Improving early life survival of priority grazers

This report contains information on experiments conducted in FY 24-25 intended to address attenuation of production potential at critical life stages for the reef grazers Diadema antillarum and Maguimithrax spinosissimus. The sea urchin Diadema antillarum is a Western Atlantic coral reef herbivore important in maintaining productive reef community structure. Due to a die-off event in the 1980s, numbers of D. antillarum are exceedingly low on Florida’s Coral Reef, but may be supplemented with aquacultured individuals. However, limitations in knowledge on culture methods hinders output. Understanding diet requirements for early post-metamorphosis juveniles may improve growth and survival of a culture. Current methods utilizing natural biofilm are less than ideal for rearing large numbers of urchins to a releasable size. Three experiments were run covering the first twelve weeks post-metamorphosis. Experiments 1 and 2 used replicable diets including a benthic diatom and flocculated live microalgae. The benthic diatom outperformed biofilms during the first three weeks post-metamorphosis in terms of growth, while flocc out performed biofilm during the 4-7th weeks post-metamorphosis. Mortality was consistent among treatments aside from the no-feed control in experiment 1, which experienced significant mortality by week three post-metamorphosis. Test diameter variation was highest in the treatments with the best growth. In Experiment 3, data strongly suggested that D. antillarum are unable to consume a macroalgae food source until ~1.5 mm test diameter and that they should remain on microalgae-based diets until this point. The Caribbean king crab, Maguimithrax spinosissimus, is the largest brachyuran crab in the western Atlantic. It is uncommon but ubiquitous on coral reefs and shallow hardbottom habitats throughout Florida, gulf waters, and the Caribbean region. It is an omnivore, but benthic algae, including chemically-defended and calcified algae that many herbivores tend to avoid, comprise the bulk of its natural diet. At sufficient densities, these crabs reduce benthic algal cover dramatically and lead to cascading effects of increased fish abundance and richness and significant increases in coral recruitment. The species has been identified as one targeted for stocking to facilitate improved coral reef restoration outcomes. Production of the species for this purpose is established in Florida and several other locations throughout the region, but post-larval settlement and metamorphosis, as with many aquacultured species, is a bottleneck to mass production. Two experiments were conducted to evaluate the effect of substrate type on settlement rates and settlement preference. Experiment 1 evaluated the effect of substrate type on settlement rate relative to no-substrate controls. All candidate substrates tested increased settlement over control treatments with marginal, if any, differences between treatments. Experiment 2 evaluated settlement preference for each of the candidate substrates in Experiment 1. Interestingly, there were no discernible differences in preference among candidate substrates or controls, but in an unanticipated result, vertical surfaces were a clear preference for M. spinosissimus post-larvae. Both experiments suggest that substrate(s) should be incorporated into M. spinosissimus production efforts but that those substrates should be oriented vertically in the culture vessel. By expanding the current understanding of these delicate life stages in aquaculture of these two important coral reef grazers, culture success can be improved to the benefit of restoration efforts for Florida’s Coral Reef.