Innovative techniques could be key for sea urchin recovery

Deterioration of Caribbean coral reefs begun in the 1970s when diseases devastated the most important reef building corals in the region. These issues were accelerated, when the most abundant herbivore, the long-spined sea urchins (Diadema antillarum), suffered massive die-offs in the 1980s. Within days after the Diadema die-off, macroalgae started to dominate Caribbean coral reefs. This shift from coral dominated to macroalgae dominated reefs have had devastating effects on the biodiversity and overall reef health.

Assisted natural recovery

Natural recovery of the long-spined sea urchin has been slow. A new method for sea urchin restoration, coined Assisted Natural Recovery (ANR), employs similar techniques as those used in terrestrial reforestation. The concept is centered around the idea that by removing barriers to natural recovery, succession can be accelerated. Earlier studies suggested that recovery of Diadema sea urchins might be limited by a lack of suitable settlement substrate for their larvae. With the reefs overgrown with algae, clean substrate with a fresh biofilm, which is thought to be the main cue for settlement, is lacking.

Bio ball streamers

With ANR the researchers provided new settlement substrate in the form of streamers attached to the reef. In total, nine streamers, made up of thirty bio balls tied on fishing line, were attached to the reef. Bio balls are normally used in aquarium filters and have a large surface area. A previous study, conducted in the same project, showed that this material is very effective to monitor sea urchin settlement. These streamers provided areas for sea urchin larvae to settle, protecting them until they grew large enough to migrate to the reef. In April 2021, six patch reefs were selected off the coast of Saba, of which three were provided with bio ball streamers and three were left untouched and served as control.

At the end of the study, it was demonstrated that reefs with bio ball streamers had significantly higher Diadema urchin recruit densities than those without, highlighting the importance of settlement substrate in natural recovery. Unfortunately, the improved recruitment rate did not equate to a recovered population. The recruits rarely grew larger than 20 millimeter, suggesting that predation on recruits is also limiting natural recovery.  Once a recruit becomes larger than 5 millimeter they outgrow the bio ball and need to move to the reef, where they are vulnerable to predation.

Implications

With the latest news of massive Diadema die offs, park authority managers are desperately looking for ways to protect the future of sea urchin populations in the region. The new approach of ANR might be one of the ways to assist Diadema sea urchin recovery in the future. Researchers from the RAAK PRO Diadema project are now working on optimization of the method. However, future recovery of this sea urchin species will mostly depend on the scale and lethality of a new die-off that is currently happening in the Caribbean. Already accounts of these die-offs have been recorded on Saba, St. Eustatius, St. Maarten and Curacao. Threatened reefs are struggling to combat the spread of macroalgae, and the continued loss of a herbivores would only exacerbate these issues.

More information and you can help

• You can help authorities to track sea urchin health in the Caribbean by reporting your observations of healthy, sick or dead urchins on the AGRRA website. 
• You'll find more information about the latest sea urchin recovery methods in the article Assisted Natural Recovery: A novel approach to enhance Diadema antillarum recruitment or by following the RAAK PRO Diadema project online.

Text: DCNA, Van Hall Larenstein, Wageningen University and Wageningen Marine Research
Photo's: Alwin Hylkema