In the vast expanse of the Atlantic Ocean, finding the precise area in the deep sea where young sharks are born and grow – a nursery, it’s called – is like thrusting a shovel into a field the size of Co Cork and hitting a bar of gold. It’s an extremely rare and almost unimaginable find.
In 2018, however, the fortunes of a small team aboard the Marine Institute’s Celtic Explorer – Ireland’s national marine research vessel – took a turn for the better when the marine biologists unexpectedly discovered thousands of baby sharks swimming in the deep, dark Atlantic waters. According to Dr David O’Sullivan, one of the scientists present, it was “incredible – real David Attenborough stuff”.
If you board a boat and set off from the west of Ireland – say, from Louisburgh in Mayo – the seabed 100m below remains flat for about 90km. Then, suddenly, a large cliff drops down nearly 4km. This is where the cold, nutrient-rich waters of the Atlantic meet the warmer, shallower waters, creating ideal conditions for marine species to gather. It’s like an oceanic city – an underwater New York – teeming with life.
It was in this part of the ocean in 2018 that O’Sullivan and his colleagues directed their focus. Floating along on the water’s surface in the safety of the Celtic Explorer, they deployed the Holland 1, a small submarine named after Liscannor engineer John Philip Holland, who designed the first modern submarines for the US navy.
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This remotely operated vehicle, unmanned but tethered to the ship by a cord, can be lowered 3,000m to the sea floor. Equipped with a range of sensors and lights, it transmits live images of the seabed, capturing five metres on either side: a mere sliver of what’s down there, but this snapshot of deep-sea life is vital for scientists to understand where species live and breed.
As the Holland 1 motored along the sea floor, the scientists on the Celtic Explorer recorded everything they saw – from fish life to deepwater coral reefs. What they didn’t expect to find – partly because it had never been observed before – were thousands of young blackmouth catsharks, a small deepwater species. As O’Sullivan recalls, it was a “Jesus, where did these come from?” moment as more and more sharks appeared, slowly circling each other. Beneath them, the seabed was covered in thousands of egg cases, known as “mermaid’s purses”. Dark brown and black, about 5cm long, with curly tendrils at each corner to anchor them to the cold water corals – preventing them from drifting away – each case contained a single shark embryo.
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For more than an hour the Holland 1 drifted among the sharks, its bright light illuminating the scene for the scientists 800m above. They could hardly believe their eyes – no one had previously known where this species of shark bred. They also spotted sea urchins and starfish, attracted, no doubt, to the mermaid’s purses, which are tempting mouthfuls for potential predators. By the time the scientists returned to land, having sent some of the footage to international shark experts, they knew they had witnessed a “rare find”.
Citizens can do their bit to help scientists better understand where sharks and skates might lay their eggs by embarking on an ‘egg hunt’ for mermaid’s purses
As the Irish Government moves forward with plans for offshore wind farms along the Atlantic coast – the aim is 5,000 megawatts of offshore wind energy by 2030, while also committing to conserve 30 per cent of Irish waters and seafloor by the same year – understanding where sharks and skates replenish their populations becomes essential if we are to have any chance of effectively protecting marine life.
Some species, such as the diamond-shaped flapper skate, were once common in Irish waters but are now critically endangered, with only a few remaining hotspots. They are the largest saltwater skates in the world, growing nearly three metres long and inhabiting deep waters. The females live up to 16 years before they begin to reproduce; she will then lay about 40 eggs, which take up to 18 months to develop. Scientists still don’t know where these eggs are laid; if they did, they could confidently prioritise these specific areas for conservation.
Advancements in AI will undoubtedly enhance the precision of ocean exploration but, as it stands, vessels such as the Celtic Explorer can’t possibly cover every inch of the seabed in search of nurseries. So citizens can do their bit to help scientists better understand where sharks and skates might lay their eggs by embarking on an “egg hunt” for mermaid’s purses.
These cases resemble leathery capsules with Batman-like horns at both ends, looking like something you might expect to see in the Star Wars universe. Each mermaid’s purse is unique to its species; for example, the flapper skate’s egg case is large, pouch-like, dark grey, and has jagged edges, while the black mouth catsharks’ egg case is smaller, more tapered at the ends, and has a smoother surface.
After several months of development in the mermaid’s purses, the embryos transform into young fish. They exit the egg case by flapping their tails to circulate oxygen so it opens and the young shark or skate is freed. At this point, the empty shell-like cases float in the ocean and may eventually wash up on beaches, tangled up in lengths of seaweed and kelp.
In collaboration with the Irish NGO Marine Dimensions, scientists at University College Cork are leading the “Purse Search Ireland” initiative and encourage everyone to log any sightings.
