An experiment designed to investigate lobster predation on sea urchins unexpectedly found that Port Jackson and crested horn sharks ate the spiky animals instead.
The research, led by University of Newcastle marine ecologist Jeremy Day, involved tethering sea urchins at the entrance to a lobster den – home to at least 20 large eastern rock lobsters – near Wollongong on the south coast of New South Wales.
“We expected that would result in higher rates of predation by lobsters, but it’s not what we found,” he said.
“Sharks came in and smashed them.”
The study, published in Frontiers in Marine Science, was designed to test the assumption that lobsters played a key role in regulating sea urchin numbers. The results highlighted the significance of overlooked predators – like sharks.
Over 25 nights in the spring of 2023, researchers tethered 50 long-spined and 50 short-spined urchins outside the den, as underwater cameras captured the urchins’ fate.
Sharks were the main predator, eating 45%, while lobsters ate less than a tenth of that (4%).
While they are native to NSW, long-spined urchins have extended their range into Victoria and Tasmania, where they pose a threat to local ecosystems and fisheries, as waters have warmed due to climate change.
Dr John Keane is a fisheries scientist who researches urchins at the Institute for Marine and Antarctic Studies at the University of Tasmania.
An estimated 20m to 30m sea urchins in Tasmania waters ate kelp and created bare rock barrens that threatened local ecosystems as well as recreational and commercial fisheries, impacts that were the focus of a senate inquiry in 2023, he said.
“It is a massive ecological issue, particularly for the southern states – here in Tasmania, where we’ve lost 15% of our east coast reefs – with the same problem across eastern Victoria.”
Keane’s research has focused on managing urchins by harvesting them for human rather than shark or lobster consumption. Urchin roe, particularly during March and May, was highly sought after for its unique umami and nutty flavour, he said.
While humans preferred the roe, Day said sharks ate the urchins whole undeterred by their long spines, even though some measured up to 18cm in diameter with their spines extended.
That can be comparable to the size of a fully grown Port Jackson shark’s head.
“Port Jackson sharks handle very big urchins with no apparent feeding limitation,” Day said. When urchins were “put on a platter” by being tethered, sharks appeared willing to eat them regardless of size and species.
“Urchins have white muscle tissue around the Aristotle’s lantern, which is their mouth. They’ve got delicious gut material of algae and invertebrates, because now we know they’re omnivores. They’ve also got roe, the gonad part,” he said.
“Sharks, they just smash the whole thing and crunch it to pieces.”
• This article was amended on 4 October 2024 to clarify 15% of east coast reefs have been lost, not 50% as an earlier version said.
