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Starfish Are Walking Heads with No "Bodies"
The ocean is a horror show.
John Carpenter’s The Thing famously features an alien menace capable of mimicking any creature (human or otherwise) that it has ever encountered. The creature uses that ability to hide in plain sight in the form of a dog or a trusted friend, meanwhile making moves to achieve its aims: namely the conquest of our planet.
Scene by scene, the titular Thing transforms itself in new and novel ways, but its brief takeover of Norris (Charles Hallahan) was the height of spine-tingling biological innovation. Norris succumbs to an apparent heart attack and is taken to the station’s threadbare infirmary. The doctor tries to revive him using a defibrillator, but on the second charge, Norris’ chest opens up revealing alien teeth which sever the doctor’s arms. MacReady (Kurt Russell) attacks the transformed Norris with a flamethrower, prompting the head to drop to the floor, sprout spider legs, and make a run for it.
Starfish are Disembodied Heads Patrolling the Seafloor
The image – a disembodied head slowly slinking across the floor – works so well for horror because it breaks the rules of how a body, even a nonhuman one, is supposed to work. When we look at an animal, even an animal we have never seen before, we can usually make heads and tails of it. That’s because most creatures are bilaterally symmetrical, with a dividing line down their middle, separating two sides which are more or less the same. Whether you’re looking at a fish, an insect, or a person, you can usually break them up into right and left halves with equal numbers of limbs, digits, eyes, ears, etc. We can also usually tell which end is the tail and which end is the end, so to speak. No so with starfish and other echinoderms (sand dollars, urchins).
Instead, starfish have pentaradial bodies with five-sided symmetry branching from a central hub. Biologists believe that starfish and other echinoderms evolved from an earlier ancestor which was bilaterally symmetrical like us. Somewhere along the way, they changed so dramatically that the ways in which their five-sided bodies relate to ours have been an enduring mystery. Now, a new study published in the journal Nature may have solved the mystery.
“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. 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.” said Dr. Jeff Thompson, co-author on the study, in a statement.
Because there is no obvious morphological connection between the parts of the starfish body and bilaterally symmetrical animals, researchers relied on gene expression to reveal what’s going on inside. Scientists relied on molecular markers to map how genes are expressed in different parts of the starfish body and compare those to the same markers found in bilateral critters.
They looked in particular at markers for the nervous system and skin, which correlate nicely with body plan orientation in bilateral animals. Using those markers, researchers identified a symmetry along each arm of the starfish body with the midline correlated to the anterior (front) of the body and the fringes correlated with the posterior (back). What they didn’t find were genes correlating to the trunk of the body. Those genes were either missing or not expressed.
“It’s as if the sea star is completely missing a trunk and is best described as just a head crawling along the seafloor,” said Laurent Formery, lead author of the study, in a statement.
It seems as though deep in their evolution, echinoderms ditched the genes needed to build the body’s trunk, instead attaching a few hundred tiny suction-cup feet directly to their necks. Having ditched the body, they were no longer bound by bilateral symmetry, freeing them up to evolve heads with five (sometimes more) identical sides. Not even John Carpenter himself could have dreamed that up.
Catch The Thing, available now from Universal Pictures.