Off the north-western coast of Australia, near the remote coral atolls of Rowley Shoals, ghost catsharks are slinking through the dim water and searching for bushy colonies of corals growing between 400 and 500 metres (1,300-1,600ft) down.
This is where the elusive sharks lay their egg cases and leave them hanging like Christmas tree ornaments.
A scientific survey 40 years ago collected one of these cases and for years it sat on a museum shelf. More recently, scientists examined it again and realised it was unlike any other shark eggs, with unique, T-shaped ridges running along its 5cm (2in) length.
This finding sparked a shark egg hunt through museum collections, which eventually led to a new species being discovered. This is Apristurus ovicorrugatus – the ghost catshark with corrugated eggs.
A lot of sharks give birth to live young, and some – including more than 40 species of ghost catsharks – lay eggs in which the pups gestate for as long as a year before wriggling out into the sea.
When the unusual Western Australian egg case was first examined in 2011, scientists found a tiny embryo preserved inside and identified it as a ghost catshark.
But they did not know which of a number of species it might be. They ruled out several Australian varieties that lay smooth egg cases, including the Pinocchio and bulldog catsharks, and narrowed it down to either a bighead or freckled catshark.
In 2017, Will White, an ichthyologist with the Australian National Fish Collection, picked up the trail of the enigmatic fish. He and his team searched collections and found more ridged egg cases that had been caught in the same region off north-western Australia, as well as a preserved female catshark.
Inside her body was an unborn egg case. They carefully took it out and saw it was a perfect match for the other ridged cases, proving this was the species they were looking for.
“We were lucky in the end with this one that we managed to find all the pieces of the puzzle,” says White.
The half-metre-long female had previously been identified as a freckled ghost catshark. When White and the other researchers took a closer look they decided it was something new, partly because of the shining white irises in her cat-like, oval eyes.
There are various ideas as to why these ghost catsharks evolved deeply ridged egg cases. “It’s such a unique design and a very strong design,” says White. The ridges are similar in shape to iron girders. “It definitely adds a lot of strength to the walls of those egg cases.”
The ghost catsharks may have evolved this shape to evade predators. Sea snails are known to attack shark egg cases, drilling a hole then slurping out the contents. “There’s no way a snail would be able to stick on to the surface of a ridged egg case,” White says.
As for their habit of laying eggs on tree-like corals, White thinks this probably prevents the eggs from getting buried in organic detritus, known as marine snow, which floats down into the deep sea and smothers the seabed.
But this puts the ghost catsharks at high risk from human activities that disturb seabed ecosystems, such as deep-sea trawling and mining.
“If [trawling and mining are] wiping out all the corals, then there’s a lot of potential risk to the species, even though it might not be getting caught itself,” says White.
