Evaluating Cittarium pica for reef restoration: From grazing effects to post-release retention

Caribbean coral reefs have experienced extensive coral loss, accompanied by increased algal cover which inhibits coral recovery. Enhancing native herbivore populations is being explored as a means of controlling excessive algal growth. The gastropod Cittarium pica (C. pica) may represent an option, but its suitability for reef restoration is unknown. We therefore evaluated its grazing, survival, and behavior across depths, effects on benthic community composition, and retention after release on Saba, Caribbean Netherlands. C. pica remained active and grazed across the full 2–20 m experimental depth range, with algal height declining at all tested depths in enclosures with C. pica, whereas it generally increased in control enclosures. In enclosures on natural reefs, benthic community trajectories differed between enclosures with C. pica and control treatments. C. pica grazing reduced turf algae cover and increased crustose coralline algae (CCA) and bare substrate cover, while total macroalgal cover showed no consistent treatment-specific pattern. Retention after release declined rapidly in two separate experiments, reaching 7.3% after 25 days following enclosure removal in one experiment and 1.2% after 43 days in the other. Migration and mortality contributed to low retention. Follow-up experiments showed that overturned C. pica were unable to right themselves on sand, potentially increasing their vulnerability to predation and starvation. Thus, despite beneficial benthic effects under enclosure conditions, the practical feasibility of C. pica for reef restoration remains uncertain and depends on achieving sufficient survival and retention under unrestricted reef conditions.