Sea urchins on Caribbean coral reefs

The RAAK PRO Diadema project aimed to restore populations of Diadema sea urchins (scientific name Diadema antillarum) on the coral reefs around Saba and St Eustatius. Diadema sea urchins were the main herbivores on Caribbean coral reefs. In 1983, more than 95% of the sea urchins died due to an unknown disease. Without the sea urchins grazing, algal cover increased dramatically, affecting mature corals and hindering the establishment of young corals. Today, more than 35 years after the mass mortality, sea urchins are still very rare. They are sometimes common in shallow waters, such as harbours, but are rarely seen on the deeper coral reef.

Reducing local threats

Globally, coral reefs face many threats that are difficult to tackle locally, such as global warming and ocean acidification. This makes it all the more urgent to reduce as many local threats as possible. Restoring sea urchin populations will reduce overgrowing algae and make reefs more resilient to other threats they face.

Researchers

VHL researcher Alwin Hylkema was the lead researcher on the project. Other researchers in the project included Tom Wijers (VHL), Oliver Klokman (VHL), Dolfi Debrot (WUR) and many others from various partner institutes.

Participating organisations

Van Hall Larenstein University of Applied Sciences (VHL) collaborated in this RAAK PRO Diadema project with STENAPA, Saba Conservation Foundation, Wageningen Marine Research, Marine Animal Ecology of Wageningen University, Caribbean Netherlands Science Institute, University of Applied Sciences HZ, ISER Caribe, Root Product Design and Golden Rock Dive Centre.

Funding

The project was partly funded by the Netherlands Organisation for Scientific Research (NWO).

• The project studied natural settlement of sea urchins around Saba and St Eustatius. A new settlement collector was developed, made of bioballs:Hylkema, A., Debrot, A. O., Pistor, M., Postma, E., Williams, S. M., & Kitson-Walters, K. (2022). High peak settlement of Diadema antillarum on different artificial collectors in the Eastern Caribbean. Journal of Experimental Marine Biology and Ecology, 549, 151693.
• Based on the high natural settlement, a new method was developed to increase recruitment on the reef, which we called 'Assisted Natural Recovery':Hylkema, A., Debrot, A. O., Van De Pas, E. E., Osinga, R., & Murk, A. J. (2022). Assisted natural recovery: A novel approach to enhance diadema antillarum recruitment. Frontiers in Marine Science, 9, 929355.
• At the same time, a new breeding method was developed in collaboration with the University of Florida. For the first time in almost 30 years, Diadema sea urchins could be cultured consistently. We called this new approach the 'shaker bottle' method: Wijers, T., Hylkema, A., Pilnick, A. R., Murk, A. J., & Patterson, J. T. (2023). Novel shaker bottle cultivation method for the long spined sea urchin (Diadema antillarum; Philippi, 1845) results in high larval survival and settlement rates. Aquaculture, 562, 738855.
• Because we were finally able to produce a significant amount of larvae, we were able to study the effect of different algal species on larval settlement behaviour: Wijers, T., van Herpen, B., Mattijssen, D., Murk, A.J., Patterson, J.T. & Hylkema, A. (2024) Implications of changing Caribbean coral reefs on Diadema antillarum larvae settlement. Marine Biology 171, 48.
• We also worked on introducing Diadema sea urchins to the reef and studied their main predators: de Breuyn, M., van der Last, A. J., Klokman, O. J., & Hylkema, A. (2023). Diurnal predators of restocked lab-reared and wild Diadema antillarum near artificial reefs in Saba. PeerJ, 11, e16189.
• In early 2022, Diadema sea urchins underwent another mass mortality. Together with other regional experts, HVHL researchers established the Diadema Response Network. Together, we described symptoms, mortality rates and distribution of mass mortality in the Caribbean. Hylkema, A., Kitson-Walters, K., Kramer, P. R., Patterson, J. T., Roth, L., Sevier, M. L., ... & Lang, J. C. (2023). The 2022 Diadema antillarum die-off event: Comparisons with the 1983-1984 mass mortality. Frontiers in Marine Science, 9, 2654.
• With a team of more than 40 researchers, we were able to find the cause of the 2022 Diadema mass die-off. The results were published in Science Advances:Hewson, I., Ritchie, I. ., Evans, J. ., Altera, A., Behringer, D., Bowman, E., Brandt, M., Budd, K. ., Camacho, R. ., Cornwell, T. ., Countway, P. ., Croquer, A., Delgado, G. ., DeRito, C., Duermit-Moreau, E., Francis-Floyd, R., Gittens, S., Henderson, L., Hylkema, A., … Breitbart, M. (2023). A scuticociliate causes mass mortality of Diadema antillarum in the Caribbean Sea. Science Advances, 9(16).

Diadema II project

In the RAAK-PRO Diadema II project (2024-2027), we will continue to work on developing methods to restore coral reefs through increased herbivory.