Starfish or to give them their correct name sea stars are familiar sea creatures often seen on our coastlines and admired for their bright and beautiful colours and elegant symmetrical shape. There are over 1800 species of sea stars. They are distributed extensively throughout the world’s oceans, and can be found in a wide variety of marine habitats, ranging from the shallow intertidal zone, to the sea floor at depths in excess of 6000 metres. Sea stars are an ecologically important keystone species in the marine environment.
Sea stars are echinoderms (meaning spiny skin) which belong to the class asteroidea. Their characteristic structure is arms or rays radiating symmetrically from a central disc. Most sea stars have 5 arms but some species may have up to 50.
Sea stars do not have eyes but at the end of each arm there is a microscopic ocellus that can distinguish light and dark. Although sea stars do not have a central brain they do have a nervous system which is sensitive to touch, temperature, orientation, chemicals and odours.
Sea stars do not have a heart or blood, instead they have a hydraulic water vascular system that circulates water through channels and is used to feed, transport waste and move. Locomotion is through tiny projections called tube feet. When a tube foot presses against a surface, muscles withdraw water creating suction, when the muscles return water to the foot the force of suction is broken, adhesive chemicals can also play a part in attaching and detaching tube feet.
Sea star may be carnivorous predators, grazers, suspension feeders or scavengers depending on the species. The mouth of sea stars is toothless and located on their underside. Sea stars have two stomachs, a cardiac stomach and a pyloric stomach; the cardiac stomach can be everted outside the body. Some species of sea stars pry open shellfish, such as oysters, with their tube feet then slide their stomachs inside the shellfish where digestive enzymes puree the shellfish flesh. The stomach and digested shellfish is then retracted back into the sea star’s body.
Reproduction methods vary depending on the species – some species are hermaphrodites, others are separate sexes, some may change sex. Most species fertilise externally and produce prolific numbers of tiny larvae, which join floating zooplankton. A small number of species are livebearers, the young develop in a sac inside the parent before emerging as tiny fully formed starfish. Sea stars can also reproduce by fragmentation - a detached arm and portion of the central disc can develop into an independent individual.
Lifespan varies between species; larger species tend to live longer and may reach over 34 years.
Sea stars are vulnerable to pollution and over harvesting but may become pests if transported to another marine environment in ships ballast water eg the Northern Pacific Sea star is now a major pest in the Derwent River.
- Written by Nadia O’Carroll