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Seastar Life History |
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Seastar Biology The Study Bibliography and Weblinks
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Seastars, previously mistakenly called starfish are not really fish at all – they lack both a vertebral column and fins. In addition to the common characteristics they share with other echinoderms, seastars (members of the class Asteroidea) possess channels on each arm called ambulacral grooves and spiny, pincer-like organs called pedicellariae covering the aboral surface. Each arm of the seastar has a short sensory tentacle at its end that responds to chemicals and vibrations in the water. For this reason seastars often elevate one of their limbs to detect light and movement. Seastars are able to regenerate lost limbs and in some cases a severed arm can even grow into a complete seastar. Seastars are predators and feed upon a variety of intertidal and subtidal organisms, including barnacles, snails, sea urchins, clams and mussels. Seastars that feed upon bivalves such as mussels, oysters and clams consume them by wrapping their arms around the bivalve, gripping with its tube feet and creating a slight opening in their shell with sustained powerful suction. Only a tiny crack in the shell of around 0.25mm is needed for the seastar to insert its stomach through the opening into the shell. The seastar then digests the prey inside the shell, absorbing the soft inner tissues.
Seastars reproduce through external fertilization, eggs and sperm being discharged freely in the water. The swimming, bilaterally symmetric larvae then settle and undergo a sessile stage while metamorphosing into a free-living pentaradially symmetric adult. Female seastars may produce over 2 million eggs in one spawn, but because many marine organisms feed on these planktonic larvae, few survive to adulthood.
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