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June 9, 2005
Dr. Hays Cummins
Sea cucumbers are cylinder, or cucumber, hence the name-shaped invertebrate animals that live in many different ranges of the ocean. They can be found from warm tropical waters to cold deep sea trenches. They are most diverse to tropical shallow-water coral reefs. The biggest sea cucumber is the tigerÕs tail and can reach lengths of up to 2 m long. Most sea cucumbers are about 20 cm in length, while some adults may not grow to be over a centimeter. Sea cucumbers enjoy the life-span of five to ten years. Despite at first appearing as a plant or mineral on the ocean floor the sea cucumber, or Holothuroid, is an animal.
The sea cucumber is in the Kingdom Animalia, Phylum Echinodermata, Subphylum Asterozoa (with sea urchins, sand dollars, sea stars, and basket stars). Within the class Holothuroidea there are six orders which are home to around 1,500 species of sea cucumbers. The holothuroids are a unique group of echinoderms. They are the only class that is soft bodied rather than hard. Their axis is horizontal unlike other echinoderms. Their mouth and anus are located at separate ends of their elongated body, which is also not typical of echinoderms. The body form and characteristics of the sea cucumber are so distinct from other echinoderms that they were long separated from them and classified as worms. Sea cucumbers are still echinoderms because they have pentameral symmetry (five-rayed). They have five rows of tube feet running from the mouth along the body. They also retain the skeleton of echinoderms, although in most species of sea cucumbers the skeletal plates are microscopic. This contributes to the difficulty in studying body fossils of the creatures.
The body of the sea cucumber lacks arms, instead the body is covered in tube feet. Surrounding the mouth are eight to thirty (depending on the order) modified tube feet called tentacles. Five double rows of tube feet run along the body. Each tube has a tiny suction cup on the end, which allows the animal to feel and move. The sea cucumber tends to be quite sluggish. Some sea cucumbers will burrow into the sea floor sand as this is where they feed.
Holothuroids use their tentacles to feed. The difference in tentacle structure is so great between holothuroids that it is used to separate between subclass and order levels. While some tentacles function in suspension feeding, others function in deposit feeding, or capture of prey. Feeding is usually continuous for the sea cucumber as they digest decaying organic material in the sand or floating around them. Suspension feeders are restricted to areas that are home to suspended animal or plant material. Deposit feeders do not need to depend upon food being brought to them. They are vagrant and move randomly, feeding simultaneously. Many of these sea cucumbers feed on or in the sand beneath them. Reef dwelling sea cucumbers are most often suspension feeders, as currents continually bring in food.
One very important feature of the sea cucumber is the calcareous ring that encircles the throat. This ring serves as an attachment point for muscles operating the oral tentacles. The lack of skeleton is an advantage here, as the tentacles are flexible, and can easily grab up food. Once the food enters the sea cucumberÕs mouth it travels to a simplistic stomach and through intestines where basic nutrients are absorbed, before the waste is displaced out of the anus. Often times the sea cucumber acts as a sea floor janitor, ingesting large amounts of sand, absorbing any organic matter before releasing the ÒcleanedÓ sand.
The sea cucumber has three sites for oxygen uptake: the tube feet, the body wall, and the respiratory tree. Respiratory trees are unique to sea cucumbers and are not found in any other echinoderm. All species of Holothuroids must use at least one of these organs to breath. Respiratory trees are often fed oxygen by the sea cucumber actually breathing through their anus. They not only excrete waste from their anus, but expand and contract their muscular body walls in a slow rhythm, which in turn draws in and expels water. This is where the respiratory trees extract the oxygen. There are a few species of sea cucumbers that also allow small fishes to enter and exit the anus. These fish find safety within the sea cucumberÕs intestines.
For other sea cucumbers, such as Apodids, breathing must occur through the body wall as they lack both respiratory trees, as well as tube feet. These sea cucumbers have an extremely thin body wall that allows oxygen to pass through.
Finally sea cucumbers breathe using there tube feet. It is quite common for these animals to have a number of modified tube feet that are adapted to oxygen uptake. Some of these respiratory feet can be withdrawn into the body to transfer the oxygen. Other respiratory tube feet extend a long tube which uptakes the oxygen.
Suspension-feeding holothuroids show little movement as it is not necessary to their daily life. Surface-feeding or deposit-feeding holothuroids, in contrast, must move to obtain food. Rapid motion is not necessary. The moving sea cucumber can use its tube feet and tentacles to slowly move or to tunnel into the sand to locate food. Only tube feet with suckers tend to be used for motion.
Sea cucumbers are prey to many predators such as sea turtles, crustaceans, many fish, and people. There are few features of the holothuroid that can be related directly to protection from predators. Except for the feeding tentacles and tube feet there are no extensions of the body that are vulnerable to predators. Therefore, the individual sea cucumber must be ingested whole or a portion of the body must be bitten off. The sea cucumber is not a preferred food because of the presence of toxins in the body. Furthermore, a threatened, or wounded holothuroid will contract its body exposing the small skeletal bones that make up the body wall, which can act as hooks to the mouths of predators. For most species, the connective tissue that makes up the greater part of the body wall is the primary deterrent to predators. The contracted body not only makes a more compact body to bite, but increases stiffness of the body wall. The soft, flexible tentacles seem to be the most vulnerable part of the sea cucumber, and thus it is not surprising that most species can withdraw their tentacles within their body.
The sea cucumber also has a very unusual defense mechanism. Many species use the Cuvierian tubules, which are located in the digestive system of the animal to confuse would be predators. The sea cucumber, when stimulated, will raise the anus, point it in the direction of the stimulation, and contract the body wall to expel the tubules through the anal opening. The Cuvierian tubules are sticky and toxic, and are used to simply confuse the predator. These digestive tubules are lost when the sea cucumber takes this action, and can be re-grown later. In at least two species of holothuroids auto-explosion of the Cuvierian tubules occurs seasonally, but it is not known exactly why.
The majority of holothuroids have sexual reproduction with male and females of equal size. They are very rarely hermaphroditic. The sea cucumber will produce small eggs which are released into the water and fertilized externally. When an egg is fertilized and hatches, the tiny planktonic larva drifts with the ocean currents. It will eventually settle onto the sea floor and develop into an adult.
Although sea cucumbers do not have eyes, many of them are light sensitive. Therefore many sea cucumbers that live in shallow waters tend to hide out during daylight hours, coming out at night to feed. They will also quickly retract their tentacles when approached.
Some other interesting facts about sea cucumbers are that the toxins within their bodies are of great interest as drugs for people. The sea cucumber is also considered a delicacy by many people around the world. In some deep ocean trenches sea cucumbers are so prevalent that they make up around 90% of the total biomass for that trench.
A few common types of sea cucumbers are California, Sweet Potato, Warty , White, Orange, and Slipper. The sea cucumber can be almost any color, and is often bright and beautiful. The sea cucumberÕs close relatives are sea stars, brittle stars, sea urchins, and sand dollars. I am looking forward to discovering and studying more about sea cucumbers in the Keys, and the Bahamas.
1. Hyman. L.H. 1955 The Invertebrates. Vol. 4. Echinodermata. New York. McGraw Hill.
2. Binyon. John. 1972 Physiology of Echinoderms. Oxford. Pergamon Press.
3. Lawrence. John. 1987 A Functional Biology of Echinoderms. Baltimore. The Johns Hopkins Press.
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