Effect of diets and light regimes on calcification and somatic growth of the sea urchin Tripneustes gratilla elatensis

The effects of Ulva lactuca (cultivated in the outlet of an Integrated Multi-Trophic Aquaculture [IMTA] system) vs. formulated diets on the somatic growth of the sea urchin Tripneustes gratilla elatensis were evaluated under three light regimes (continuous light, continuous darkness and ambient dark-light cycle). Total CaCO₃ precipitation and calcification rate, test diameter and surface area were used as growth parameters. The different light regimes tested did not appear to have a significant effect on any of these parameters. Sea urchins exposed to ambient light had significantly faster growth (measured as wet weight) than those exposed to continuous darkness or continuous light, a phenomenon attributed to gonad growth at the expense of shell growth. IMTA-produced U. lactuca was the preferential food source for promoting calcification, as average calcification rates in sea urchins fed formulated and U. lactuca diets, (1.31 ± 0.69 and 2.1 ± 0.31 mg h⁻¹SU⁻¹, respectively), were significantly higher in the urchins fed U. lactuca. Calcification rates in adult T. gratilla elatensis, measured as percentage of skeleton added per day, decreased with the juveniles' increase in size, as larger sea urchins allocated most of the CaCO₃ in favour of skeleton density rather than somatic growth. A 3D laser scanner was used to measure test surface area, from which significant correlation could be extrapolated extremely accurately between test diameter and the sea urchin surface area, volume, and skeletal and test dry weight, while alkalinity depletion in the surrounding sea water was used to calculate total calcium carbonate precipitation and calcification rates in the sea urchin. Both measurements applied for the first time to sea urchins, can be used to monitor the progress of growth in an industrial echinoculture setting.