restoration

On October 22st, 2021, the first ever recorded Diadema sea urchins in the Caribbean were cultured on Saba. Diadema sea urchins are important grazers and can facilitate corals by reducing their competition with algae. By culturing them, researchers from University of Applied Sciences Van Hall Larenstein set an important step in restocking sea urchins on Saba’s coral reefs.

Long-spined urchins (Diadema antillarum). Photo credit: Hans Leijnse

The long spined black sea urchin, also known as Diadema, has been absent on most coral reefs throughout the Caribbean, since 99% of all populations died to an unknow disease in the early 80s. Before the die-off these sea urchins were the major herbivore in the Caribbean, by scraping off and eating the seaweeds. After the mass mortality, Diadema sea urchin populations never really recovered and most reefs nowadays are dominated by macroalgae. Restoring Diadema populations is therefore seen as key priority in Caribbean coral reef management.

Culturing Diadema and releasing them in the wild would speed up the recovery of this keystone species. Unfortunately, the culture of Diadema is very hard, due to the sensitive nature of the larvae. It has been tried several times, especially in Florida, but despite some successes, a consistent method was never developed. In 2020 a new culture method was developed in The Netherlands by researchers from University of Applied Sciences Van Hall Larenstein, making it possible to consistently culture Diadema from tiny larvae to juvenile sea urchins.

In July of this year the research team moved their culture efforts from the cold and not so tropical Netherlands to the Dutch Caribbean island of Saba, in an attempt to culture juveniles near the reefs where they are much needed. With help of the Saba Conservation Foundation, the first larvae have successfully been cultured and settled, resulting in the first 19 Caribbean cultured Diadema sea urchin juveniles! The first 19 cultured Diadema are raised in captivity until they are big enough for release in the wild, where they can graze away the algae that are smothering the corals and prevent new corals from settling.

First 19 cultured Diadema sea urchins in the Caribbean. Squares are 1×1 mm. Source: Tom Wijers and Alwin Hylkema

The next step is to upscale cultivation, with currently over 3000 larvae being cultured. If this approach is proven to be effective on Saba, it can be copied throughout the Caribbean. By removing their most important competitors, Diadema sea urchins can help coral reefs to cope with other stressors like climate change and pollution.

More information: https://www.vhluniversity.com/research/research-projects/diadema

Or contact researchers Tom Wijers (tom.wijers@hvhl.nl) or Alwin Hylkema (alwin.hylkema@hvhl.nl)

https://www.dcbd.nl/document/new-diadema-project-aims-restore-sea-urchin...

Article published in BioNews 50

The massive die-off of the herbivorous sea urchin Diadema antillarum in 1983 and 1984 resulted in phase shifts on Caribbean coral reefs, where macroalgae replaced coral as the most dominant benthic group. Since then, D. antillarum recovery has been slow to non-existent on most reefs. Studying settlement rates can provide insight into the mechanisms constraining the recovery of D. antillarum, while efficient settlement collectors can be used to identify locations with high settlement rates and to collect settlers for restoration practices. The aim of this study was to compare pre and post die-off settlement rates and to determine possible settlement peaks in the Eastern Caribbean island of St. Eustatius. Additionally, we aimed to determine the effectiveness and reproducibility of five different settlement collectors for D. antillarum. D. antillarum settlement around St. Eustatius was highest in May, June and August and low during the rest of the study. Before the die-off, settlement recorded for Curaçao was high throughout the year and was characterized by multiple settlement peaks. Even though peak settlement rates in this study were in the same order of magnitude as in Curaçao before the die-off, overall yearly settlement rates around St. Eustatius were still lower. As no juvenile or adult D. antillarum were observed on the reefs around the settlement collectors, it is likely that other factors are hindering the recovery of the island's D. antillarum populations. Of all five materials tested, bio ball collectors were the most effective and reproducible method to monitor D. antillarum settlement. Panels yielded the least numbers of settlers, which can partly be explained by their position close to the seabed. Settler collection was higher in mid-water layers compared to close to the bottom and maximized when strings of bio balls were used instead of clumps. We recommend research into the feasibility of aiding D. antillarum recovery by providing suitable settlement substrate during the peak of the settlement season and adequate shelter to increase post-settlement survival of settlers. The bio ball collectors could serve as a suitable settlement substrate for this new approach of assisted natural recovery.

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