How the starfish got its five arms
How did starfish get their five arms? This is a question that puzzles zoologists. Biologist Dr. Marc de Lussanet from the Institute of Psychology at Münster University has now found an explanation. His theory is that the earliest ancestors of starfish had six rays and were thus symmetrical. The sixth ray, however, got lost during an early stage of evolution. His work has been published in the current issue of the journal "Evolution & Development".
One look at the animals' family tree shows that starfish belong to the class of echinoderms. Just like vertebrates, echinoderms belong to the category of deuterostomes, which basically exhibit bilateral symmetry. However, this symmetry is missing in the case of adult echinoderms, which include not only starfish but also brittlestars, sea urchins and sea lilies. The larvae of echinoderms are very different from the adult animals, though, displaying the bilateral symmetry which is actually typical of this group of animals.
"Over the past few decades it has become clear to scientists that the five-ray body form found in adult echinoderms must have had its origins at a very early stage of their evolution," as Marc de Lussanet explains. "Those species of starfish with more than five arms are also descended from five-armed ancestors." There are a number of possible explanations for how the body form of adult starfish came about. What they all have in common is the assumption that the body axis in adult animals is very dissimilar to the bilateral symmetrical axis of the larvae. "Up to now the assumption has been that the adult animal grows out of the left side of the young animal, with the larva being dissolved," says de Lussanet.
However, in his work the Münster biologist demonstrates that the larval body form is not dissolved, but rather goes through a series of complicated turns. "In this process the belly first moves away from the right side of the body," he explains. In many of the extinct forms the right side formed a shaft with which it could fasten itself to the subsurface – much like sea lilies in existence today, and like young starfish which anchor themselves to the sea-bed during a certain phase of their development. "During this re-structuring, one ray of the six-ray structure of the young animal gets lost. Five rays are preserved. They group themselves around the mouth which, in the case of the adult starfish, is located in the middle of the belly side," de Lussanet explains. As he points out, "This work shows not only why starfish have five arms. It also explains that the widely varying forms of different echinoderms are based on one common design – and that they do actually have a bilateral symmetrical body axis."