A sea creature humans assumed had five limbs could actually be walking on its “disembodied head” with its bum sticking in the air.
Boffins have long scratched their heads over the unusual five-part body layout of starfish and sea urchins. Creatures with that body plan, known as echinoderms, are thought to be the ancestral relatives of creatures with a two-part body plan seen in a number of vertebrae and insects today. They even begin life as two-fold larvae but at some point, the “trunk” part of the body was lost by echinoderms through evolution.
The findings come following a new report from academics at the University of Southampton, who published their research in the academic paperNature. One of its co-authors, Dr Jeff Thompson, has since spoken out on the remarkable new information – and revealed just how little was known about starfish beforehand.
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“How any bit of an echinoderm related to any bit of any other organism in terms of its general body plan was really unclear.” He said. He and the team reckon that given the two-fold start to life echinoderm larvae have, the odd body shape of adults isn’t something that comes to be from the central body. Instead, they think they grow to have five identical parts.
Dr Thompson added: “It doesn’t look like the trunk is there at all”.
The team looked at a particular type of starfish, Patiria miniata, and assessed which genes were switched on its outer layers and then compared these genes to a type of worm, acorn worms, and vertebrates. This is because animals with twofold symmetry are thought to be similarly related to those with echinoderms.
They found that the genes switched on in the outer layers of the starfish body were only similar to those found in the heads of the worms and vertebrae. Parts of what was thought of as the arms of the starfish matched up with parts of the heads of the other creatures.
For example, the front part of the head matched up with the middle part of the “arm” of the starfish. The rear part of the head was found to match more closely with the edges of the starfish “arms”.
Dr Thompson added: “The arms of a starfish are not like our own arms, but more like extensions of the head. 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.”
Thurston Lacalli, of the University of Victoria in British Colombia, wrote an opinion piece to accompany the report, describing the creatures 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.”
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