To Correct Some Fishy Anatomy, Researchers at the National Museum of Natural History Get Inside the Head of a Coelacanth

The new work adds to the legacy of Dave Johnson, a long-time museum curator famed for his detail-oriented research on fishes

If any fish possesses the ability to surprise, it would be the coelacanth. Researchers assumed that these fish died out with the dinosaurs millions of years ago. However, in 1938, a trawl off the coast of South Africa caught a coelacanth, revealing that these ancient fish were still very much alive.

In the decades since, researchers have exhaustively studied coelacanths. But while these fish have become evolutionary icons, it turns out that even basic aspects of coelacanth anatomy were in need of a fact check.

Ichthyologists Aléssio Datovo and Dave Johnson, a longtime curator of fishes at the National Museum of Natural History, recently used pickled coelacanth specimens to study the muscles packed inside the fish’s head. Their findings, published today in the journal Science Advances, reveal that the fish lacks 11 jaw muscles researchers previously thought it possessed. The scientists also discovered three undescribed muscles inside the fish’s head.

The findings shed light on the evolution of jawed vertebrates as a whole. “Coelacanths are very important because of where they are placed on the vertebrate tree of life,” said Datovo, a former research fellow at the museum who now works at the Museum of Zoology of the University of São Paulo in Brazil. “You need to make sense of coelacanths to understand how other bony fish evolved.”

The coelacanth’s 65-million-year disappearing act has led many to refer to the fish as a ‘living fossil.’ Actual fossils reveal that these fish have changed little over the eons, conserving a body plan that originated during the Devonian period around 420 million years ago.

Coelacanths are one of the few living fish species that possess fleshy, limb-like lobe fins. Ancient fish had similar fins that eventually gave rise to the legs of early tetrapods. The coelacanth possesses a suite of other unique features, including electroreceptors in its snout and a hinge in its skull that allows the fish to open its mouth extremely wide.

Datovo, who studies the cranial morphology of several groups of fishes, thought that the coelacanth’s odd features would make for an intriguing subject. However, when he began sifting through the scientific literature, he was struck by numerous contradictions. “I’d read one paper talking about a muscle attached to a bone and then I would read another paper that would say there were no muscles attached to that same bone,” Datovo recalled.

Datovo knew that the only way to clear up the confusion would be to dissect a coelacanth himself. But few believed that the coelacanth needed to be re-examined. “The coelacanth is possibly the most studied vertebrate out there, so it was very difficult to convince people that I needed to dissect one,” Datovo said. “Dave was the only one who believed that I could find something new and interesting in a coelacanth.”

Over the course of his year-long fellowship, Datovo worked with Johnson to dissect two African coelacanth (Latimeria chalumnae) specimens preserved in alcohol. They used tweezers and scalpels to carefully separate the muscles surrounding the fish’s skull.

They discovered that the muscles they picked apart differed dramatically from the accepted anatomy of the fish’s head. Several structures that had previously been described as muscles were actually ligaments.

As Datovo and Johnson carefully examined the muscles, bones and cartilage packed inside the coelacanths’ heads, they discovered three previously unknown muscles. Two of the muscles help the fish snap its jaw shut while the third muscle controls the spiracle, a cavity located behind many fish’s eyes that leads to the respiratory system. Because coelacanths do not use their spiracles, this muscle is likely a vestigial relic.

In total, the scientists outlined nine new evolutionary features in the coelacanth that have emerged in several major groups of vertebrates. This spurred Datovo and Johnson to compare the coelacanth’s musculature to fossil reconstructions of several ancient fish lineages.

This confirmed that the head muscles of lobe-finned fishes like coelacanths have changed little over evolutionary time. The cranial muscles of cartilaginous fishes like sharks have undergone slightly more changes. However, the head muscles of ray-finned fishes have experienced a more dramatic overhaul. This giant group, which includes everything from anglerfish to tuna, contains a diverse assemblage of different jaw and head shapes. Datovo and Johnson posit that some of these changes are tied to the development of unique feeding styles, such as suction-feeding.

According to Carole Baldwin, another of the museum’s fish curators, the new findings are emblematic of Johnson’s detail-oriented research. Johnson, who passed away in November 2024, was among the world’s preeminent experts on fish anatomy. He had an exacting eye for minute details and little patience for sloppy mistakes at the microscope.

“We would call them ‘Davisms’ and the first one was ‘always get it right,’” recalled Baldwin, who knew Johnson for nearly 45 years. In 1992, Johnson brought Baldwin to the museum as a postdoctoral researcher just days after she defended her dissertation.

Johnson was a pioneer and global leader in the use of marine fish larvae in evolutionary studies, taking advantage of the fact that this planktonic phase often differs dramatically from the fishes’ adult form. By carefully studying odd larval forms, which often have extensive head spines, oversized fins and translucent skin, Johnson could glean insights into how fish evolved.

According to Baldwin, this work helped Johnson and his colleague Ralf Britz, an ichthyologist at the Natural History Museum in London, solve “one of the greatest mysteries in fish embryology”: how did remoras evolve their unique suction disk? Johnson and Britz studied the various developmental stages of the fish, which use the disc to fasten themselves to sharks, and discovered that this specialized feature was actually a highly modified dorsal fin.

While Johnson’s research was second to none, his colleagues remember him more for his good spirits and supportive mentorship. “Dave was not only the best fish anatomist of his time, but he was also just fun to work with,” Datovo said.

Over the course of his career, Johnson published papers on every major group of fishes. Except the coelacanth. Today’s paper (which coincidentally published at the same time as a memorial service for Johnson at the museum) finally fills this coelacanth-sized hole in Johnson’s career opus.

“This paper is a fitting exit for him,” Baldwin said. “It corrects the record of something as iconic as a coelacanth, which 70 years of scientific literature got wrong.”

Jack Tamisiea | | Read More

Jack Tamisiea is a Science Communications Specialist at the Smithsonian's National Museum of Natural History. In addition to covering all things natural history for the museum's blog, Smithsonian Voices, he tracks media coverage and coordinates filming activities for the museum's Office of Communications and Public Affairs. Jack recently completed his masters in science writing at Johns Hopkins University and his writing has appeared in the New York Times, Scientific American, National Geographic and other science-focused publications. In his free time, he loves exploring the outdoors with a sketchbook and camera. You can read more of Jack's work at https://jacktamisiea.com.