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Cal Poly Humboldt scans of 10 shark species find inner-ear shape tracks kinship over habitat
Cal Poly Humboldt researchers CT-scanned 10 shark species and found inner-ear shape varied widely, matching relatedness more than diet or habitat. That cuts against a pattern seen in other vertebrates, on a sample too small to fully separate ancestry from lifestyle.
The Scientist · Science desk
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What happened
- Cal Poly Humboldt students Kaci Dodd and Isamar Lopez-Argueta led the work with biology professor Allison Bronson.
- The team had expected diet and habitat to shape the ears, as reported in other vertebrates, and looked for links to prey hearing or life at depth.
- The scans are part of a six-year effort, with 15 undergraduates so far, to document inner ears in every living group of sharks and rays.
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Why it matters
- constraint If ancestry dominates, a shark's ear shape says more about its relatives than its diet or depth, so it becomes a weaker clue for inferring how a species lives.
- constraint Ten species from squalomorph sharks alone cannot show whether lineage beats ecology across sharks generally, or by how much.
- capability Digital dissection of museum specimens lets a small undergraduate lab compare rare deep-sea sharks held in collections around the world.
- precedent The lab's survey of every living shark and ray group is set up to test whether the squalomorph result holds across the rest of the tree.
The team expected sharks to follow the pattern reported in other vertebrates, where inner-ear shape tracks ecology [1]. They asked whether the ears might differ with the need to hear particular prey, or with life at great depth [1]. The sample was chosen to test that. It included bioluminescent lantern sharks and gulper sharks from the deep sea, the frilled shark, the flat, ambush-hunting angel shark, and sixgill and sevengill sharks [8]. If lifestyle shaped the ear, sharks that live alike should have ears that look alike.
According to the release, they did not. Sharks with similar diets and habitats had very different ear shapes, and closer relatives had similar ears even when they lived in different environments [2]. The release states the conclusion as a possibility: ear structure may be more strongly shaped by evolutionary history [2].
The thing this doesn't tell you is how strong the pattern is. The release does not report the statistics behind the comparison. Ten species is a small denominator for pulling apart two explanations. Each species has one ancestry and one lifestyle, and with so few data points a single cluster of relatives can set the result. The paper's title also narrows the scope. It describes the skeletal labyrinth in squalomorph sharks only [12]. Whether the same holds in other sharks, or in rays, is outside what this sample can show.
The method is what made the sample possible. CT scanning let the team digitally dissect the heads and examine the inner ears in three dimensions [7]. The specimens came through publicly available specimen data, the Cal Poly Humboldt Fish Collection Database and museums around the world [14]. Tracing the structures meant hours over grayscale images, a process Bronson likens to a digital coloring book for adults [9].
"This work was possible because we could combine CT technology and a global network of natural history collections," Bronson said [10]. On the result itself, she said: "The findings are significant because they give us a new way to understand shark evolution and further explore how their sensory system evolved." [3]
I'd treat the finding as a strong hint within one group of sharks until the larger survey reports. The lab has spent six years documenting the inner-ear anatomy of every living group of sharks and rays, with 15 undergraduates involved so far [11]. Its updated collection of ear models is posted on Sketchfab [13].
What to watch
- Whether the full Anatomical Record paper reports a formal test of lineage against ecology, with effect sizes, beyond the qualitative comparison in the release.
- Results from the lab's survey of other shark groups and rays, which would show whether the kinship pattern extends beyond squalomorph sharks.