Scientists in Albania have confirmed the first ever case of identical twins in small spotted catsharks.
In February 2024, a commercial trawler operating from Vlorë, Albania, brought up 216 dead small spotted catsharks (Scyliorhinus canicula) as bycatch – meaning the catsharks weren’t the fishery’s target species. 31 of the sharks unintentionally caught were adult females.
Scientists at the Sharklab ADRIA conducted autopsies on the specimens – to further their understanding of the species – and moved any viable developed egg cases from the dead females into incubation tanks. Their aim was to release the pups back into the wild when they hatched.
In one of the egg casings, they saw something strange. “I had to do a double take,” says study author Andrej A. Gajić, general director of the Sharklab ADRIA. He noticed that twins were forming inside one of the eggs.
As someone who examines thousands of shark eggs every year, seeing two embryos sharing a single yolk sac "was completely unexpected and remarkable,” Gajić says. "Even after thousands of dissections per year, I encounter something that truly stops me in my tracks – this was one of those moments.”
Gajić has shared remarkable footage of the two little sharks wriggling within the egg. Their shared yolk sac, which provides them with nutrients, is clearly visible.
This is the first confirmed case of monozygotic (identical) twins in the species.
“Monozygotic twinning occurs when a single egg is fertilised and subsequently divides to form two genetically identical embryos – a rare event across most vertebrate species,” says Gajić.
The development of twins (especially identical ones) is very rare in sharks. Although researchers don’t fully understand what causes this during the development of the embryo, it probably isn’t beneficial, he explains.
Twins have been seen in some other species of elasmobranchs – that is, sharks, skates and rays – such as big skate (Beringraja binoculata) and mottled skate (Beringraja pulchra). But in these instances, each of the three or four embryos within the egg have their own yolk sac.
“That’s very different from our case,” he says. “These catshark twins were sharing a single yolk sac, which makes survival much more difficult.”
Sadly, these little twins didn’t make it. They started developing normally but, in the end, one yolk sac wasn’t enough to support them both.
“The egg case couldn’t provide enough oxygen for both embryos,” Gajić explains. “One twin died first, and as it began to decompose, it likely created toxic conditions – like ammonia buildup – that led to the death of the second embryo.”
Thankfully, there was a happier story for many of the other egg casings they incubated, which developed and hatched into healthy baby sharks.
“Our chief technician, Emina, takes care of them through hatching and ensures they’re in good condition prior to be released back into suitable wild habitats,” he says.
Although this case of twins is incredibly rare, “nature still finds ways to surprise us,” he adds. These new insights into shark reproduction could help researchers find ways to better preserve and protect these animals.
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