The long spined sea urchin Diadema antillarum used to be a keystone herbivore on Caribbean reefs, with an average density 5-10 individuals per square meter. In 1983-84, an unknown disease wiped out 93-98% population resulting an ecological shift from hard coral to macroalgae dominated reef. The recovery of natural populations has been very slow over subsequent decades and Diadema restocking has emerged as a strategy of interest to improve local recovery of coral reefs.
Our lab has been working to develop techniques for hatchery propagation of Diadema larvae over the past four years. Diadema larvae have a lengthy larval cycle (35-90 days) to reach competency for settlement. Negative buoyancy and fragile body features pose challenges to keep larvae suspended in the culture system using water circulation. At the late stages, larvae succumb to a progression of dark pigmentation in the body tissue, which is believed to be an immune response to bacterial infection.
A novel recirculating system has been developed in our lab, which to date has produced settled urchins in multiple cohorts. A recent trial has indicated higher growth and survival of larvae with mixed diet Rhodomonas lens and Chaetoceros calcitrans than single species diet R. lens . In addition, inclusion of 10 µM ethylenediaminetetraacetic acid in the larvae culture system improved growth, survival, and health status. In a recent batch, over 500 settled Diadema juvenile were produced from ~9,000 competent larvae .
We are now trying to identify settlement cues using histamine, potassium chloride, macroalgae and coralline algae. In addition, the efficacy of axenic microalgae food on larval performance will be tested using the next batch of larvae. Systematic improvement of larvae culture parameters has the potential to reach a production goal of thousands of settled juveniles from fertilized eggs.