We found this starfish with four arms about 100 kilometres south of Broome on the north-west coast of Australia (see photo). Is it unusual? How did it evolve? How many or how few arms can starfish have?
• The answer is simple: it has lost one arm. The wound has healed and a new arm will slowly grow to replace the lost one. Starfish belong to the phylum Echinodermata, whose members have amazing regenerative powers.
Much more fascinating, though, is the question of why starfish have five arms in the first place. There are five classes of echinoderms: starfish, brittlestars, sea urchins, sea cucumbers and crinoids, all of which have fivefold symmetry. Some starfish have evolved more than five arms, notably the crown-of-thorns starfish which has about 15, but this is a secondary development.
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Echinoderms develop from a ball of cells into a larva, which typically has twofold, or bilateral, symmetry. Over many millions of years this line of evolution gave rise to the ancestors of the vertebrates and ultimately to groups that include dinosaurs and humans.
The extraordinary thing is that echinoderms have normal bilateral larvae, or bipinnaria, and then suddenly – almost like a parasite – a five-armed creature develops within, not from, the larva.
“Starfish have normal bilateral larvae and suddenly a five-armed creature develops within”
It is far from clear how an animal’s cells could divide to produce this primitive condition of five axes in the body. Nearly all other animals have a head end and a segmental body where basic body parts such as muscle blocks and nerve ganglia are repeated. Over time, the nerve ganglia become at least partly concentrated in the front, and various levels of sophisticated brains evolved to coordinate the whole animal’s behaviour.
With fivefold symmetry it wasn’t possible to develop a brain, so echinoderms have segmental arms. Each segment has a collection of nerve cells forming a nerve cord in each arm. These cords are connected via a nerve ring around the mouth.
If you confront a starfish with a problem, such as turning it upside down, one arm will take the lead in trying to turn the animal over. If this does not work then dominance is transferred to another arm, which takes over the task, and this will continue until one arm succeeds.
I suppose an advantage of not having a brain but instead a chain of equal ganglia connected around the mouth is that the animal can suffer serious damage from predators or environmental forces and still completely rebuild itself.
These facts pose a question that I brooded over throughout the years I worked on echinoderms – where did the five-armed growth inside the normal larvae come from? These animals evolved during the great Cambrian explosion of multicellular animal life around 530 million years ago, and we now know from the wonderful fossils of the Burgess Shale in Canada that all sorts of weird and wonderful evolutionary experiments took place.
What follows is probably scientific heresy; it is certainly my personal speculation. I believe another quite different organism in some way hijacked a bilateral bipinnaria larva and grew to independence as a unique five-rayed animal. Such animals evolved very armoured bodies and have been highly successful ever since, leaving an excellent fossil record.
It is difficult to think up a good reason as to why pentamery has been so successful, although it seems to be an evolutionary straitjacket because basic echinoderms have not evolved significantly for hundreds of millions of years.
James Cobb, Retired echinoderm researcher, University of St Andrews, Kingsbarns, Fife, UK
Such “heresy” has occurred to other researchers too. It was thesubject of a recent 91av feature on metamorphosis (24 September 2011, p 56) – Ed
• I suspect that this four-armed individual is a five-armed specimen that has been in an accident and hasn’t yet regenerated its missing part. This can take up to a year.
One problem for humans that arises from this ability to regenerate was seen after an early attempt at controlling the devastating plague of crown-of-thorns starfish on Australia’s Great Barrier Reef. The attempt consisted of catching the starfish, chopping them up, and dropping them back in the sea. They happily regrew.
Barbara May, Winston Hills, New South Wales, Australia