Warming sea temperatures over the last 50 years have forced the Longspined Sea Urchin to extend its range hundreds of kilometres south into Tasmanian waters. While this species is on the move, it’s turning reefs into barren landscapes.
So to what extent can Southern Rock Lobster populations manage this ecological threat?
That was the question driving Dr Jennifer Smith’s PhD research, where she investigated the level at which the primary native predator on Tasmanian reefs would consume this range-extending urchin in the presence of alternative prey types.
“We wanted to explore the effectiveness of lobsters as a control agent, what they're eating and whether they like to eat the Longspined Sea Urchin,” the University of Tasmania’s Institute for Marine and Antarctic Studies (IMAS) PhD graduate said.
“We wanted to explore the effectiveness of lobsters as a control agent, what they're eating and whether they like to eat the Longspined Sea Urchin,” the University of Tasmania’s Institute for Marine and Antarctic Studies (IMAS) PhD graduate said.
Dr Smith spent weeks setting up aquaria, and getting out in the field collecting urchins and lobsters at various sites on the East Coast and at Taroona.
“The major experiment was a prey preference trial, which was conducted with lobsters collected from barren habitat where Longspined Sea Urchins were abundant, and then lobsters from a healthy reef and no Longspined Sea Urchins,” she said.
“We did something I call an ‘all-you-can-eat’ food trial in a ‘lobster hotel’ where the lobsters were offered different food options. I monitored what they ate and whether the size of lobster or where they came from made a difference."
“We found that all size classes of lobsters can eat the Longspined Sea Urchin, not just the large ones, but the larger ones ate more. However, all of them would rather eat native fauna if they're on offer, like abalone, Shortspined Sea Urchin and periwinkles. There wasn't a favourite, but the lobsters preferred the native options over the Longspined Sea Urchin.
“We’re still researching why there is a preference for species other than the Longspined Sea Urchin. We found that handling time was longer for the Longspined Sea Urchin compared to the shortspined species, which suggests it’s more energetically demanding for lobsters to eat Longspined Sea Urchins. Lobster naivety, in not recognising the species as a food, could potentially play a role as well.”
The majority of lobsters that ate Longspined Urchins in captivity had already encountered them in the wild in the barrens they were collected from, indicating learned behaviour may be important.
Results also showed that when lobsters predated on sea urchins, it was generally on the smaller urchins, with larger urchins reaching a size refugia.
“The lobsters are likely to be most helpful in controlling smaller and medium sized Longspined Sea Urchins and preventing barren formation in the first place,” Dr Smith said.
“In the early stages of urchin inundation of an area, the urchins are small so lobsters can play a large role, assuming they learn to see them as a food option, and they are in high enough abundance.
“In areas where the sea urchins have become large and over-abundant, such as long-term barren areas, other methods like commercial harvests, or as a last resort culling, will be required to assist the control of large urchins.”
Dr Smith’s thesis highlights the complex nature of managing the range-extending sea urchin, where rebuilding predator populations needs to be accomplished in conjunction with direct human control.
One of the highlights of Dr Smith’s research, who graduated with her PhD at the end of last year, was the ability to dive and collect the lobsters and see the barrens.
“Using diving as a tool for the research was an incredible opportunity,” she said.
"Diving allows me to be completely immersed in the environment, offering a deeper understanding of what’s happening beneath the surface.
“Researching the study site and witnessing firsthand the impact that sea urchins have on the ecosystem is a powerful experience. Seeing this with my own eyes makes me feel a strong responsibility to share what I’ve learned with others and increase awareness of this challenge.”
Predator and prey fast facts
- Lobsters of all sizes, including sub-legal, legal, and large (greater than 140mm), were shown to predate on Longspined Sea Urchins in laboratory studies, and the urchin was found to contribute to lobster diet of every size class in field studies.
- Lobster predation may be less effective at sites where most Longspined Sea Urchin individuals have reached a size refuge (i.e. large) than at sites dominated by small urchins.
- Lobster predation may provide a useful control for smaller Longspined Sea Urchins, but top-down predatory control may be constrained at sites dominated by urchins that exceed the size suitable for lobster predation.
- The nocturnal emergence and movement of small urchins on open reef spaces makes them accessible to nocturnal predators, such as the Southern Rock Lobster.
- This time-space overlap of predator and prey implies that rock lobsters (including small lobsters) may be inflicting higher predatory pressure than previously considered on juvenile sea urchins.
Cover image: Matt Testoni IMAS