Ex situ co culturing of the sea urchin, Mespilia globulus and the coral Acropora millepora enhances early post-settlement survivorship

Abstract

Reef restoration efforts, utilising sexual coral propagation need up-scaling to have ecologically meaningful impact. Post-settlement survival bottlenecks, in part due to competitive benthic algae interactions should be addressed, to improve productivity for these initiatives. Sea urchins are keystone grazers in reef ecosystems, yet feeding behaviour of adults causes physical damage and mortality to developing coral spat. To investigate if microherbivory can be utilised for co-culture, we quantitatively assessed how varying densities of juvenile sea urchins Mespilia globulus (Linnaeus, 1758), reared alongside the coral Acropora millepora (Ehrenberg, 1834) effected survival and growth of coral recruits. Spawning of both species were induced ex situ. A comparison of A. millepora spat reared in three M. globulus densities (low 16.67 m−2, medium 37.50 m−2, high 75.00 m−2) and a non-grazed control indicated coral survival is significantly influenced by grazing activity (p < 0.001) and was highest in the highest density treatment (39.65 ± 10.88%, mean ± s.d). Urchin grazing also significantly (p < 0.001) influenced coral size (compared to non-grazing control), with colonies in the medium and high-densities growing the largest (21.13 ± 1.02 mm & 20.80 ± 0.82, mean ± s.e.m). Increased urchin density did however have a negative influence on urchin growth, a result of limited food availability.

Native Herbivores Improve Sexual Propagation of Threatened Staghorn Coral Acropora cervicornis

“…All data including live snail counts for each replicate aquarium at the beginning and end of all 21-day sampling intervals are available in the Supplementary Materials. While other studies have found increasing coral survival up to the highest herbivore density tested (Villanueva et al, 2013;Craggs et al, 2019), the present study clearly surpassed the maximum density required to control turf algae. Further research to optimize herbivore density in relation to system characteristics is warranted.…”

“…More recently, juvenile sea urchins Mespilia globulus were shown to improve growth and survival of Acropora millepora sexual recruits (Craggs et al, 2019). Multiple herbivore densities were tested, and increasing densities produced higher growth and survival up to the maximum density tested (75 urchins per 10,000 cm 2 tank surface area; Craggs et al, 2019).…”

“…Relying on snails rather than manual cleaning of these surfaces may have resulted in more food resources and lower rates of mortality. Toh et al (2013), Villanueva et al (2013), and Craggs et al (2019) are the only other studies we are aware of to test grazing herbivores in ex situ culture of coral sexual recruits, albeit in Pacific rather than Caribbean species. However, herbivory as a tool for ex situ culture of A. cervicornis asexual fragments has also been explored (Serafy et al, 2013).…”

“…Recovery of A. cervicornis will require scalable sexual reproduction to foster genetic diversity within restored populations (Hunt and Sharp, 2014). In recent years, great advancements in ex situ sexual propagation of corals have made this a viable restoration strategy (Nakamura et al, 2011;Chamberland et al, 2016;Craggs et al, 2019). Thus, improving grow-out methods for sexual recruits in land-based nurseries is becoming increasingly relevant to the active restoration of A. cervicornis and maintenance of genetic diversity in out-planted populations (Hunt and Sharp, 2014).…”

“…Algae stresses recruits and inhibits growth, making it a factor that must be addressed in culture scenarios (Toh et al, 2013). Grazing herbivores may help control algae to maintain systems and improve coral development (Serafy et al, 2013;Toh et al, 2013;Villanueva et al, 2013;Craggs et al, 2019). This interaction mirrors natural systems, where herbivores play an important role in controlling benthic algal succession by maintaining small epilithic algae in a grazed state (Lirman, 2001;Suchley and Alvarez-Filip, 2017).…”