SEA CUCUMBERS: Holothuroidea

PHYSICAL CHARACTERISTICS

Sea cucumbers are soft-bodied sea animals that have a circle of five to twenty tentacles around the mouth. Some sea cucumbers have a thick, muscular body wall, but others are clear and jellylike. Most sea cucumbers look like thick worms or slugs, but some are U-shaped. Depending on the species, the tentacles are simple, fingerlike, featherlike, or flat and shovel- or shieldlike. The large tube feet of some sea cucumbers make them look prickly. Most sea cucumbers are about 20 inches (51 centimeters) long, although some are only a fraction of an inch (millimeters) long and others are longer than 10 feet (3 meters). Sea cucumbers that live in warm, shallow water usually are bright green, red, orange, or yellow. Most sea cucumbers that live in deep, open water are dark. Those that live in deep water but burrow in the bottom are pale gray to white, although some are clear, light purple, or pink.

The water-circulating system that powers the tube feet of sea cucumbers is made up of a ring canal around the throat and long canals that run from the ring to the rear of the animal. In sea stars and other relatives of sea cucumbers, the water-circulating system exchanges water with the environment through a strainer plate that opens to the outside. In most sea cucumbers, however, the strainer plate is inside the animal and opens into the body cavity.

In almost all sea cucumbers, the skeleton of the body wall is made up of microscopic bonelike parts that look like rods, roses, crosses, buttons, tables, or wheels and anchors, among other shapes. In some sea cucumbers, these parts are large and platelike and make the animal rigid. Another important feature of the skeleton is a hard ring around the throat. This ring is an attachment surface for the muscles that move the mouth tentacles and for the forward ends of the muscles that contract the body lengthwise.

GEOGRAPHIC RANGE

Sea cucumbers live all over the world.

HABITAT

Sea cucumbers live in all sea habitats, ranging from areas exposed at low tide to the bottom of the deepest part of the ocean. Some species live on wave-hammered reef crests and rocky shorelines. Many species live on hard bottoms. Others burrow in sand or mud. Some species swim miles above the sea floor.

DIET

Sea cucumbers eat algae, plankton, bacteria, and waste particles. Algae (AL-jee) are plantlike growths that live in water and have no true roots, stems, or leaves. Plankton is microscopic plants and animals drifting in water.

BEHAVIOR AND REPRODUCTION

Most sea cucumbers are slow-moving animals. Some rear up and extend their front ends into the water when releasing eggs and sperm. Some twist violently or inflate when they meet a predator. Some deep-sea species can swim. Bottom dwellers wander in an apparently random way as they feed. Many tropical species of sea cucumbers are active at night, staying in crevices or under the sand during the day.

DID YOU KNOW?

Regions deeper than about 18,000 feet (5,500 meters) are only about one percent of the area of the ocean floor. These depths consist of trenches almost 7 miles (11 kilometers) deep. Sea cucumbers dominate the animal life of this region. One species of sea cucumber lives in waters as shallow as 230 feet (70 meters) and as deep as 33,000 feet (10,000 meters).

Some sea cucumbers have toxins in their body wall that generate a taste that keeps fishes away. Other sea cucumbers defend themselves by shooting tubes out their anus (AY-nuhs). The tubes become very long and sticky, entangling predators or scaring them away. Some sea cucumbers defend themselves by ejecting their internal organs—some through the head by breaking off the tentacle crown and others through the anus. The sea cucumbers survive the organ ejection, and the organs grow back.

Sea cucumbers use their tentacles for eating. Some sea cucumbers scoop up sand or mud with shovel-shaped tentacles. Others lash the surface with featherlike tentacles. Still others scoop up food particles as they burrow with fingerlike tentacles. Some sea cucumbers have branched tentacles that are lightly coated in mucus and extend into the current to capture algae and plankton. Food sticks to the mucus, and the sea cucumber brings the tentacles into its mouth one at a time to wipe them clean by contracting muscles around its throat.

Some species of sea cucumbers have separate sexes, and others make both sperm and eggs. At least one deep-sea species forms pairs. Some sea cucumbers, mainly those that live in warm water and those that live in the deep sea, release eggs and sperm into the water, where they unite and larvae develop. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults. In other species, mainly those that live near the shore or in cold water, females use their tentacles to gather up the eggs as they are being released and keep them on their bellies or in special pouches for development. In a few species the larvae develop inside the female's body cavity.

Some sea cucumber larvae are non-feeding but have fat stored in them and develop directly into adults. Other sea cucumber larvae have a feeding stage in which they drift in the water. They then transform from a two-sided to a five-armed body plan and settle on the bottom as miniature adults. Adults of some warm-water species of sea cucumbers may also reproduce by splitting in half.

SEA CUCUMBERS AND PEOPLE

People in several Asian and Pacific Island countries eat sea cucumbers in a dish called trepang. Sea cucumbers also are used in aquariums. The toxins in sea cucumber skin are being studied for use as drugs.

TREPANG

Dried and processed sea cucumbers, called trepang (trih-PANG) or bêche-de-mer (besh-duh-MEHR) are sold as a delicacy in Asia, where they form the basis of a multimillion dollar industry. Demand for trepang is increasing, and overfishing is a threat in many areas. The most valuable species for trepang are slow-growing, long-lived sea cucumbers that live in warm, shallow water, where they are easily harvested.

CONSERVATION STATUS

Sea cucumbers are not considered threatened or endangered.

CANDY CANE SEA CUCUMBER (Thelenota rubralineata): SPECIES ACCOUNTS

Physical characteristics: Candy cane sea cucumbers are colorful and large, as long as 20 inches (51 centimeters). The entire body is covered with pointed tube feet. Candy cane sea cucumbers have a bright red pattern of stripes on a white background and have about twenty dull red shield-shaped tentacles.

Geographic range: Candy cane sea cucumbers live in the southwestern part of the Pacific Ocean.

Habitat: Candy cane sea cucumbers live on sand patches on reefs 20 to 200 feet (6 to 60 meters) deep.

Diet: Candy cane sea cucumbers eat food particles they find in the sand.

Behavior and reproduction: Candy cane sea cucumbers crawl exposed on the reef during the day and at night. When disturbed they curl up by bringing their front and back ends together. Candy cane sea cucumbers release their eggs and sperm into the water, where they unite and develop into larvae, which transform into young sea cucumbers that develop into adults.

Candy cane sea cucumbers and people: Candy cane sea cucumbers are harvested by accident with sea cucumbers that are used to make trepang.

Conservation status: Candy cane sea cucumbers are not considered threatened or endangered. ∎

SEA APPLE (Pseudocolochirus violaceus): SPECIES ACCOUNTS

Physical characteristics: Sea apples are colorful sea cucumbers about 7 inches (18 centimeters) long. They usually are purple. Three rows of tube feet run along the bottom side of the animal. The top side has two rows of tube feet as well as small scattered tube feet. The body is curved so that the mouth and anus point upward. The ten tentacles are bushy purple to red and have white tips. The pieces of the body wall skeleton are rounded, smooth plates with a few holes.

Geographic range: Sea apples live in the Indian Ocean and the western part of the Pacific Ocean.

Habitat: Sea apples live on hard material, such as coral reefs, in water as deep as 40 feet (12 meters) in areas with a current.

Diet: Sea apples eat plant plankton.

Behavior and reproduction: Sea apples live partly hidden to fully exposed with tentacles expanded, even during the day. They feed continuously, capturing large food particles with outstretched branching tentacles that are lightly coated in mucus. Sea apples have separate sexes. Males release sperm and females release eggs into the water, where they unite and where the larvae develop. The larvae transform into young sea apples that develop into adults.

Sea apples and people: Sea apples have no known importance to people.

Conservation status: Sea apples are not considered threatened or endangered. ∎

SEA PIG (Scotoplanes globosa): SPECIES ACCOUNTS

Physical characteristics: Sea pigs are clear sea cucumbers 2 to 4 inches (5 to 10 centimeters) long. They have ten tentacles and a few large tube feet. The tube feet on the top side of sea pigs are two widely spaced antennalike pairs. The other tube feet are arranged in a row around the edge of the bottom side of the animal. The pieces of the body wall skeleton are smooth to spiny rods and smaller C-shaped rods. Sea pigs are also called sea cows because the tube feet on the top side of the body look like cattle horns.

Geographic range: Sea pigs live all over the world except the northern part of the Atlantic Ocean and the eastern part of the Pacific Ocean near Central and South America.

Habitat: Sea pigs live in the deep ocean.

Diet: Sea pigs eat food particles they find in the sand.

Behavior and reproduction: Sea pigs move above the bottom using long tube feet. These sea cucumbers form large groups. Sea pigs feed by using their tentacles to push sand or mud into their mouth. Scientists do not know how sea pigs reproduce.

Sea pigs and people: Sea pigs have no known importance to people.

Conservation status: Sea pigs are not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Carson, Rachel. The Edge of the Sea. 1955. Reprint, Boston: Mariner, 1998.

Niesen, Thomas M. The Marine Biology Coloring Book. 2nd ed. New York: HarperResource, 2000.

Periodicals:

Summers, Adam. "Catch and Release: Sea Cucumbers Might Put a Torn Achilles Tendon Back Together Again." Natural History (November 2003): 36–37.

Web sites:

"Frequently Asked Questions of the Sea Cucumber." Charles Darwin Research Station.http://www.darwinfoundation.org/marine/FAQcuke.html (accessed on March 14, 2005).

"Sea Cucumbers." Thinkquest.http://library.thinkquest.org/J001418/seacuc.html (accessed on March 2, 2005).