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All about starfish...
Phylum: Echinodermata
Subphylum: Asterozoa
Class: Asteroidea
Starfish are members of a broad and diverse clan, the Echinoderms, a term which translates as spiny skin. This group includes sea stars, brittle stars, sea cucumbers and sea urchins. In fact, 6,500 species lurk on the branches of this unusual family tree.
Starfish are not listed in the IUCN Red List of Threatened Species but the Crown of Thorns is a major threat to corals. |
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What links the members of this marine group together is their five-point radial symmetry. Their bodies consist of five equal segments, each containing duplicate sets of internal organs, which radiate from a central body mass. This is fairly obvious in a starfish, less so in their relatives. They have no heart, brain or eyes, but some seem to have light sensitive parts on their arms. Mostly their mouth is situated on their underside and their anus on top.
Starfish motion is slower than the proverbial tortoise. They use a complex hydraulic system to move around or cling to rocks. On their base side a mass of tiny tubes run along a distinctive groove in each arm. At the tip of each is a diminutive suction pad - or foot - which is hydraulically controlled by a remarkable system where water is sucked in through a sieve plate on top of their body, then pulsed through interconnecting canals to the toes.
Most incredible is their defence mechanism - the capability to regenerate. Should they lose an arm (or is it a leg) to a predator, then no problem! They just grow another. Or if an arm gets misplaced, it can regenerate itself into a whole new creature.
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THE CROWN OF THORNS STARFISH
The best known predator in the starfish group is the much-maligned Crown of Thorns starfish. This bewitching creature is best known for being able to wipe out whole colonies of hard coral. They feed on coral polyps by sliding their stomach out and suffocating a whole colony beneath, leaving just the bleached, dead skeleton.
This image was taken at Sangalaki Island during a period where the Crown of Thorrns had become highly prolific. They were seen on almost every dive and always in multiples of three or four. At the time, the water temperature was consistenty over 30°C or 86°F.
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