Subclass Asteroidea
sea stars and starfishes

There are approximately 1500 extant species in the Asteroidea. (Brusca and Brusca, 2003; Canada's Aquatic Environments, 2002)

Mainly a marine group, asteroids are found in all the oceans. (Brusca and Brusca, 2003; Canada's Aquatic Environments, 2002)

Except for a few species which inhabit brackish waters, asteroids are benthic organisms found in marine environments. (Brusca and Brusca, 2003; Canada's Aquatic Environments, 2002; Waggoner, 1994)

Members of the Asteroidea are often called "true starfish" and are closely related to the Ophiuroidea, having a common ancestor in the Ordovican. Phylogenic hypotheses including fossil groups place them as being closely related. Phylogenic hypotheses with only extant groups put them as sister groups with the five-rayed body plan as a synapomorphy. (Brusca and Brusca, 2003; Waggoner, 1994)

Asteroids can range from less than 2 cm to over one m in diameter, although the majority are 12 to 24 cm. Arms extend from the body from a central disk and can be short or long. A majority have 5 arms, although some can have up to 40. Calcareous ossicles make up the internal skeleton.

The water vascular system of the sea stars open up at the madreporite, a perforated opening in the central part of the animal. Internally, the madreporite leads to a stone canal, made up of skeletal deposits. The stone canal is attached to a ring canal which leads to each of the five (or more) radial canals. Tiedemann’s bodies and polian vescicles are pouches on the ring canal whose function may be osmoregulation or hydraulic regulation within the water vascular system. Each radial canal ends in a terminal tube foot, which has a sensory function.

Each radial canal has a series of lateral canals that terminates at a tube foot. Each tube foot is made of an ampulla, podium, and usually a sucker.

The oral surface, under the central disc, is where the mouth is located. The hemal system parallels the water vascular system and probably distributes nutrients from the digestive tract. Hemal channels extend to the gonads.

Larvae are bilaterally symmetrical and adults are radially symmetrical. (Brusca and Brusca, 2003; Canada's Aquatic Environments, 2002)

Asteroids are deuterostomes. Fertilized eggs develop into bilaterally symmetrical planktonic larvae, which have 3-part paired coeloms. Embryonic coelomic structures have specific fates as the bilaterally symmetrical larvae metamorphose into radially symmetric adults. Adult pheromones may attract larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered by adult pheromones. After settling, the larvae go through a sessile stage and metamorphose. (Anonymous, 2005; Barnes, 1987; Brusca and Brusca, 2003; Canada's Aquatic Environments, 2002)

Some sea stars can live up to 35 years. (Canada's Aquatic Environments, 2002)

Asteroids move by using their water vascular system. Internal muscle contractions squeeze fluid to the tube feet, which then elongate. Cilia within the epithelium of the water vascular system moves the water. The end of the tube feet have suckers, which chemically adhere to the substrate. Another chemical bond is secreted to release the tube feet. Sea stars living on soft bottom substrates have pointed tube feet (rather than suckers) to help them move. (Brusca and Brusca, 2003; Canada's Aquatic Environments, 2002)

The non-centralized nervous system allows echinoderms to sense their environment from all sides. Sensory cells on the epidermis sense light, contact, chemicals and water currents. Higher densities of sensory cells are found in the tube feet and along feeding canal margins.

Red pigmented eye spots are found on the end of each arm. These function as photoreceptors and are clusters of pigment-cup occelli.

Adult pheromones may attract larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered by adult pheromones. (Brusca and Brusca, 2003; Canada's Aquatic Environments, 2002)

Asteroids are mainly scavengers and carnivores. In many areas where they are found they are high level predators. Asteroids feed on slow moving prey, including gastropods, bivalves, barnacles, polychaetes and other invertebrates. They feed by grasping the prey, then everting their stomach and secreting primary enzymes on the prey. The digestive juices break down the tissue of the prey, which the asteroids then suck up.

Some asteroids are suspension feeders. Plankton and organic detritus sticks to mucus on the body surface and is moved by cilia to the mouth. A few species that use their pedicellariae to capture prey may even feed on fish.

Asteroids have a complete digestive system. The mouth leads to the cardiac stomach, which is what the sea star everts to digest its prey. The cardiac stomach leads to a pyloric stomach. Digestive glands, or pyloric ceca located in each arm. Enzymes are secreted through pyloric ducts. A short intestine follows the pyloric stomach and leads to the anus. (Brusca and Brusca, 2003; Waggoner, 1994)

Planktonic larval stages are probably the most vulnerable to predation. Calcareous ossicles probably discourage predation of the adults. Other predators include Hyperoodon ampullatus, the northern bottlenose whale. Asteroids can lose arms to predators and regenerate the arms later. (Brusca and Brusca, 2003)

Some asteroids, such as Acanthaster planci, have increased in population and damage coral reefs as a result of their grazing. Many asteroids are top predators and keystone species. (Brusca and Brusca, 2003; Canada's Aquatic Environments, 2002; Waggoner, 1994)

Asteroids can damage coral reefs. They can also be a problem to commercial oyster beds. (Canada's Aquatic Environments, 2002; Waggoner, 1994)