Bull sharks are coastal creatures, but at least two of the animals were able to make it as far inland as St. Louis by swimming up the Mississippi River, according to a team of researchers who looked at the shark’s fossil record and reported sightings over the years.
The research duo—Ryan Shell, a paleontologist at the Cincinnati Museum Center and Nicholas Gardner, a librarian at WVU Potomac State College with degrees in ecology and evolutionary biology—scrutinized hundreds of reports of sharks in the Mississippi River and compared those historical records with archaeological and paleontological evidence for bull sharks moving in those waterways in the distant past. Their results were published in the journal Marine & Fishery Sciences.
“I believe Ryan mentioned something like ‘hey, bull sharks can go up the Mississippi,’ and my first thought was, ‘bull****,’” Gardner wrote in an email.
The sharks are known to make forays into freshwater beyond the African, Asian, Australian, and American coasts they inhabit, and the fossil record holds evidence of bull sharks in the river, but not in its upper basin. Some teeth are found inland, but not in natural fossil beds and in association with teeth from other species, suggesting they may have been traded inland.
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No sightings are documented before the turn of the 20th century, but Shell and Gardner found two confirmed bull shark catches in the historical record: One in 1937, in Alton, Illinois, and another just outside of St. Louis in 1995. The 1937 shark was an 84-pounder and about 5 feet long, according to contemporary reporting by the Alton Evening Telegraph, which said the fish got stuck in a fisherman’s seine. The 1995 specimen was retrieved from the screen of a power plant intake canal, according to a 2004 report on the waterway’s ecology; the same report ascribed both the 1937 and 1995 events to waif, or accidental, one-off wandering fish.
Besides those, the sighting record is littered with incomplete or erroneous reports, which often were parroted from news source to news source. These piggybacked sightings made up the majority of the reports Shell and Gardner examined.
“Anybody with computer access can make an authoritative looking website, and if someone doesn’t do the due diligence they need to do, boom, misinformation slips in,” Gardner said. “We dealt with the biology side as best we can, now I’m curious about what drives hoaxes, misidentifications, and more.”
The lack of bull shark fossil evidence in freshwater deposits could mean that the sharks’ venturing into freshwater is a relatively recent development in their behavior. More likely, the researchers believe, occasional ventures into the freshwater have been a habit of the creature for millions of years, but it’s just not been seen in the fossil record. Their third theory is that venturing into the Mississippi is so uncommon for the fish that the two caught specimens can be viewed as anomalous events.
Whatever the ancestral case, Gardner is hopeful that environmental DNA (called eDNA for short) can help scientists figure out just how common the behavior is. eDNA allows biologists to sample water, soil, and even air to learn what organisms live in that environment, based on the microscopic amounts of genetic information that fall off animals as they move through their habitat.