After Centuries of Confusion, Unique Bones Help Scientists Place Catfish

Left side view of the Hypophthalmus marinates collected from the Suriname River.

Since at least 1829, a big-eyed, silvery South American catfish with a peculiar skeleton has confounded scientists attempting to fit it into the tree of life. 

Now, a pair of researchers from the Academy of Natural Sciences of Drexel University believes that by using high resolution CATSCAN imaging and keying in on unique structures in the vertebrae and swim bladder of Hypophthalmus, they’ve finally accurately described the unusual fish.

“Hypophthalmus is so very different from other catfishes that it has had a checkerboard-like history of bouncing around from group to group, even outside and apart from all other catfishes at times,” said John Lundberg one of the Academy’s researchers who is an emeritus curator and emeritus professor in Drexel’s College of Arts and Sciences. “It was clear from other studies that major pieces were either missing or way off the mark. I believe we got it right at last.”

Lundberg teamed up with Kyle Luckenbill, an Ichthyology research and curatorial assistant as well as imaging specialist at the Academy, to publish their findings in “The Extraordinary Occipito-Vertebral Skeleton and Swim Bladder of South American Hypophthalmus Catfish (Siluriformes: Pimelodidae): Improved Illustration, Description, and Interpretation.” It was just chosen as the best paper in ichthyology for the latest annual [or 2015] volume of the journal Copeia, published by the American Society of Ichthyologists and Herpetologists. 

Previous anatomical studies of Hypophthalmus misinterpreted or even completely missed distinctive parts of the catfish’s bone structure that would’ve provided clues to its identity and history.

Specifically, scientists mischaracterized the area where the skull of Hypophthalmus joins the spinal column, as well as its exceedingly rare “bi-lobed,” bone-encased swim bladder.

“The swim bladder of fishes is ancestrally an unpaired air or gas-filled sac that functions in adjusting buoyancy,” Lundberg said. “A number of catfishes have left/right bi-lobed swim bladders of reduced size that are partially separate but generally connected via a restricted tube.” 

That’s where Hypophthalmus becomes completely distinct from other catfish and, likely, all fish.

“Hypophthalmus has the ultimate separation into the left and right sacs,” Lundberg said. “They each have a distinctive mineralized — or bony — outer capsule around the thin-walled inner sac.”

Basically, Hypophthalmus has two distinct, small bladders that are almost completely enclosed in bone. It’s a trait that they’ve likely kept through many phases of evolution, though it’s not clear why.

“Small, reduced gas bladders are most commonly found in benthic species [those that reside on river and lake bottoms], especially those in fast, shallow streams. So that can be viewed as adaptive to life where it is best to be on the bottom and out of the current,” Lundberg said. “But Hypophthalmus are not benthic — they swim in the midwater. One might think — correctly — that they should be living on the bottom since they don’t have larger swim bladders. So it seems their bladders don’t contribute to buoyancy control.”

With buoyancy likely out of the picture, what else would the bladders be used for?

“Tiny bladders would not work well for where the fish swim, but there are secondary biological roles,” Lundberg explained. “An important role for these bladders in catfishes and their relatives is ‘hearing’ — the amplification of sound energy impacting a fish.”

When the bladder is inflated with gas in some fish, it’s believed that it picks up sound energy from the fish’s surroundings. Lundberg said the bladders function in the same way that a taut drum transmits sound after being struck. Vibrations from sound energy transfer from the fish’s bladders through a chain of tiny bones. Although they came from different evolutionary sources, these bones are actually pretty similar to what humans have inside their ears to pick up sound. 

All of this helped to show Lundberg and Luckenbill that Hypohthalmus belong in the sorubimine clade of the Pimelodidae family. That classification makes the Hypophthalmus a relative of other long-whiskered South American catfish. But the anatomy of Hypophthalmus also showed that it significantly diverged at some point in its evolution. 

“These unusual features are limited to a small group of strange South American fishes — maybe five or six distinct species in the genus — so we are not talking about a large lineage,” Lundberg reflected. “Across all of life’s diversity, there are thousands of extreme cases of novelties and adaptations for many biological roles: take the panda’s thumb, echolocation in whales and bats, or humans’ large brains. The odd traits of Hypophthalmus are one of those. It was a puzzle and we got it figured out — at least anatomically.”