Scaling up mariculture of a native herbivore in support of “Mission: Iconic Reefs” coral reef restoration

In recent years, the potential importance of coral reef herbivory by a guild of reef-dwelling crabs in the Mithracidae family has emerged. The Caribbean King Crab (Maguimithrax spinosissimus) is widespread throughout the Caribbean Sea and graze on a wide variety of macroalgae and algal turfs. Its use for coral reef restoration as a controller of coral-alga competition has been restricted by limiting crab stocks. Years of in-situ (laboratory) production have not been successful. In this project, we studied the abiotic and biotic characteristics of two quarries located in the Lower Florida Keys (Cudjoe Key) that have the potential for semi-wild mariculture of M. spinosissimus. Between August 2024 and April 2025, surface temperatures at a depth of 1.67 meters fluctuated between 20.16°C and 33.47°C. Dissolved oxygen and salinity were 6.19 (±) 0.02 Mg/l) and 30.56 (±) 0.03 PSU, respectively, year-round for both quarries. The benthic habitats of both quarries are dominated by periphyton mats (~50%), macroalgae (Batophora, ~35%), and Acetabularia (~10%), with apparent seasonal variations. The density of crabs in these habitats is approximately 0.09 (±) 0.03 crabs/m². The low crab densities in areas with high macroalgae abundance (>90%) and a lack of predators (almost none) suggest that habitat is a limiting factor for population growth. We created three new habitats in February 2025 using plastic shipping pallets covered with concrete (1-tier to 5-tier habitats, n = 4 sets) and monitored them in March and April. One month after deployment (March), the habitats had between 15 to 25% periphyton algae cover. By April, the average periphyton cover was 27.71 (±) 0.78%, and other macroalgal species were starting to grow (e.g., Acetabularia sp.). On April 19th, four male adult crabs were found in our new habitats, suggesting that improved habitat conditions resulting from two months of macroalgal recruitment and succession- are triggering adult crab migration, which hopefully translates into enhanced crab production. Our results from this phase support the habitat limitation hypothesis for Caribbean king crab populations in the quarries. We expect to benefit from this knowledge to increase the crab population in phase II.