Scientists trying to work out where a starfish’s head is have come to a startling conclusion: it is effectively the whole animal. As well as solving this longstanding mystery, the finding will help us understand how evolution generates the dramatic diversity of animal forms on Earth.
Starfish, also known as sea stars, belong to a group of animals called echinoderms, which includes sea urchins and sea cucumbers. Their strange body plans have long puzzled biologists. Most animals, including humans, have a distinct head end and tail end, with a line of symmetry running down the middle of their body dividing it into two mirror-image halves. Animals with this two-sided symmetry are called bilaterians.
Echinoderms, on the other hand, have five lines of symmetry radiating from a central point and no physically obvious head or tail. Yet they are closely related to animals like us and evolved from a bilaterian ancestor. Even their larvae are bilaterally symmetrical, later radically re-organising their bodies as they metamorphose into adults.
These profound differences make it hard for scientists to find and compare equivalent body parts in bilaterians to work out how echinoderms evolved. “The morphology cannot tell you anything, almost,” says Laurent Formery at Stanford University in California. “It is just too weird.”
Formery and his colleagues decided to look at a set of genes known to direct the head-to-tail organisation of all bilaterians. In these animals, these genes are turned on, or expressed, in stripes in the outer layer of the developing embryo. The genes that are expressed in each stripe define which point on the head-to-tail axis it will become.
The idea was to see if the gene expression patterns would reveal a “molecular anatomy” hidden in echinoderms. “This particular suite of genes is just good for exploring animal diversity in its most extreme forms,” says team leader Chris Lowe, also at Stanford University. “I think echinoderms represent a really extreme experiment in how to use that bilateral network to produce a very, very different body plan.”
To the team’s surprise, the genes that determine the head end in bilaterians were expressed in a line running down the middle of each arm on the underside of the starfish. The next head-most genes were expressed on either side of this line, and so on.
Even more strangely, the genes normally expressed in the trunk of bilaterians were missing in the outer layer of the animal. This suggests that starfish have jettisoned their trunk regions and freed up the outer layer to evolve in new directions, says Formery.
The findings show that “the body of an echinoderm, at least in terms of the external body surface, is essentially a head walking about the seafloor on its lips”, says Thurston Lacalli at the University of Victoria in Canada, who wasn’t involved in the study. Animals like us may have kept their trunks to escape predation by swimming away. “Echinoderms hunkered down and armoured themselves, so they didn’t need a trunk,” says Lacalli.
The idea that echinoderms are “head-like” animals is “interesting and powerful”, says Andreas Heyland at the University of Guelph in Ontario, Canada. It raises some very important and fundamental questions about how ecological factors shape the evolution of anatomy, he says. “Finding underlying conserved patterns really provides critical insights into how development evolves.”