In a new study to understand how lobsters prey on sea urchins, a camera was installed in a lobster burrow and sea urchins were strategically placed.
Surprisingly, most of the sea urchins were eaten—not by lobsters—but by sharks. This fact challenges the existing beliefs that lobsters are the main predator of sea urchins, and is important to help inform the management strategies for sea urchins that are intended to protect reefs.
“Sharks are overlooked predators of sea urchins in NSW, while the role of lobsters appears to be smaller than expected. It is important to note that sharks readily handle very large sea urchins!” says Jeremey Day, the principal investigator at the University of Newcastle.
“This experiment was originally designed to show how lobsters feed on sea urchins, but sharks surprisingly entered and ate the majority (45%) of the tethered urchins, while lobsters ate few (4%),” explains.
“Lobsters are thought to be the ‘key’ predators of urchins, controlling their abundance, while sharks are not usually considered in urchin-predator models,” adds.
Methodology
Jeremy explains that they tethered 100 urchins directly to a lobster den (i.e., the place where lobsters live) in Wollongong for 25 nights. “Tethering” is when the urchins are surgically fixed to remain available for predation during the night and to be visible to our cameras, he explains, noting that they used a filtered red light to film the experiments because invertebrates do not like the white light spectrum.
Until now it was believed that there were few or no predators capable of handling very large urchins.
“In this study, we report the existence of a relatively overlooked urchin predator among Port Jackson sharks and crested sharks, and we determine that the effect of lobsters on urchins in nature is smaller than previously thought. This is important because long-term efforts to control the number of urchins, by ensuring predator numbers, have not had the expected effect in NSW, despite more than 20 years of protections.”
“This experiment provides a possible answer, since lobsters generally appeared uninterested in eating urchins, and it also increases the complexity of the model, as sharks unexpectedly emerged as the predator that ate the majority of urchins. We confirm that the range of predators that eat urchins is broader than traditionally thought,” concludes Jeremy Day.
