Whole-body light-sheet maps turn up hundreds of previously unknown hair cells on squid

Bundle length is the detail that ties squid to the human ear. In the human cochlea, thousands of hair cells carry stereocilia that are longer where low pitches are detected and shorter where high pitches are, so a cell's position sets the frequency it responds to ^[8]. Hair bundles in fish lateral lines are all one length ^[7]. Squid bundles that vary ^[6] would give researchers length-based tuning on the outside of an animal, a feature the fish system lacks. The squid seem to need it. Their survival appears to depend on detecting water movement across a range of frequencies ^[9].

The whole-body design matters for the count. A survey of the head and arms could only find cells on the head and arms ^[2]. Mapping the entire animal, lateral lines included, is what turned up the rest ^[5].

How many the rest amounts to is harder to pin down. PopSci's account does not name the species or report how many animals were imaged, the exact number of new cells, or any recording of how the cells respond to moving water ^[10]. "Hundreds more" is a difference without a baseline. It cannot tell a reader whether the known total doubled or rose by a modest fraction. The length tuning, too, is something the research suggests ^[6]. Confirming that a longer squid bundle responds to lower frequencies takes a physiological recording.

Brian McDermott, a biologist who studies hair cells and a co-author of the Current Biology paper ^[3], put the payoff in human terms. "Squid are cephalopods with a diverse population of hair cells on the surface of their bodies, which may yield insights not only into how these fascinating animals detect water movement to survive, but also into how hearing and deafness occur in humans," he said in a statement ^[11]. His argument rests on where the damage happens. "Often, when a child is born deaf or a hearing person loses their hearing, it is the hair bundle that has been damaged," he said ^[12]. "So, studying the squid's hair bundle holds promise for understanding how hearing loss occurs" ^[13].

I think the case for squid as a hearing model is reasonable, on two conditions. The bundles have to be shown to be tuned by length, and they have to fail in ways that resemble human bundles. The thing this doesn't tell you is the second. What has been shown so far is anatomy, in squid ^[5]. Testing the second condition means damaging squid bundles and comparing the result with the kind of hair-bundle injury McDermott described in deaf children ^[12].