Sea Star; Mosaic
Sea stars exist in the Phylum Echinodermata, a group of marine invertebrates including sea cucumbers, sea urchins, brittle stars and feather stars. Echinoderm translates to ‘spiny skin’, whereby the species possess an exoskeleton of calcareous or limestone plates, commonly referred to in sea stars as ‘ossicles’. Echinoderms have two other common characteristics. Firstly they have radial symmetry, where their body plan is based on multiples of five and secondly a unique water vascular system that operates tube feet which are used in echinoderms for locomotion, respiration and food collection.
Sea stars are flat, and are star-shaped with five arms radiating out from a central disc. Down the length of each arm is a row of tube feet, a pair of reproductive organs, and a branch of digestive tract. Their mouth is located on the underside of the central disc, through which they can evert their stomach to surround and digest a variety of encrusting invertebrates. They have a visible pore on the surface of their body, known as the madreporite, which controls the pumping of water though the body, inflating the animal’s tube feet which enables movement.
The mosaic sea star is a brightly coloured species, varying in colouration from yellow, orange and red, with the individual plates separated by thin white lines. It varies in colouration. The marginal plates, extending around the outside of the body, are larger than those in the middle of the body. This species grows to a maximum size of 15 centimetres across.
The mosaic sea star is found in shallow, sheltered bays and on rocky reefs to 200 metres from the Shark Bay Marine Park, WA around the southern coast of Australia and Tasmania, and up to southern Queensland.
Other common names include: Red sea star
Occurrence at the Busselton Jetty:
Mosaic Sea Stars are rarely seen from the Underwater Observatory windows, although they are a common species underneath the jetty. You may see them whilst snorkeling or diving. They are most often sighted on the piles amongst the invertebrate growth near the seafloor.
Image by: S. Teede