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Why do starfish have 5 legs or arms?
George Carsley from Warwickshire (age 5-14)
How is star fish shaped like a star?
Abid Hossain + Jack Huang + Eddy Liou from Swansea (age 5-14)
Filed under: age 5-14, Animals Big Questions, Answered Big Questions, Biology Big Questions |
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The Big Question
Starfish – or sea stars, to underline the fact that they are not related to fishes – belong to the family Asteroidea (from aster, Greek for star) and with other species like the sea urchins, they form a group called the echinoderms, the “spiky skinned”. Most of these animals show a radial symmetry of their adult body shape while other species, like humans or insects, exhibit a bilateral body symmetry (we have “left” and “right”, “back” and “front”, words that would have no meaning for a starfish).
But when you look at the larvae of echinoderms you realize that these are organized bilaterally as well, so the radial form is a secondary development that only happens in some echinoderms when they become adults (see e.g. http://www.microscopy-uk.org.uk/mag/artjul00/echino4.html and http://www.microscopy-uk.org.uk/mag/art98/janstar.html).
So, most of the starfish develop from their larval stages to adults with five “arms” but there are also species with more or less than five arms and the reason why some have five and others seven or nine is not known but might depend on different environments or feeding/locomotion strategies.
On the other side, the reason that the starfish are radically shaped might come from the fact that they don’t move fast, don’t hunt and don’t have to (or can not) run away from predators, so they simply do not need to have a front or back, a left and right, something that gives them a sense of direction.