Ground Ulva solution (GUS): A promising metamorphosis cue for Paracentrotus lividus larviculture

Sea urchin larvae have developed metamorphosis and settlement stages as an adaptation serving to increase their distribution and find more suitable habitats; for aquaculture farmers, these stages represent a bottleneck, as the cost-effective production of large numbers of metamorphosed larvae in confined conditions remains a major constraint. The aim of this study was the development of a reliable metamorphosis cue for Paracentrotus lividus larvae that is widely available and easy to prepare and use even at the hatchery scale. For this purpose, substrates of environmental origin (GUS - ground Ulva spp. solution) and chemical origin (histamine) were tested for their effectiveness, evaluating the metamorphosis rates of competent P. lividus larvae after 24-h exposure. In addition, differences in metamorphosis rates following exposure to scaled concentrations of both fresh and frozen-thawed GUS were evaluated, with a view to the latter's routine adoption as a metamorphic cue. The metamorphosis rates obtained in the presence of GUS were always around 80%, using both fresh and thawed solutions. Decreasing GUS concentrations resulted in significantly lower percentages of metamorphosed larvae. No metamorphosis occurred after exposure to histamine solutions. GUS proved to be an effective metamorphic cue and to have several other strengths. Indeed, Ulva spp. are widely available and easy to collect in the field as well as being currently cultivated, in some cases in integrated multi-trophic systems. Ulva spp. represent a common food source for natural populations of juvenile and adult P. lividus, and GUS can therefore be used in the formulation of a settlement substrate that also serves as food in the initial post-metamorphosis period. Finally, GUS is prepared by grinding fragments of Ulva spp. with FSW in a mortar and then filtering the solution, with no need for time-consuming or expensive methods. It can thus be easily produced on a commercial scale and then stored or delivered after being frozen. In conclusion, the formulation of GUS represents a promising advance in the development of protocols for echiniculture, meeting the need for tailored metamorphosis and settlement methods that support successful and cost-effective production of sea urchin juveniles. Moreover, it could represent an additional valuable method for obtaining marketable products from harvested Ulva grown in integrated multi-trophic aquaculture systems.