Amazing Sea Stars

Article | Updated 4 months ago

Nothing scares seastars. A prey too large for their tiny mouth? No problem, they eject their stomach from their body.  A damaged arm? No worries, they regenerate a new limb!

Varying in shape and size, and often brightly coloured, seastars are loved by children and prised by collectors; but do you really know what they are? Let’s take a look at these surprising aquatic animals.

Image of an orange-red sea star which belongs to the species Pentagonaster

Sea stars which belongs to the species Pentagonaster duebeni
Image copyright Clay Bryce, WA Museum 

Image of an orange-yellow sea star photographed near Cassini Island, Kimberley

Crinoid seastar photographed near Cassini Island, Kimberley
Image copyright Clay Bryce, WA Museum 

Bright red sea star which belongs to the species Himerometra robustipinna

Sea star which belongs to the species Himerometra robustipinna
Image copyright Clay Bryce, WA Museum 

Image of an yellowish sea star which belongs to the species Iconaster longimanus

Sea star which belongs to the species Iconaster longimanus
Image copyright Clay Bryce, WA Museum 

Group of sea stars which belong to the species Protoreaster nodulosus

Sea stars which belong to the species Protoreaster nodulosus
Image copyright Clay Bryce, WA Museum 

Image of an underwater scene in Dampier Archipelago

Underwater scene in Dampier Archipelago
Image copyright Clay Bryce, WA Museum 

Image of a spiny sea star which belongs to the species Acanthaster planci

Sea star which belongs to the species Acanthaster planci
Image copyright Clay Bryce, WA Museum 

Sea stars generally take the shape of either a star or pentagon, and typically have five arms although species with six, seven, ten and up to twenty limbs also exist. Despite the old common name of “starfish”, sea stars are not fish but echinoderms, which means “spiny skin”. Echinoderms include animals such as sea urchins, sand dollars and sea cucumbers. About 1,800 species occur in all the world’s oceans. They can be found from the intertidal zone down to the abyssal zone 6,000 meters under the sea and many sea stars inhabit temperate or cold waters, including in frigid polar water under the ice floe, while other species live in tropical areas.

An external digestion 

Different sea star species eat different types of food. Some species are detritivores, eating decomposing organic matter in suspension or deposited on the seabed, but most sea stars are scavengers or carnivores, preying on sponges, sea anemones, corals, snails, crustaceans, worms, sea urchins, bivalves, fish, or even other sea stars.

Detritivorous sea stars eat tiny particles with their mouth and digest them in their stomach, whereas carnivorous seastars have developed a unique feeding mechanism. To capture their prey these species use tiny tube feet located on their underside.  Tube feet usually possess suckers that aid in movement and securing seastars to rocks. Once these sea stars seize their prey with the suckers on their tube feet, they eject their stomach from their body! The stomach is turned inside-out, covers the prey and partially digests the food. This external digestion may take ten hours. Afterwards the food is pulled back into the body and the process of digestion is completed. This unique feeding mechanism enables sea stars to feed on prey larger than their mouths can eat. 

Tube feet on a sea star in the wild. Ord River Survey 1971/72

Tube feet on a sea star in the wild. Terrestrial Zoology, Ord River Survey 1971/72
Image copyright WA Museum 

Image of a dry specimen from WA Museum sea star collection

Look at the underside of this dry specimen found in the Western Australian Museum sea star collection. In the middle of each arm we can see slots within which the tube feet ending with suckers are stored. When sea stars are alive, the tube feet are mostly outside. This native Western Australian specimen belongs to the species Pseudoreaster obtusangulus; family Oreasteridae.
Image copyright Jessica Scholle, WA Museum 

Seastars, formidable bivalve eaters

If you have ever tried to open an oyster for a yummy meal you know how hard it is to pry them open! Sea stars have developed an efficient technique to open bivalves, such as oysters, mussels and clams. The sea star wraps its arms around the seashell and, helped by its suckers, pulls on both parts of the shell. Over time this pressure will force the mollusc to slightly open its valves to get oxygen. When the bivalve opens, even a fraction of a millimetre, the seastar inserts its stomach into the crack to begin digestion of the mollusc. 

Armed for defence

Sea stars are formidable predators with clever adaptations to avoid being eaten. Although they have few enemies, seastars have developed quite novel features for their defence.

Many coastal species, especially in tropical seas, have bright colours that make them very visible to potential predators. These colours announce that they are not edible. In general these bright coloured sea stars may be toxic but during the slow process of evolution, some edible sea stars have adopted the bright colours of poisonous species to deceive their predators.

Shed a limb to save a life

Some species of sea stars have developed an amazing means of defence: they intentionally lose parts of their bodies when they are attacked or threatened by a predator. The loss of a limb occurs at a predetermined level of threat and is achieved by the quick softening of the arm’s tissue in response to nervous signals. The arm is abandoned to the predator and the sea star flees or buries itself as quickly as it can to escape the threat. The loss of the limb has minimal impact on the animal which can still move with its remaining arms and feed, as its central disc is intact. Sea stars are then able to regrow a new limb even if the mechanisms may take from several months to a year.

Image of a dry sea star specimen

Dry sea star specimen with two missing arms. This native Australian specimen belongs to the species Echinaster luzonicus, family Echinasteridae.
Image copyright Jessica Scholle, WA Museum 

Image of the underside of a dry sea star specimen

Underside of a dry sea star specimen on which we can see an arm in regeneration. Species Echinaster luzonicus, family Echinasteridae.
Image copyright Jessica Scholle, WA Museum 

Image of a dry sea star specimen with arm in regeneration

Dry sea star specimen on which we can see an arm in regeneration. Species Echinaster luzonicus, family Echinasteridae.
Image copyright Jessica Scholle, WA Museum 

Imagae of dry sea star specimen

Look at this original-shaped sea star specimen. We can see two different-sized arms and three minute arms in regeneration. Species Mithrodia clavigera, family Mithrodiidae.
Image copyright Jessica Scholle, WA Museum 

Furthermore a few species, such as Asteria rubens, can also regenerate an entirely new seastar from an arm on the condition that a portion of the central disc remains attached to the limb. On the other hand, the species which belong to the genus Linckia are able to regrow a complete new disc and an entire seastar from a single arm. Thus several small arms regrow on the isolated limb, giving the regenerating seastars an appearance of a shooting star or comet and leading to their common name ‘comet’. 

Image of a dry 'comet' sea star

A ‘comet’ which belongs to the species Echinaster luzonicus, family Echinasteridae
Image copyright 
Jessica Scholle, WA Museum 

Imagae of dry sea star specimens

A ‘comet’ (right) compared to its usual shape (left). These dry specimens belong to the species Echinaster luzonicus, family Echinasteridae.
Image copyright 
Jessica Scholle, WA Museum 

Image of the underside of a dry 'comet' sea star

Underside of a comet which belongs to the species Echinaster luzonicus, family Echinasteridae
Image copyright Jessica Scholle, WA Museum 

In some species the body divides to produce two seastars and not only as a defence response. In these cases, seastars divide their body in half, the two halves pulling away from each other. These two parts will then regenerate the portions of their bodies that are missing.

Further Information

Find out more information about sea stars on our fact sheet.