a slimy, leathery skin in which are embedded little limy secretions of remarkable variety. Some are microscopic plates perforated by many holes. Others are knobby rods, or anchor-shaped, or resembling a concrete bird bath or a wheel with spokes but no rim. Each species has its own distinctive limy granules. Only a few kinds lack them altogether.
Many of the larger sea cucumbers that live close to shore supposedly discourage attack by fish and crabs through the presence of a poison (holothurin) in their skins. If extracts of it are injected into mice, they die quickly. The presence of certain sea cucumbers in an aquarium tank may be enough to poison any fish present. With some of the large subtropical and tropical cucumbers, the effect sometimes persists in a tank for weeks after the echinoderm has been removed and the water changed repeatedly.
Large cucumbers belonging to the genera Holothuria and Actinopyga have ready a truly astonishing defense against animals that molest them. Associated with the region where their respiratory trees open into the cloaca they have short tubules of red, pink, or white color. If the echinoderm is disturbed seriously or repeatedly, it slowly turns its body until the cloacal opening faces the molester, then performs a general contraction and proceeds to send out the slender tubules in great numbers. The blind ends of the tubules may be enlarged; almost always they are very sticky. And as they emerge from the cloacal opening, they become darting, adhesive threads that, in a minute or less, can so enmesh a crab or lobster that it is immobilized. The cucumber frees itself from the tubules and moves slowly away as though nothing had happened.
With provocation these and many related sea cucumbers will perform a far more amazing trick. With a single powerful contraction they turn themselves partly inside out–throwing out the respiratory trees, the reproductive organs, and sometimes some of the intestine as well. All of these emerge suddenly through the cloacal opening as a tangled mass over and around a crab or fish. From these too the cucumber separates itself, as one of the most spectacular instances of self-mutilation and evisceration in the animal kingdom. Until new organs are regenerated, the sea cucumber continues its breathing movements, drawing sea water directly into its body cavity. In six weeks or so, the animal recovers completely and is ready to repeat the performance if irritated sufficiently.
In many parts of the South Pacific and along Oriental coasts, people deliberately annoy these large sea cucumbers and gather up the extruded organs (particularly the ovaries of a female) as meat for the soup pot or delicacies to be eaten raw. More widespread is the custom of preparing holothurians as "trepang" or "'beche-de-mer." Usually the animal is eviscerated, its body wall boiled, then dried or smoked. In the Indo-Pacific region the product is very popular as an ingredient for soups or as gelatinous tidbits. Great quantities of trepang are sold commercially to the Chinese.
Trepang from the Mediterranean is almost two-thirds protein, whereas that from the Indo-Pacific averages between one-third and one-half protein. Apparently the protein constituents are completely di- gestible, and the method of preparation removes all toxic materials.
Since the sea cucumbers in which the little pearl fish Carapus seems an unwelcome guest are exactly the ones producing fish poison and sticky threads and eviscerating themselves when irritated, a person can only marvel that the pearl fish is able to use the cucumber's cloaca as a refuge. Actually, Carapus gets enough space for its body by sliding its tapered tail well up into one of the cucumber's respiratory trees. Yet the fish seems never to trigger the common responses and is completely immune to the poison.
The potency of the poison to fish in general is well known among natives on many South Sea islands. On Guam, for example, people cut the common black sea cucumber in two and wring the contents of its body cavity into tidal pools to drive the fish to the surface. In the Marshall Islands, similar sea cucumbers are pounded and the mangled remains dropped into pools at low tide, stupefying the fish enough that they can be caught easily. Yet the poison is not feared by the natives. It is harmless to human skin, and fish caught through its use are often eaten raw with no ill effects.
About 500 different kinds of sea cucumbers have been found, living almost exclusively on or in the bottom sediments. Most of them are dull colored, and only a few have contrasting spots or stripes. Yet they pursue their lethargic way of life on minute food in so many different levels of the sea that a surprising variety of form and body build is represented.
Something in common can be seen between a child solemnly licking its fingers to clean them of jam, and a big sea cucumber in its normal method of feeding with ten or more profusely branched tentacles, each like a shrubby tree. The cucumber spreads its tentacles over the sea bottom and rubs them around, gathering food particles in the mucus coating. Then, one at a time, the animal thrusts a loaded tentacle into its mouth, closes fleshy lips around it, and pulls out the tentacle all clean and ready for reloading.
Sea cucumbers acting in this way can be found in cooler waters between low-tide mark and 1200 feet below the surface. Cucumaria frondosa, found in tide pools along rocky coasts on both sides of the North Atlantic, is one that presents a particularly magnificent set of bushy tentacles when fully expanded. Along the body of a Cucumaria, five lengthwise tracts of short tube-feet show the five-parted symmetry so obvious in most echinoderms.
In Thyone, the whole body is studded with tube- feet and curved into a broad U. Ordinarily these ani- mals bury themselves in the bottom with only the cloacal opening and the bushy tentacles exposed. If a Thyone is dug out and then placed on the sea floor, it usually needs three to four hours to work itself into the hidden position again.
Some other sea cucumbers with bushy tentacles have a scale covering. Usually these animals rest on a solelike area of the lower surface, and give the general appearance of an armored slug with tentacles instead of gills. They creep from place to place, and can climb the vertical walls of a glass aquarium at fair speed. Psolus has tube-feet only around and under the creeping sole, whereas Psolidium extends degenerate tube-feet that lack sucker tips through holes in the body scales.
Psolus antarcticus carries as many as 22 young along with it, holding to smooth areas of the creeping sole. Cucumaria parva has been seen holding plant material against its body, helping keep young in place. Other species of these two genera have pockets in the body wall, usually around the anterior end, in which the eggs develop.
Large tropical and subtropical sea cucumbers usually have twenty tentacles, but each of these feeding organs has an expanded tip and cannot be withdrawn into the body as is done by cucumbers with bushy tentacles. Holothuria is one genus of particularly inert and sausage-like sea cucumbers, with no obvious flattened surface to indicate a ventral side. Actinopyga has a creeping sole, as has Stichopus. Both of these live in exposed positions on mudflats, reaching record lengths of 40 inches and a diameter of 8 inches. Actinopyga differs from Stichopus in that the anus opens into the cloaca through an armament of five limy teeth. Stichopus lacks these teeth, but has the ability to raise its body in waves of movement, like a giant caterpillar walking, and shift the animal far more rapidly than use of its tube-feet would permit.
Close relatives of these cucumbers live in the great depths of the ocean. There Bathyplotes appears to drift well above the bottom for most of its life, supported by a float extending around the rim of its creeping sole. Mesothuria intestinalis, a grayish white animal often tinged with pink or violet, is sometimes found also near the surface. It covers its body with debris, as though to hide from enemies, and has been found to begin adult life as a male, later transforming into an egg-laying female.
Molpadonias are sea cucumbers with a conspicuous tail, often found buried in the bottom mud with only the tail tip and cloacal opening exposed. These animals lack tube-feet, or have them only around the anus, perhaps used there in keeping the cloaca free of sediments. The feeding tentacles are fleshy, sometimes with a few finger-like extensions at the ends.
According to Japanese scientists, molpadonias feed particularly rapidly. An individual may move from 125 to 150 pounds of bottom sediments through its 7-inch body annually in extracting nourishment. One of this type of cucumber is Caudina arenata, found from Rhode Island to the Gulf of St. Lawrence between 100 feet below the surface and low-tide mark. Its tail tip can be found exposed from the sandy mud, and used to capture the buried cylindrical animal. The body may be 1 inch in diameter and 7 in length, in hues ranging from deep purple to flesh-color.
Some sea cucumbers are wormlike, lacking tube- feet and respiratory trees. Usually the body wall is very thin, often translucent, and the animal itself is more active than most other holothurians. Several kinds with this shape of body burrow in the mud and can bury themselves in five to six minutes. Others, while only partly grown, swim to the surface at night by a curious twitching movement of the body, suggesting a scissors kick.
Synaptula is one of the commoner wormlike sea cucumbers. It can be found clambering among sea- weeds and through coral reefs. When fully extended a Synaptula may reach a length of 3 feet, yet be no more than 1/2 of an inch in diameter. Some members of this genus have openings through the wall of the intestine in the female, through which sperms from sea water reach the eggs in the body cavity. Thus fertilization is internal, and the embryos develop for some time in the body cavity before being cast out into the sea. The young of Chiridota rotifera, a wormlike sea cucumber of shallow water in the West Indies, reach the same body form as the parent before they emerge, and this sea cucumber is truly viviparous.