Starfish
Starfish, also known as sea stars, are marine invertebrates belonging to the class Asteroidea within the phylum Echinodermata. They are characterized by their distinctive starlike shape, typically with five or more arms, and are covered in spiny, leathery skin. Found at various ocean depths across all oceans except the polar regions, starfish exhibit a remarkable ability to regenerate lost limbs and can reproduce both sexually and asexually. Their diet primarily consists of bivalves like clams and mussels, which they can open using their tube feet and digestive processes. Some species, such as the sunflower starfish and crown-of-thorns starfish, have specific habitats and feeding behaviors, with the latter known for its impact on coral reefs. Starfish possess a simple nervous system without a brain, enabling basic sensory functions but limited complex behaviors. While many species are stable, some, like the sunflower starfish, face threats to their populations. Overall, starfish play an integral role in marine ecosystems and highlight the diversity of life found beneath the ocean's surface.
Starfish
Starfish Facts
Classification:
Kingdom: Animalia
Phylum: Echinodermata
Class: Asteroidea
Orders: Include Forcipulatida and Spinulosida, amongst others
Geographical location: All ocean depths in all oceans, except near the North and South Poles
Habitat: Ocean bottoms, from shallow water to great depths
Gestational period: No true gestation; sperms and eggs are spawned into oceans, where the eggs are fertilized; larvae drift, settle to the ocean floor, and become adults
Life span: Dependent on species but generally around thirty-five years
Special anatomy: Radially symmetrical arms, locomotor tube feet, mouth, and stomach cavity that can turn inside out, protective spines
Starfish are carnivorous ocean animals that usually have five starlike arms. They are related to brittle stars, sea urchins, and sea cucumbers, all of which are known as echinoderms. Like these animals, starfish have spiny skins. Starfish comprise several orders—between 1,600 and 2,000 species—of marine invertebrates, the class Asteroidea of the phylum Echinodermata. They have radially arranged arms that hold locomotor tube feet and reach diameters from six inches to four feet. They are abundant at all ocean depths and occur in all oceans except near the North and South Poles. Often, a starfish escapes its enemies by breaking off one or more of its arms, which regenerates. If a starfish is halved, quartered, or cut into smaller pieces, each becomes a new individual.
The Physical Characteristics of Starfish
Stiff-bodied starfish move by crawling slowly. Their bodies may have five arms or many more. Starfish skin is leathery and has protective spines, which point upward. The spines are made of lime and develop from the skeleton. The animal also has a large gut, a complex system of body cavities, and a nervous system but no brain.
The central body of a starfish is located where its arms join. Its bottom surface holds a mouth at the point where all the arms meet. A groove also stretches from the mouth to each arm tip. There are rows of holes in the grooves from which tube feet can push out. The feet, with sucker ends, enable crawling. They are supported and moved by an internal hydraulic system inflated with seawater.
Most of the central body is a baglike stomach into which the mouth opens. Starfish also have well-developed senses of touch, smell, and taste and respond to light. However, they lack the complex behavior patterns of animals having brains.
The Life of the Starfish
Starfish spend much of their lives searching for food, mainly clams, mussels, and oysters. To eat bivalve mollusks, a starfish uses its tube feet to open shell halves. It surrounds a mollusk, attaches the tube feet, and uses them to pull in opposite directions. Once the mollusk tires of resisting the starfish’s force, the shell opens. Then, the starfish pushes its stomach inside out through the mouth and surrounds the mollusk body. Its stomach releases digestive fluid, and as the mollusk softens, the starfish eats its flesh and releases the shell. A similar technique is used with other foods, such as coral. Coral polyps (individual corals) are eaten. First, a starfish climbs onto a polyp and presses its stomach out through its mouth. Then, digestive juice softens the polyp’s shell and turns the polyp into a soupy liquid, which the starfish eats.
Starfish themselves are eaten by fish and snails. Their most successful predators are giant carnivorous sea snails, such as tritons. A triton rips starfish open and eats their soft tissues. Fish also eat starfish by flipping them over and biting off and eating their soft centers. Other predators of the starfish include crabs, lobsters, and seagulls.
Mating occurs in spring or summer, depending on species and habitat. Most species of starfish prefer to mate in warmer temperatures. Most starfish have two sexes and mate by secreting sperm and eggs into the ocean (spawning). External fertilization follows, and the eggs become larvae. Larvae float in the ocean and sink to its bottom to become adults. Some species of starfish may reproduce asexually. Most starfish regenerate body parts. In some starfish, this is also their mode of reproduction: a bud grows and pinches off into a new starfish.
Three Sample Starfish Species
Sunflower starfish (Pycnopodia helianthodes), amongst the largest species of starfish, resemble sunflowers because of their many arms. They grow to diameters up to 3.3 feet and can have up to twenty-four arms. These starfish inhabit the west coast of North America from Alaska to California, from shallows to deep water. They eat bivalves, sponges, coral, worms, crustaceans, other starfish, and small fish. They mate in spring by spawning.
Crown-of-thorns starfish (Acanthaster planci) are star-shaped with many body spikes (thorns). They grow to over two-foot diameters and have up between eight and twenty-one arms. A crown-of-thorns starfish also has a large, round midsection, which holds internal organs and many tube feet. Crown-of-thorns starfish inhabit tropical West Pacific and Indian Oceans. Feeding on coral, they live on or near coral reefs, such as Australia’s Great Barrier Reef, where they hide during the day and feed at night. A single crown-of-thorns starfish can destroy one and a half feet of coral reef per week.
European starfish (Asterias rubens) are five-armed starfish of the European and African coasts. They grow to one-and-a-half-foot diameters and have tube feet. They feed on bivalves, sponges, corals, worms, crustaceans, other starfish, and small fish, and they find food via chemical signals picked up by tube feet. Their spring mating is by means of spawning.
The main foods of starfish are mussels, oysters, and clams. Starfish are thus serious pests on oyster and clam farms. In addition, they eat coral and can damage reefs. For example, crown-of-thorns starfish sometimes overgrow and damage coral reefs by eating too much coral. Some species of starfish, like the sunflower starfish, are endangered, while other species are not threatened.
Principal Terms
Bivalve: a mollusk having two shell halves
Carnivore: an animal that eats only other animals
Invertebrate: an animal lacking an internal skeleton
Bibliography
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