Cryopreservation of the collector urchin embryo, Tripneustes gratilla

The collector urchin, Tripneustes gratilla, is an ecologically important member of the grazing community of Hawai'i's coral reefs. Beyond its ability to maintain balance between native seaweeds and corals, T. gratilla has also been used as a food source and a biocontrol agent against alien invasive algae species. Due to overexploitation, habitat degradation, and other stressors, their populations face local extirpation. However, artificial reproductive techniques, such as cryopreservation, could provide more consistent seedstock throughout the year to supplement aquaculture efforts. Although the sperm and larvae of temperate urchins have been successfully cryopreserved, tropical urchins living on coral reefs have not. Here, we investigated the urchin embryos' tolerance to various cryoprotectants and cooling rates to develop a cryopreservation protocol for T. gratilla. We found that using 1 M Me2SO with a cooling rate of 9.7 °C/min on gastrula stage embryos produced the best results with survival rates of up to 85.5% and up to 50.8% maturation to the 4-arm echinopluteus stage, assessed three days after thawing. Continued research could see cryopreservation added to the repertoire of artificial reproductive techniques for T. gratilla, thereby assisting in the preservation of this ecologically important urchin, all while augmenting aquaculture efforts that contribute to coral reef restoration.