Long-spine sea urchin population study in Cozumel

by Ines Faure

The goals of this study are to observe the behavior and geographic distribution of the long-spine sea urchin, D. antillarum, in the Villa Blanca reef, and to consider solutions to boost the current population.

Two mass mortality events (1983 and 2022) have affected this species, causing significant ecosystem disruption in Caribbean reefs. Without the grazing activities of this herbivore, macroalgae spread and form a lawn that covers the reefs. Algae become the dominant benthic group and gradually replace corals. This leads to recruitment failure (the process by which new coral colonies form from larvae that attach themselves to a substrate). Over time, as algae replaced corals, the living coral cover decreased, reducing the roughness and complexity of the reefs. This is bad news for sea urchins, as they prefer high relief, which allows them to hide from predators. So we're stuck in a negative feedback loop.

Since D. antillarum is a nocturnal species, a night dive was conducted on August 18th, 2025, and revealed distinct age cohorts in different areas. The juvenile population is geographically distant from the various populations of mature adults.

adult rock entered population

adult boll fish population

juvenile live rock population

To reduce the effect of predation on juveniles, shelters were installed (made from terracotta flower pots with a diameter of 25 cm). This installation also serves to keep the juveniles in place and to establish an adult population in the area, which requires significant grazing. The shelters were installed in the angles of a 1 m by 1 m square, as D. antillarum is a gonochoric species. Male and female individuals must be close to each other to ensure reproductive success. 



An in situ nursery prototype was also built but not yet implemented. It consists of a larva collector and a grow-out cage. Its purpose is to reduce predation on larvae in order to boost the population.

right: construction process for the grow-out cage

The Villa Blanca reef can count on volunteers from the CCRRP who take care of it, notably by cleaning it and removing algae daily. Restoring the population of D. antillarum would reduce the CCRRP's workload, provide natural maintenance for the reef, and re-establish the mutualistic relationship between sea urchins and corals.