Seastars (also known as starfish) are a distinctive group of marine invertebrate animals, often encountered intertidally and by divers. They are readily recognized by their stellate (star) shaped profiles with five or more tapering arms radiating from the central body, although some species have a more cushion-like pentagonal shape with arms reduced.

Asteroid gallery
Approximately 30 species of seastar have been recorded from the Sydney region to depths of 30 metres, these are images of some examples from the seastars of the Sydney region project image gallery. Photographed by Adrienne Gatt and Roger Springthorpe, compiled by Stephen Keable Image: Roger Springthorpe, Adrienne Gatt
© Australian Museum

Seastars are also ecologically and commercially significant, as shown by the examples of the impact of the Crown-of-Thorns starfish (Acanthaster planci) on the Great Barrier Reef where it destroys coral, and the introduction of the Northern Pacific Seastar (Asterias amurensis) to parts of Australia where it preys upon native species.

Mass spawning of crown-of-thorns starfish
Mass spawning of crown-of-thorns starfish at Lizard Island Image: Lyle Vail and Anne Hoggett
© Australian Museum

They are classified with sea cucumbers, sea urchins, feather stars, brittle stars, basket stars and sea daisies in the phylum Echinodermata (from the Greek words for spiny skin). Characteristics these animals share include their basic body symmetry, an internal calcareous skeleton and a water vascular system composed of fluid filled canals that are often evident as external tube feet which are used for locomotion. However, body shape and other features separate seastars into the class Asteroidea. Brittle stars and basket stars (class Ophiuroidea) are similar but typically have a circular central body from which the arms are more strongly demarcated.

Up-to-date information on seastars occurring in the Sydney area, the most populated and one of the most highly visited parts of Australia, is scattered. Therefore, enquiries which are often received at the Australian Museum are sometimes difficult to answer easily and invasive starfish species may be hard to recognise. 

To provide tools to aid in the identification of seastar species likely to be encountered in depths to 30 metres in the Sydney region a project has been underway over the last few years, recently culminating in a series of web pages. The Australian Museum Marine Invertebrate Collection registration database was used to generate a list of species, specimens from the collection were also used to provide images and these were supplemented with field collections to obtain photographs of living animals.These web pages are still under development but there are plans to develop a more comprehensive guide with identification keys, images of live specimens of all species and other information.This will allow both scientists and the general public to better understand and appreciate these brilliant celebrities of the Sydney marine scene.