Amazing “Vacuum Cleaners” of the Sea
By Awake! writer in Fiji
They “ooze along the bottom at the speed of an hour-hand, or slurp their way through rich organic mud. They are everywhere from the intertidal zone to the deepest parts of the ocean. Like a miniature herd of gnus trundling across the abyssal plains, they graze on the rich organic snow that has drifted down from above.”—Philip Lambert, a curator of the Royal British Columbia Museum.
YOU might be surprised that anyone would wax lyrical with respect to the lowly sea cucumber, also known as the holothurian. Why, this creature has been described as “a sort of headless rubbery sausage.” Is there more to the sea cucumber than meets the eye?
Sea cucumbers are said to be relatives of starfish and sea urchins. Although seemingly sluglike, they are quite different from true sea slugs, which are marine snails without shells. So far, more than 1,100 holothurian species have been identified. Of these, many—including the edible species—are plain. Others are decked in the most exotic finery. Projections on the skin of many varieties cause them to resemble cucumbers with warts.
Some sea cucumbers are microscopic, while others can reach 15 feet [5 m] in length. Most, however, measure between 4 and 12 inches [10 and 30 cm]. It is estimated that sea cucumbers constitute more than 90 percent of the mass of all creatures living at a depth of 26,000 feet [8,000 m], making them the dominant organism in some ocean trenches. Though the majority of them live on the seabed, a few deep-water species can swim.
Sea cucumbers are found in all the world’s oceans, steadily grazing on organic sludge. Like teams of vacuum cleaners, they clean seabed mud and ooze by swallowing large quantities of sediment, filtering out the organic matter, and leaving clean sand in their wake. As many as 2,000 sea cucumbers may live in one acre [5,000/1 ha] of coral reef.
The food of the sea cucumber consists of microscopic organisms and detritus resting on the seabed or passing by in the current. Up to 30 featherlike tentacles with special nerve endings at their tips are used to sense and seize food particles. Each tentacle, in turn, is “licked-off” before returning to continue the search.
Some varieties of sea cucumbers act as “hosts” that are entertaining “guests.” The “guests” are fish and other creatures that live in the gut of the host and emerge at night to feed. These include 27 species of pearlfish, or cucumber fish, of the Carapidae family. When alarmed, they may be seen scurrying back to their hiding place. On occasion they are known to feed on their landlord’s reproductive and respiratory organs. This does not harm the host, however, since the sea cucumber is able to regenerate lost tissue.
Resourceful if Threatened
Sea cucumbers clearly merit a second glance if you should come across them on your next visit to the ocean. But be warned! If they feel threatened, these “vacuum cleaners” of the sea have a range of bewildering tricks up their sleeves. Some, for example, eject a mass of long sticky threads that entangle or distract predators. The sticky material hardens rapidly, leaving hapless human victims with little choice but to shave off any hair with which the gummy strands have come into contact.
Other sea cucumbers produce a toxin named holothurin. This substance is lethal to many kinds of fish. Though dangerous to the eyes and likely to cause a skin rash, it otherwise appears to be harmless to humans. For generations, islanders have used this substance to poison, kill, or stupefy fish for the table, and it is an effective shark repellent. Research indicates that toxins produced by sea cucumbers may have potential for development as pharmaceuticals in the treatment of cancer and infections. Various extracts prepared from sea cucumbers have been used by practitioners of alternative medicine in treating arthritis, rebuilding cartilage, and lowering blood pressure. The animals are also processed to produce vitamin and mineral supplements.
But the sea cucumber has one more surprise held back for the most threatening of situations, a reaction that takes the issue of self-defense to a new dimension. As you try to move it, the sea cucumber auto-eviscerates—that is, it literally splits its sides and disembowels itself. Appalled at this shocking overreaction, you wonder what you did to cause such a ghastly end to the poor creature! But relax. Likely, you did not take its life. Rather, you were witness to an extraordinary escape device. This resilient animal will grow a new set of internal organs within weeks!
Still in Demand
The ancient occupation of fishing for sea cucumbers is still alive and well in both temperate and tropical waters. In fact, some divers will risk life and limb to increase their catch. The bulk of it is destined for China and other parts of the Orient—just as it has been for centuries. Production of dried sea cucumbers involves boiling them in salt water and then gutting, smoking, and sun-drying them for market. Today, sea cucumbers are also available frozen.
Will a sea cucumber find its way to your dinner table? Perhaps it will. Despite its being called a cucumber, however, you are unlikely to find it in your salad. Once cooked, the sea cucumber is gelatinous and almost transparent. It is used as a thickening and flavoring agent for soups. In Fiji, local varieties are prepared with coconut cream in traditional island style, and the result is fishy, tasty, and chewy.
Still, these unobtrusive, quiet achievers are much more than just a tasty dish. We are indeed indebted to sea cucumbers for their ceaseless janitorial activities in maintaining the health of our seas. Call them what you will—holothurians or sea cucumbers—these amazing “vacuum cleaners” of the sea give silent praise to the One who created them!—Psalm 104:24, 25.
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Peculiarities of Sea Cucumbers
▪ The respiratory system of the sea cucumber differs greatly from what we would consider the norm. Water is drawn in through the gut, and oxygen is absorbed into the walls of two respiratory trees. Certain deep-sea species use their entire body surface to collect oxygen. Some even breathe through the skin of their feet.
▪ Ossicles in place of bones give many sea cucumbers their characteristic cucumberlike appearance. When viewed through electron microscopes, the ossicles’ fascinating designs are seen to be wheel-, anchor-, and hook-shaped spikes embedded in the surface of their skin, giving it a leathery consistency. These intricate, minute crystals of calcium carbonate have characteristics unique to each species and are therefore useful for identification.
▪ Sea cucumbers move under hydraulic power. Hundreds of hollow tube feet, or podiums, controlled by a valved water-vascular system operate in synchronization. By the expansion and contraction of chambers, water is forced into the feet to extend them in the proper order so as to achieve the motion required.
▪ Reproduction is usually by external fertilization, after which free-swimming embryos settle to the seabed. However, some species have been noted to use a more drastic method. They literally tear themselves apart, dividing transversely into two sections. Amazingly, they then regenerate the missing body parts. This method of asexual reproduction requires considerable reorganization of the tissues in each of the torn pieces.
From top to bottom: Courtesy Bruce Carlson, Georgia Aquarium; courtesy of UC Museum of Paleontology, www.ucmp.berkeley.edu; © Houseman/BIODIDAC; Ocean Sky Diving, Hong Kong
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Sea cucumber ejecting sticky defensive threads
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Warty sea cucumber
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Sea cucumber with one feeding tentacle in its mouth
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Sea cucumber prepared in the traditional island style, with coconut cream
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Top inset: © David Wrobel/Visuals Unlimited; background: © Phillip Colla/SeaPics.com; bottom left: © Doug Perrine/SeaPics.com