For centuries, naturalists have puzzled over what might constitute the head of starfish. When looking at a worm, or a fish, it’s clear which end is the head and which is the tail. But with their five identical arms, it’s been anybody’s guess how to determine the front end of the organism from the back. This unusual body plan has led many to conclude that starfish perhaps don’t have a head at all. But now, biologists from Stanford University and elsewhere have found that the truth is closer to the absolute reverse. While they detected gene signatures associated with head development just about everywhere in juvenile starfish, expression of genes that code for an animal’s torso and tail sections were largely missing.
Starfish (sea stars) belong to a group of animals called echinoderms, which also includes sea urchins and sand dollars.
Echinoderms have a unique ‘fivefold symmetric’ body plan, which means that their body parts are arranged in five equal sections.
This is very different from their bilateral ancestors, which have a left- and right-hand side which mirror one another, as in humans and many other animals.
“How the different body parts of the echinoderms relate to those we see in other animal groups has been a mystery to scientists for as long as we’ve been studying them,” said University of Southampton’s Dr. Jeff Thompson.
“In their bilateral relatives, the body is divided into a head, trunk, and tail. But just looking at a starfish, it’s impossible to see how these sections relate to the bodies of bilateral animals.”
“This has been a zoological mystery for centuries,” said Stanford University’s Professor Chris Lowe.
“How can you go from a bilateral body plan to a pentaradial plan, and how can you compare any part of the starfish to our own body plan?”
In the new study, the researchers compared the molecular markers of the sea star Patiria miniata to other deuterostomes — a wider animal group that includes…
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