The spooky science of shark mummies
John O’Sullivan, the Aquarium’s Director of Collections, was in Mexico on a mission. A young white shark equipped with an electronic tag had traveled over 650 nautical miles south from its release point in Monterey Bay, and the tag had popped off somewhere along the central coast of Baja California. The tag contained a complete data set documenting the shark’s movements and physiology since its release, and John aimed to recover it.
Instead his guide, a local fisherman, led John to a shark graveyard.
A grisly grimace
Sometimes, commercial and sport fishermen accidentally ensnare juvenile white sharks off the coasts of California and Mexico. But locals in some communities consider it bad luck to discard the unmarketable parts, such as the heads, back into the ocean. Instead, they deposit these shark parts at dump sites in the Mexican desert.
In central Baja, just north of Guerrero Negro, John and a team of local Mexicans encountered hundreds of shark heads, in various stages of decay. Some were fresh; others were rotting. Some had skin that was dry and well-preserved—in other words, mummified—in this arid location.
Many of us would turn away from that gruesome sight. But John and his colleagues looked into the mouths of the shark-head mummies and saw an opportunity.
White sharks—unlike makos, blue sharks, and other Eastern Pacific relatives—have serrations on both their top and bottom teeth. White shark jaws are also wider and broader than other sharks’ jaws. And, in comparison with the state of the sharks’ mummified flesh, the teeth in the shark graveyard were not noticeably damaged, and stood out starkly against the leathered hides.
“White sharks have very unique dentition,” John says, “so I could identify their heads without DNA.”
Laid out flat on a blanket to be catalogued, the razor-sharp rows of white shark teeth seemed to grimace back at the researchers.
John never found that satellite tag he was looking for on the rugged Baja coastline back in the early 2000s. But he did collect about 35 white shark heads—and he already had plans for those specimens.
On the Pacific coast of northern Baja, the phone rang inside the home of Oscar Sosa-Nishizaki, a research colleague of John’s who works at the Center for Scientific Research and Higher Education at Ensenada (CICESE).
“John said ‘Hey Oscar, I have a present for you,’” Oscar recalls. “I went and picked it up at a nearby gas station. It was an ice chest full of stinky shark heads.”
To protect a predator
The Aquarium’s researchers, together with colleagues from Stanford University, Aquatic Research Consultants, and California State University-Long Beach, have been tracking young white sharks—using a combination of electronic tracking tags and fin identification—since 2002. They’ve discovered that sharks between 1 and 5 years old travel up and down the Eastern Pacific coast, exploring as far north as Alaska and as far south as Baja California Sur, including the Gulf of California in Mexico.
The data they’ve collected are critical to white shark conservation. Under the International Union for Conservation of Nature’s Red List, white sharks are recognized as a species “vulnerable” to exploitation. And since 2004, white sharks have been included on Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).
The CITES listing for white sharks helps regulate the trade of white shark body parts, including fins, jaws and teeth. It recognizes that while white sharks are not near extinction, increased trade could threaten their populations. White sharks are not listed as an endangered or threatened species in the U.S. under the federal Endangered Species Act, but off the coast of California, white sharks have been protected since 1994. This means that under authority of California Fish and Wildlife, white sharks may not be purposely caught or killed.
The Aquarium and our colleagues in Mexico are part of an international collaboration to protect these threatened animals. To protect them effectively, we need to understand where and how far they travel.
But there are some white shark mysteries that even tracking data can’t crack. John and Oscar teamed up with two more researchers—Píndaro Díaz-Jaimes of the University of Mexico, Mexico City; and Carol Reeb, a fisheries geneticist, marine biologist and research associate at Stanford’s Hopkins Marine Station—to take a closer look at the genes in those mummified white shark heads.
On the DNA trail
Tagging studies have made it clear: White sharks are international travelers that need protection across borders. But researchers were dogged by a nagging question: Are the white sharks off the coasts of California and Mexico one sub-population, or two?
Experts have historically assumed all the white sharks of the Eastern Pacific are close relatives. Because researchers don’t know where white sharks give birth and where their mothers come from, it’s possible that sharks found in Mexico could be genetically distinct from those found in California.
Adult white sharks from Mexico and central California travel to the same offshore locations each spring. But a 2010 paper, published by Carol Reeb and Sal Jorgensen, the Aquarium’s senior white shark researcher, demonstrated that sharks off Central California tend to repeatedly come back to their original neighborhoods. Similarly, sharks tagged in Mexico tend to return to Mexico. Were they two different sub-populations? The answer could be important for white shark conservation.
“If you’re trying to protect white sharks, and your whole view is that they’re all from a single population, your management strategy might go down one path,” John explains. “If it turns out that there are sub-populations with different breeding males and females, within the borders of two different countries, you might need to manage their overall protection in a different way.”
The mummified shark heads offered a lead. Returning to the dump site, Oscar gathered almost 50 more putrid, decaying heads. Then he sent samples of tissue and teeth to Píndaro’s team in Mexico City. The tissue was too rotten, and contaminated by other shark species in the dump sites, to be useful. Not so the teeth. After Píndaro dissolved away the enamel, he was able to access the clean, soft dentine inside—and from there, he extracted the DNA.
Researchers compared genetic codes from the mummified heads with DNA from central Californian adult sharks collected by Sal’s team. They found that the samples were genetically indistinguishable, suggesting sharks in California and Mexico represent a single population.
The shark-mummy study is a breakthrough on several fronts. First, the ability to extract DNA from mummified shark heads allows researchers to track genetic diversity in shark populations. If the diversity declines over time, that may indicate overfishing or other threats.
Second, they can now apply the same DNA extraction technique to other kinds of preserved samples.
“When you get a fresh sample, you’re looking at an animal that’s alive now,” Sal says. “But when you sample a mummy, you’re looking at genetic material from the past. This opens the door to sample things like museum collections. It allows us to sample across time.”
White sharks, it turns out, have quite the afterlife.
Learn more about the Aquarium’s white shark research.
Featured image: Commercial fisherman and guide Roberto Ocaña displays a mummified white shark head.
All photos by Monterey Bay Aquarium/John O’Sullivan except tagging photo by Steve McNicholas, Yes/No Productions