Earlier this year, James Gelsleichter got a call from the Bahamas. A group of researchers had been tracking the movements of some oceanic whitetip sharks off the coast of Cat Island, a long, thin stretch of land in the center of the Bahamas. Maybe, they thought, the sharks were moving certain directions because they were pregnant and looking for a place to give birth. Gelsleichter was the right guy to find out–a shark biology professor at the University of North Florida, he’s been studying the reproductive lives of sharks for over 20 years. He’s seen everything from the barbed, hooked penises of male sharks to the intrauterine cannibalism of your darkest deep-sea nightmares (yes, it’s a thing).

But even though Gelsleichter has dedicated his entire professional life to studying the weird, barbaric sex lives of sharks, he still couldn’t answer the most basic question about the sharks off of Cat Island: Are they pregnant? While a simple hormone blood test can confirm pregnancy in humans and a host of other mammals, no such test exists for sharks.

Researchers like Gelsleichter need to be able to study shark reproduction in order to protect their dwindling populations from overfishing and climate change. “Sharks are ancient, and they have the most diverse reproductive strategies under the sun,” says Marah Hardt, a biologist who specializes in marine reproduction. “Some sharks have the ability to store sperm or reject it, so we don’t really know when they’re mating or when they’re fertilizing their eggs.” Answering those questions would help scientists regulate shark fishing to ensure the animals have time to reproduce. That’s why Gelsleichter is trying to make a shark pregnancy test himself–completely from scratch.

Like Virtual Reality, But for Shark Uteri

Back in the day, Gelsleichter and other shark biologists studied reproduction by dissecting dead animals. But now that many of the sharks he studies are seriously threatened, he mostly uses ultrasound. When Gelsleichter and his students went down to the Bahamas to figure out if the whitetips were pregnant, they took a portable, waterproof ultrasound machine called the Ibex Pro. Wearing a set of VR-style goggles, they held a shark against the side of a boat with a large sling, flipped her over onto her back, and ran an ultrasound wand along her belly to look for signs of a growing fetus.

That method, as you might guess, is a bit cumbersome–and it leaves lots of room for mistakes. Dean Grubbs, a researcher from Florida State University who also studies shark reproduction, remembers trying to ultrasound a bluntnose sixgill shark for the first time. At about 17 feet long, they’re one of the biggest sharks in the sea–apex predators with stomach muscles so thick that most ultrasound wands can’t penetrate them. When Grubbs and his team managed to catch one of the beasts and saddle it alongside the boat, he crawled on top of her belly and started moving the ultrasound wand along her gigantic belly. “I just couldn’t see a thing,” says Grubbs.

Even when you’re studying smaller sharks that are easier to catch, “the conditions under which you can actually successfully ultrasound and get meaningful data really have to be perfect,” says Gelsleichter. In a place like Florida, perfect weather comes and goes within minutes. And even if the weather cooperates, accurately interpreting an ultrasound takes a lot of experience–even Gelsleichter gets stumped sometimes. Researchers need a better way to collect data on rare and important species to monitor how quickly they’re reproducing. “Having a pregnancy test that would allow us to assess those types of species would be really useful,” says Grubbs.

That’s why, every time Gelsleichter and his students have caught a shark for an ultrasound, they’ve also drawn a bit of blood to bring back to the lab. Most pregnancy tests are based on the idea that a fertilized egg produces a surge of hormones: Humans, dogs, cats, and a bunch of other mammals all have different pregnancy hormones. Gelsleichter has amassed blood samples from over 50 different shark species, both male and female and–most importantly–pregnant and not pregnant. Out of all that blood, he knew there must be a differentiating chemical marker somewhere in the pregnant shark blood samples.

To find it, he and his students built a comprehensive list of hormones produced by the placentas of pregnant animals, from humans to pandas. Many mammals share pregnancy markers, so they tested the shark blood samples for each of the known hormones. “We were leaving the lab at 2:00 in the morning for weeks,” says Gelsleichter. “And then we got nothing.” If there was any hormone marker at all in pregnant sharks, it was unique. He would have to start all over.

The Stubborn Mysteries of Shark Blood

If Gelsleichter couldn’t find a hormone marker with an intelligent search, he’d have to brute force the process. Now, he just needed to map out the entire protein composition of female shark blood. Gelsleichter sent his samples to a lab that turns proteins into images–one color per protein, with big spots representing high concentrations and smaller spots representing just a pinch. Once he had a map of a pregnant shark’s blood composition, he could hold it up against a non-pregnant shark’s blood map to look for differences. Once you spot a difference between two blood samples, you can identify the protein in the spot and design a test that scans for it.

When I first spoke with Gelsleichter, he’d just sent in his samples and was full of hope that the lab would be able to find something valuable. But when I checked on him a few weeks later, the tone had changed. The lab’s spot maps had illustrated some visible differences between the pregnant and non-pregnant blood samples, but after decades studying shark reproductive hormones, Gelsleichter warned that those visible differences don’t guarantee that he’s found the marker he’s been looking for. Any two samples could be different for a handful of reasons that have nothing to do with pregnancy, Gelsleichter points out–like the time at which the samples were taken, the shark’s age, or its overall health–which means he could be right back to square one after the analysis is done.

Meanwhile, there’s the looming need for a tool that will let him and scientists like Grubbs collect large-scale data about shark pregnancies worldwide, data that Grubb says will let them “accurately predict when and where sharks are giving birth and if they give birth every two years or every year.” The lack of an accurate pregnancy test is a major barrier to preserving the ancient underwater species that underpin the ocean’s delicate ecosystem. If researchers knew that those oceanic whitetips were in fact moving along the coast of Cat Island to find a place to give birth, they could work with fisheries and shrimp trolling companies to make sure they stay away from breeding or birthing grounds during certain times of the year.

Until then, they’ll keep relying on the slow art of ultrasound to collect a limited supply of data. What happens next depends on what he can find in those protein maps. “It’s either a novel protein or it’s a known protein with a unique structure,” says Gelsleichter. “Ultimately, I really don’t care what it is–so long as I can make a test for it.” If he can’t, he says, he’ll go back to the beginning and start over. Again.

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