Sea stars are one of the best known invertebrates. They are also ecologically important because they can be important predators of other invertebrates like clams, barnacles and other attached marine life. They can be brightly coloured and, in many species, show considerable variability in colour within one species.

• To be an echinoderm with a flattened, star-shaped body.
• To have 5 arms or multiples of 5 arms.
• Spines or reduced spines on your outer surface.
• To walk using many tube feet that have suckers on their ends.
• A mouth on bottom (oral) surface and anus on the upper (aboral) surface of your body.
• To eat both plants and animals (an omnivore).

Often called

Sea stars are relatively mobile echinoderms that do not have the long spines of sea urchins. The exception to this rule is the Crown-of-Thorns sea star which has spines that may be poisonous as well. Sea stars come in a huge range of colours which may vary greatly within a species. Their legs are fleshy and relatively inflexible. The grooves that run under each arm (ambulacral grooves) are lined with tube feet that usually have suckers for attachment to the substratum. Some sea stars are very small (1-2 cm) while others like the Crown-of-Thorns can be up to 60 cm across.

Sea stars live in the tropical, temperate and polar oceans. They are mobile and are mostly found associated with hard (rock, coral) substrates. There are species, however, that live in association with sand. These species are interesting in that they have lost the suckers that normally tip the ends of their tube feet. They also have special ossicles on their body surface so that sand does not clog the gills they use to breathe.

Sea stars are either carnivorous predators or scavenging omnivores. Sea stars feed in a very distinctive way which involves digesting food outside their bodies (extra-corporeal digestion). They hunt or scavenge their prey, probably using their sense of smell (chemo-detection). They then position themselves over the prey and literally throw part of their stomachs over it. The stomach secretes digestive enzymes which dissolve the prey and reabsorb the digested products as food - what a way to go!

There are a number of predators on sea stars, including fish. We know that Red Emperors fish eat juvenile Crown-of-Thorns sea stars, and that this species of fish may play a role in preventing outbreaks of sea stars on the Great Barrier Reef. This is only an hypothesis and there may be a number of other reasons to the outbreaks Crown-of-Thorns happen every 17 years on average. Molluscs are also thought to be important predators on Crown -of-Thorns, with large whelks like the Giant Triton Trumpet snail eating large starfish. Changes in the numbers of predators on the Crown-of-Thorns is an active area of coral reef research

One of the most amazing facts about starfish is the number of eggs they produce each year. The northern Pacific sea star (Asterias amurensis) produces a massive 19 million eggs each year and the Crown-of-Thorns has been estimated to put out 1-2 million eggs each season! Sea stars have go through several planktonic larval stages (indirect development) which are tiny and look nothing like the adult. Some of these can be quite long lived, for example the larvae of the northern Pacific sea star remain in the plankton for around 90 days, by which time they can travel quite a long way in ocean currents. The fate of the enormous numbers of eggs produced by sea stars can have an important effect on the ecology of reefs and other marine habitats. Usually, only a very tiny fraction of the eggs produced survive to become adults. Should that fraction increase only slightly (even by chance alone), many, many more sea stars could be present on a reef, and their feeding activities could greatly alter the ecology on the reefs where they settle.

The parasites of starfish have not largely been documented. There are a range of symbionts (crustaceans, polychaete worms, flatworms) which live on the surface of the sea stars. Here they usually eat substances that settle on the skin of the sea star. This is beneficial for the sea star, which needs to keep its body surfaces clear of substances so that it can carry out the normal activities of gas exchange and excretion.

Most starfish are harmless, but the Crown of Thorns has sharp poisonous spines. Crown-of-Thorns outbreaks have recently devastated reef areas world-wide but have apparently been doing so for centuries. Although in the short term outbreaks may be disastrous for tourist operators they may actually be beneficial for the diversity of coral species. Fast growing corals like staghorn and table corals (Acropora spp.) can dominate huge areas of reefs, out competing the slower growing massive corals like Porites spp.. Crown-of-Thorns sea stars prefer to eat staghorn corals and this preference possibly allows the slower growing corals to be more competitive in the long term, leading to greater biodiversity. Another sea star which has been in the news is the northern Pacific sea star. This species has been introduced into Australian waters in the ballast water of ships from the northern hemisphere. At present it is restricted to the south of Tasmania. Because it is a voracious predator of mussels, scallops and clams it could prove bad news for the fishing and mariculture industries. Some species of sea stars may be in danger from over-harvesting for the marine curio trade.