First Record and Massive Spread of Tropical Long-Spined Sea Urchin Diadema antillarum in the Mediterranean Sea (Syrian Coast)

The biological structure of the Syrian marine environment still shows a rapid and steady increase in the number of warm water species, many of which have become dominant and invasive. In the latest field data during 2020-2021, the presence of the black sea urchin Diadema antillarum (Philippi, 1845) was recorded for the first time in several locations in Latakia and Ras al-Bassit coast, and its massive spread at depths of 5-8 m, accompanied by invasive species of macroalgae, sponges, crustaceans and mollusks.. It is noteworthy that this species is one of the most widespread and important species of sea urchins in some tropical areas, and this is the first record in the Mediterranean Sea and the Syrian coast It is important as it prey on macroalgae and has an effective role in the health of some of the most important areas of biodiversity in the world.

A novel system for intensive Diadema antillarum propagation as a step towards population enhancement

The long-spined sea urchin Diadema antillarum was once an abundant reef grazing herbivore throughout the Caribbean. During the early 1980s, D. antillarum populations were reduced by > 93% due to an undescribed disease. This event resulted in a lack of functional reef herbivory and contributed to ongoing ecological shifts from hard coral towards macroalgae dominated reefs. Limited natural recovery has increased interest in a range of strategies for augmenting herbivory. An area of focus has been developing scalable ex situ methods for rearing D. antillarum from gametes. The ultimate use of such a tool would be exploring hatchery origin restocking strategies. Intensive ex situ aquaculture is a potentially viable, yet difficult, method for producing D. antillarum at scales necessary to facilitate restocking. Here we describe a purpose-built, novel recirculating aquaculture system and the broodstock management and larval culture process that has produced multiple D. antillarum cohorts, and which has the potential for practical application in a dedicated hatchery setting. Adult animals held in captivity can be induced to spawn year-round, with some evidence for annual and lunar periodicity. Fecundity and fertilization rates are both consistently very high, yet challenges persist in both late stage larval development and early post-settlement survival. Initial success was realized with production of 100 juvenile D. antillarum from ~ 1200 competent larvae. While the system we describe requires a significant level of investment and technical expertise, this work advances D. antillarum culture efforts in potential future hatchery settings and improves the viability of scalable ex situ production for population enhancement.

The ecological importance of habitat complexity to the Caribbean coral reef herbivore Diadema antillarum: three lines of evidence

When Caribbean long-spined sea urchins, Diadema antillarum, are stable at high population densities, their grazing facilitates scleractinian coral dominance. Today, populations remain suppressed after a mass mortality in 1983–1984 caused a loss of their ecosystem functions, and led to widespread declines in ecosystem health. This study provides three lines of evidence to support the assertion that a lack of habitat complexity on Caribbean coral reefs contributes to their recovery failure. Firstly, we extracted fractal dimension (D) measurements, used as a proxy for habitat complexity, from 3D models to demonstrate that urchins preferentially inhabit areas of above average complexity at ecologically relevant spatial scales. Secondly, controlled behaviour experiments showed that an energetically expensive predator avoidance behaviour is reduced by 52% in complex habitats, potentially enabling increased resource allocation to reproduction. Thirdly, we deployed a network of simple and cost-effective artificial structures on a heavily degraded reef system in Honduras. Over a 24-month period the adult D. antillarum population around the artificial reefs increased by 320% from 0.05 ± 0.01 to 0.21 ± 0.04 m−2 and the juvenile D. antillarum population increased by 750% from 0.08 ± 0.02 to 0.68 ± 0.07 m−2. This study emphasises the important role of habitat structure in the ecology of D. antillarum and as a barrier to its widespread recovery.

Predation on a newly-stocked long-spined sea urchin (Diadema antillarum) by the batwing coral crab (Carpilius corallinus) on a coral reef restoration site

This photograph documents a batwing coral crab (Carpilius corallinus) preying on the sea urchin (Diadema antillarum) on a coral reef site enhanced with artificial shelter and staghorn coral. This interaction illustrates an interaction that to be better understood to develop a restoration strategy that harnesses positive ecological processes.