Stephen Dowling/ BBC:

It was a mystery that began with an egg. In 1989, scientists in Australia found a curious kind of “mermaid’s purse” – a leathery egg case, which some species of sharks lay instead of giving birth to live young. The empty egg cases had one almost unique feature – a row of prominent ridges along the top. The eggs had been found off the Rowley Shoals, a group of atolls on the edge of a continental shelf in the East Timor Sea, a few hundred kilometres off the north-eastern coast of Australia. They offered up more questions than they did answers. What had laid them? Where did it live? And why did its egg cases have such a distinctive appearance? It would be more than 30 years before scientists would finally find out the most basic of these questions – and in doing so discover a completely new shark species. More than two decades into the 21st Century, humanity is still finding new species of the ocean’s most impressive hunters. As recently as the mid-1980s, science had settled on around 360 species of shark, ranging from deep sea featherweights like the 20cm (8in)-long dwarf lanternshark to the enormous plankton-feeding whale shark, the largest species of fish in the oceans. But in little more than 40 years this number has jumped by nearly 40%. There are more than 500 known species, and the numbers of new species show no signs of dropping off.
This new wave of discovery rivals that of the golden ages of exploration. It is as much a by-product of painstaking work in the archives of museum collections as it is peering into the deep waters of the world’s oceans. Take the shark that laid the mysterious ridged egg cases, for instance. Will White, the senior curator of Australian National Fish Collection at CSIRO (Commonwealth Scientific and Industrial Research Organisation) in Hobart, Australia, was part of the team that connected the dots. After being discovered during a survey at Rowley Shoals, the egg cases had been disseminated to museum archives without anyone looking much further into the cases’ strange ridges. In 2011, a researcher called Brett Human was volunteering at the Western Australian Museum in Perth, when he came across the ridged shark egg case. While it was similar to eggs laid by another species of shark, that animal had never been found in Australian waters. Human linked the egg case to other eggs which had been found off Australia, and narrowed down the species to possibly being a member of the catshark family. But he could not determine the exact species. “He first described the case and did a very good effort of trying to narrow down… he did a lot better than what a lot of people would have, actually,” says White. “Nothing happened on that until we started looking at a case with a colleague Helen O’Neill. Even then I was like, ‘I think I’m barking up the wrong tree, looking at cases. No one’s really done it, there’s probably a reason why’.” It turned out that CSIRO had also been sent examples of the egg cases in the 1980s – and no-one had done further research. “It wasn’t until I started looking at the collection data that I realised they’re actually from the same surveys. It was collected from the same location on the same day.”
White and his colleagues knew the eggs recovered in the 1980s had come from a certain depth – between 410m (1,345ft) and 504m (1,640ft) – and started looking for sharks which had been caught at the same depth. The CSIRO collection had what was thought to have been a South China catshark, which turned out to have been pregnant when it was caught. The scientists dissected it and found a developing embryo inside an egg case with the same tell-tale ridges as the ones discovered years before in Rowley Shoals. The detective work proved it was a completely new species, and the mystery that Australian shark scientists had been scratching their heads over for the last 30 years was finally solved. “It took us, like, two days to work it out,” says White. “The thing is, you’ve got to be looking specifically for it. The adults of that group, in particular, are very subtly different,” he says. The species now has a scientific name (a common name is due to follow), and was unveiled in the Journal of Fish Biology in April 2023. This new variety of demon catshark, otherwise known as Apristurus ovicorrugatus, is thought to dwell around 700m (2,297ft) beneath the surface, laying its egg cases over coral in water too deep for sunlight to penetrate. White says this family of sharks is particularly hard to research because they are so elusive and because the differences between species can be so subtle. “With this group, in particular, we’re actually down into looking at aspects of the digestive tract and liver shape and stuff like that. You’re definitely getting desperate when you start getting into characters like that,” White says. He says the current experts on this group of sharks all “liaise with each other and talk with each other” because identifying new species can be such a challenge. “The other thing, unfortunately, is you do get some specimens in collections that all you got is the fixed [preserved] specimen, you don’t actually have a fresh photo; that demon catshark we described was lucky because we had a fresh photo as well.” The demon catshark was not the only discovery White had a part in identifying recently. Another species, a type of horn shark, was caught in surprisingly deep waters off Western Australia in a survey last year. Horn sharks tend to like living in shallow waters, often sitting on the seafloor in kelp forests, but this new species was found at 150m (500ft). But that’s nothing compared to what may lie ahead, White says. The demon catshark is not the only new catshark species to have turned up.