Skip to main content

Echinoderms: Starfish, Brittle Stars, and Feather Stars

The familiar starfish (sea star) and its relatives are members of a group of spiny-skinned organisms, the echinoderms. Close relatives include brittle stars, basket stars, sea urchins, sea cucumbers, feather stars, and sea lilies. All of the animals in the starfish family have five or more arms radiating from a central body, shown in the lower color insert on page C-5. The mouth of an echinoderm is located on the underside and the anus on top of the body. Echinoderms may be carnivores, detritivores, or herbivores, depending on the species.

Tentacle-like structures called tube feet, visible in Figure below, help echinoderms move slowly across the reef. The tube feet act like suction pads, grasping and releasing surfaces. A hydraulically controlled vascular system supplies water to each tube foot through small muscular tubes. As the feet press against an object, water is withdrawn, creating suction. When water is returned to the cups, the suction is broken, and they release.

On its ventral sides, a starfish, or sea star, has a centrally located mouth surrounded by five arms with tube feet. (Courtesy of Dr. James P. McVey, NOAA Sea Grant Program)
On its ventral sides, a starfish, or sea
star, has a centrally located mouth surrounded by five arms
with tube feet. (Courtesy of Dr. James P. McVey,
NOAA Sea Grant Program)

To reproduce, echinoderms release sperm and eggs into water. These fuse to form zygotes that develop into larvae. After some time in the plankton, a larva settles to the bottom and takes on typical echinoderm features. Most echinoderms can also reproduce asexually. If part of the animal breaks off, it may grow into a complete, new organism. All are capable of regenerating missing limbs, spines, and in some cases, intestines.

Starfish are common on the reefs. Most are carnivorous, feeding on sponges and small invertebrates. One species, the crown of thorns (Acanthaster planci), eats live coral polyps. Crown of thorns can reach 20 inches (50 cm) in width and may have 10 to 20 spiny arms. As is the case for all starfish, it eats by extruding its stomach over its prey, then drenching it in digestive juices. After the prey tissues liquefy, the crown of thorns pulls its stomach and the partially digested food back into the body.

A brittle star has a small central disk from which radiate five snakelike arms. Unlike starfish, brittle stars lack an anus, so waste products are eliminated through the mouth. On the underside of a brittle star, there are 10 slitlike openings at the base of the arm that function in breathing, as well as releasing eggs and sperm. Brittle stars often hide in the crevices in coral reefs, emerging at night to feed on plankton. If a predator happens to find and grab a brittle star, it has trouble hanging on. As its name suggests, this animal’s arms break off easily,
allowing for easy escape. A broken arm quickly regenerates.
 
Feather stars, or crinoids, are cup-shaped animals that have five to 200 feathery arms projecting upward from a central disk. The arms are coated with sticky material that helps them catch planktonic food. Feather stars cling to coral or rocks but can move by rolling, walking, crawling, swimming. They often establish symbiotic relationships with other animals including some species of shrimp, lobsters, and clingfish.

Popular posts from this blog

Advantages and Disadvantages of an Exoskeleton

More than 80 percent of the animal species are equipped with a hard, outer covering called an exoskeleton. The functions of exoskeletons are similar to those of other types of skeletal systems. Like the internal skeletons (endoskeletons) of amphibians, reptiles, birds, and mammals, exoskeletons support the tissues and give shape to the bodies of invertebrates. Exoskeletons offer some other advantages. Serving as a suit of armor, they are excellent protection against predators. Also, because they completely cover an animal’s tissues, exoskeletons prevent them from drying out. In addition, exoskeletons serve as points of attachment for muscles, providing animals with more leverage and mechanical advantage than an endoskeleton can offer. That is why a tiny shrimp can cut a fish in half with its claw or lift an object 50 times heavier than its own body.
Despite all their good points, exoskeletons have some drawbacks. They are heavy, so the only animals that have been successful with them …

Differences in Terrestrial and Aquatic Plants

Even though plants that live in water look dramatically different from terrestrial plants, the two groups have a lot in common. Both types of plants capture the Sun’s energy and use it to make food from raw materials. In each case, the raw materials required include carbon dioxide, water, and minerals. The differences in these two types of plants are adaptations to their specific environments.
Land plants are highly specialized for their lifestyles. They get their nutrients from two sources: soil and air. It is the job of roots to absorb water and minerals from the soil, as well as hold the plant in place. Essential materials are transported to cells in leaves by a system of tubes called vascular tissue. Leaves are in charge of taking in carbon dioxide gas from the atmosphere for photosynthesis. Once photosynthesis is complete, a second set of vascular tissue carries the food made by the leaves to the rest of the plant. Land plants are also equipped with woody stems and branches that …

Prokaryotic Cell Structure

Prokaryotic cells are about 10 times smaller than eukaryotic cells. A typical E. coli cell is about 1 μm wide and 2 to 3μm long. Structurally, prokaryotes are very simple cells when compared with eukaryotic cells, and yet they are able to perform the necessary processes of life. Reproduction of prokaryotic cells is by binary fission—the simple division of one cell into two cells, after DNA replication and the formation of a separating membrane and cell wall. All bacteria are prokaryotes, as are the archaea.

Embedded within the cytoplasm of prokaryotic cells are a chromosome, ribosomes, and other cytoplasmic particles (Fig. 1). Unlike eukaryotic cells, the cytoplasm of prokaryotic cells is not filled with internal membranes. The cytoplasm is surrounded by a cell membrane, a cell wall (usually), and sometimes a capsule or slime layer. These latter three structures make up the bacterial cell envelope. Depending on the particular species of bacterium, flagella, pili (description follows)…