evolution: starfish are all head, no trunk (Introduction)

by David Turell , Wednesday, November 01, 2023, 20:25 (178 days ago) @ David Turell

Most of the rest of us are bilaterians:

https://www.newscientist.com/article/2400256-starfish-dont-have-a-body-theyre-just-a-bi...

"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.

***

"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.'”

Comment: perhaps the way they fit into their ecosystem dictates their shape.