Starfish (Asteroidea)



Starfish are  marine invertebrates. They typically have a central disc and five arms, though some species have more than this. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have  tube feet  operated by a  hydraulic system  and a mouth at the centre of the oral or lower surface. They are  opportunisticfeeders and are mostly  predators  on  benthic  invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and  suspension feeding. They have complex  life cycles  and can reproduce both sexually and  asexually. Most can  regenerate  damaged parts or lost arms and they can shed arms as a means of defence. The Asteroidea occupy several significant  ecological  roles. Starfish, such as the  ochre sea star  (Pisaster ochraceus) and the  reef sea star  (Stichaster australis), have become widely known as examples of the  keystone speciesconcept in ecology. The tropical  crown-of-thorns starfish  (Acanthaster planci) is a voracious predator of  coral  throughout the Indo-Pacific region, and the  northern Pacific sea star  is considered to be one of the  world’s 100 worst invasive species.

The  fossil  record for starfish is ancient, dating back to the  Ordovician  around 450 million years ago, but it is rather poor, as starfish tend to disintegrate after death. Only the  ossicles  and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture. They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.

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