Spatial distribution of settlement of Diadema antillarum around Saba, Dutch Caribbean.

The mass mortality event of the herbivorous sea urchin Diadema antillarum in 1983-1984 has been a major contributor to the diminished resilience of coral reefs throughout the Caribbean. The reduction in grazing pressure resulted in algae proliferation, which inhibited coral recruitment after disturbances such as disease, hurricanes, pollution and climatic change induced marine heat waves. Natural recovery of D. antillarum after the 1983-1984 die-off has been slow. However, the few locations with recovered populations exhibit signs of improvement in coral reef health, prompting interest in D. antillarum restoration. Current restoration strategies include translocation of wild individuals, the restocking of juveniles that are either cultured from gametes or collected as settlers and head-started in a nursery, and assisted natural recovery by providing suitable settlement substrate. Both the collection of wild settlers and assisted natural recovery necessitate an understanding of the local, spatiotemporal trends in settlement. In this study, which was carried out on the Dutch Caribbean Island of Saba, artificial turf settlement collectors were deployed at nine locations around the island and monitored from June 2019 till July 2020 (13 months). The primary objective was to identify trends in larval settlement in space and time, to be able to optimize restoration efforts. Additionally, the small size of Saba allowed us to deploy settlement collectors around the island and compare D. antillarum settlement between windward and leeward sides. Our study showed that on Saba, D. antillarum settlement peaked in June and July, following similar seasonal trends observed around other islands in the Northeastern Caribbean. By far the most settlement occurred at the leeward side of the island, suggesting that hydrodynamic forces entrained D. antillarum larvae in the lee of Saba and/or calmer waters facilitated settlement. Limited settlement occurred on the more exposed windward locations. The identified high settlement locations are candidates for settler collection and restoration attempts. Continued monitoring of D. antillarum settlement, especially in light of the 2022 D. antillarum die-off, holds significance as it can provide insights into the potential of natural recovery.

Diurnal predators of restocked lab-reared and wild Diadema antillarum near artificial reefs in Saba.

The long-spined sea urchin Diadema antillarum controls reef dynamics by grazing on algae and increasing coral recruitment. Populations of Diadema never recovered after a mass-die off in 1983 and 1984, and numbers were further reduced by a more recent die-off in 2022. To restore grazing pressure and thereby the resilience of Caribbean coral reefs, multiple Diadema restocking efforts have been performed. Although results vary, relatively low retention is one of the reasons restocking is not considered more often. If causes for the low retention can be identified, suitable measures may be able to increase restocking success. In this study, we monitored restocked lab-reared and wild juvenile Diadema on artificial reefs around Saba, Caribbean Netherlands. To assess the retention of Diadema over time, we conducted diver surveys and used underwater photo time lapse during daylight. Retention of uncaged lab-reared and wild Diadema decreased steadily with less than 30% surviving after 10 days. In total, 138 predator-prey interactions were recorded, of which 99% involved the queen triggerfish Balistes vetula, although other potential predators were present in the area. None of the recorded predator-prey interactions was successful, which suggests that artificial reefs with incorporated shelters may be suitable for juveniles as daytime refuge. However, Diadema that were more often attacked during the day were more likely to be absent the next morning. Because queen triggerfish often visited the experimental site in the first or last hour of daylight, it could be that they were more successful in their attacks when it was too dark to see anything on the photos and when Diadema came out to feed or to look for better shelter opportunities. If Diadema migrated off the artificial reef, they were probably predated during the process, because no Diadema were found on surrounding reefs. Wild Diadema were attacked significantly more often than lab-reared Diadema, possibly because the wild urchins were larger, but this did not significantly affect retention. Future restocking should be performed on natural or artificial reefs with deeper shelters, so Diadema can retract farther into their crevice, and should include night-time monitoring to identify the remaining unknown factors that cause low retention, including migration and nocturnal predation. This knowledge is urgently needed to coral reef managers so they can increase Diadema restocking success by selecting reefs with a lower predator density, protect urchins during an acclimatization period and/or conduct temporary predator control measures.

A scuticociliate causes mass mortality of Diadema antillarum in the Caribbean Sea.

Echinoderm mass mortality events shape marine ecosystems by altering the dynamics among major benthic groups. The sea urchin Diadema antillarum, virtually extirpated in the Caribbean in the early 1980s by an unknown cause, recently experienced another mass mortality beginning in January 2022. We investigated the cause of this mass mortality event through combined molecular biological and veterinary pathologic approaches comparing grossly normal and abnormal animals collected from 23 sites, representing locations that were either affected or unaffected at the time of sampling. Here, we report that a scuticociliate most similar to Philaster apodigitiformis was consistently associated with abnormal urchins at affected sites but was absent from unaffected sites. Experimentally challenging naïve urchins with a Philaster culture isolated from an abnormal, field-collected specimen resulted in gross signs consistent with those of the mortality event. The same ciliate was recovered from treated specimens postmortem, thus fulfilling Koch's postulates for this microorganism. We term this condition D. antillarum scuticociliatosis.