Sea urchins play an important ecological role on reefs in the Lord Howe Island Marine Park (LHIMP). They graze on seaweed, and sometimes completely clear it, creating unique habitats known as ‘barrens’ which are instead covered in encrusting algae and coral. Some species of fish and invertebrates rely on urchin barrens, which usually form part of a natural mosaic of different reef habitats. However, as some sea urchin species move further south with climate change, these barrens are also notorious for negatively impacting kelp forests in cooler waters. On the other hand, sea urchin grazing is important for preventing corals from being overgrown by algae in warmer waters.
There are three large species of grazing sea urchin in the LHIMP that can form barrens, including the Red Sea Urchin (Heliocidaris tuberculata), Long-spined Sea Urchin (Centrostephanus rodgersii) and Lamington Sea Urchin (Tripneustes australiae). Lamington Sea Urchins, in particular, can quickly form extensive barrens due to their boom-and-bust population cycle. Population booms occur due to a sudden recruitment of larvae and fast growth rates, usually followed by a mass die-off and population ‘bust’ in following years.
Lamington Sea Urchin booms have occurred in the LHIMP around the Admiralty Islands in 2008, Balls Pyramid in 2020, and more recently a widespread boom occurred on offshore reefs surrounding Lord Howe Island earlier this year. The barrens formed during these booms can reduce the aesthetic value of these reefs for snorkelling and diving. Research is ongoing to document the recovery of seaweeds and any other impacts.
All three barren-forming species of sea urchin in the LHIMP are regionally endemic, occurring only on subtropical to temperate reefs between Eastern Australia and New Zealand, including Lord Howe Island. Therefore, it is not yet clear how their populations will respond to climate change, which may affect the supply of their larvae as well as conditions for their growth and reproduction.
To help answer these questions, the LHIMP team is assisting researchers from both sides of the Tasman Sea who are collecting tissue samples for these species. The research is led by PhD candidate Melissa Nehmens and Postdoctoral Fellow Elahe Parvizi from Associate Professor Libby Liggins’ research group at the University of Auckland. The wider team includes researchers from the University of Sydney, the University of Tasmania, Flinders University, and Macquarie University and has been supported with samples provided by the Australian Museum, the Auckland Museum, collaborating Iwi (Māori tribes), commercial fishers, and NSW Department of Primary Industries and Regional Development (DPIRD).
Genomic data from these samples will be used to understand how closely related sea urchins in the LHIMP are to other populations in the region and where their larvae likely come from. It will also help reconstruct the history of larval supply and population changes to predict future climate change scenarios. This research is further supported by long-term data on sea urchin populations collected and analysed by Reef Life Survey and specific analysis of population trends and grazing impacts of Lamington Sea Urchins in the LHIMP by Professor Maria Byrne from the University of Sydney. Results from the research are expected in the coming months and will be shared with the community.
This article appeared in The Lord Howe Island Signal, 31 December 2024.
