Starfish may appear to have a plethora of limbs, but it turns out the creatures actually resemble something akin to a disembodied head.
Experts say it has long been a conundrum how starfish, sea urchins and other animals with a fivefold body plan, known as echinoderms, evolved from an ancestor with twofold symmetry – a body plan common today in animals including insects, molluscs and vertebrates.
“How any bit of an echinoderm related to any bit of any other organism in terms of its general body plan was really unclear,” said Dr Jeff Thompson, a co-author on the study at the University of Southampton.
The situation is further complicated by the fact echinoderms begin life as larvae with twofold symmetry, before metamorphosing into their adult forms.
Now researchers say the unusual adult body plan of starfish and other echinoderms does not arise as a result of the central part of the body, or trunk, giving rise to five identical parts.
“It doesn’t look like the trunk is there at all,” said Thompson.
The team say they made their discovery by looking at which genes are switched on in outermost layers of adult Patiria miniata, a type of starfish. They then compared these with genes that are switched on in similar layers in acorn worms – animals with twofold symmetry that are closely related to echinoderms – and vertebrates.
Both acorn worms and vertebrates show clear differences in the genes that are switched on in these layers in the head and trunk.
However, writing in the journal Nature, the team explain that the majority of genes switched on in the outermost layers of the starfish body corresponded only to those activated in the heads of acorn worms and vertebrates.
What’s more, different parts of the starfish “arms” corresponded to different parts of the head, with the frontmost part located near the centre of the “arms”, and the rearmost part located nearer their edges.
“The arms of a starfish are not like our own arms, but more like extensions of the head,” said Thompson. “To summarise starfish anatomy, I would say it’s a mostly head-like animal with five projections, with a mouth that faces towards the ground and an anus on the opposite side that faces upwards.”
Thompson added that with the trunk genes lost from the outermost layers at some point in the evolution of echinoderms, research is now exploring whether animals with an intermediate body form can be found in the fossil record.
Prof Chris Lowe, of Stanford University, who led the work, said it shed light on how adult echinoderms ended up with their unusual five-fold forms.
“We showed that under all the weird organisation of the echinoderm adult body plan, we can still see the molecular anatomy shared with other bilateral animals and it gives us a unique glimpse into how this unusual animal evolved from its bilateral ancestors,” he said.
Writing an accompanying opinion piece, Thurston Lacalli from the University of Victoria in British Columbia, said the study showed how the five-fold body plan of creatures such as starfish was acquired, adding that in simple terms one could think of the body of a starfish as “a disembodied head walking about the sea floor on its lips — the lips having sprouted a fringe of tube feet, co-opted from their original function of sorting food particles, to do the walking.”
While Lacalli noted that the current work focuses on starfish, he said there was no reason to suppose the findings would not hold for other echinoderms, adding the results reveal a truly a radical transformation of the ancestral bilaterian body plan.
“Knowing how it was done means that we now have a much firmer foundation for interpreting early echinoderm fossils, and a better understanding of how the regions of our own brain compare with their echinoderm counterparts,” he wrote.
