Spawning a future: assisting coral recruitment though larval propagation

We demonstrated that assisted gene flow between genetically-distinct Caribbean coral populations, using cryopreserved sperm, could be a viable option to help restore coral reefs. We used frozen coral sperm from three genetically-distinct populations of the endangered coral Acropora palmata to fertilize A. palmata eggs collected in Curaçao, producing the first-ever pan-Caribbean coral crosses conducted in a laboratory. The frozen sperm that we used represented three genetically-distinct populations: Western Caribbean (Key Largo, FL), central or mixed Caribbean (Rincon, Puerto Rico) and Eastern Caribbean (Curacao). The Puerto Rico and Florida sperm samples, which had been frozen for up to 10 years, were sent to CARMABI Research Station in Curaçao from the USDA. The experimental crosses with fresh eggs consisted of four treatment groups: CUR (fresh sperm), CUR (frozen sperm), FL (frozen sperm), and PR (frozen sperm). All four categories of crosses were successful. Specifically, fertilization success ranged from 86% to 99% for CUR (fresh sperm), 32% to 88% for CUR (frozen sperm), 1% to 20% for FL (frozen sperm) and 0% to 24% for PR (frozen sperm). Embryos and larvae were reared at CARMABI in 1L polystyrene static bins and 10L conical rearing cones for up to 8 days post-fertilization. Then, thousands of swimming larvae were transported to The Florida Aquarium Center for Conservation and Mote Marine Laboratory for settlement and grow-out. Their respective settlement numbers were approximately 50% to 60%, producing over 10,000 settlers including over 3,500 settlers from CUR (frozen) sperm, 1,247 settlers from FL (frozen sperm), and 233 settlers from PR (frozen sperm). This is the largest living wildlife population ever created from cryopreserved sperm, demonstrating that cryopreservation can be an effective tool for assisted migration, assisted gene flow, and the preservation of genetic diversity to help restore and invigorate coral populations.

Restoration techniques using sexually propagated corals currently allow cost and labor efficient mass production and outplanting of hundreds of thousands of sexual coral recruits. It is becoming clear however that the effectiveness of these restoration efforts is often limited by the fact that many aspects of species’ reproductive biology and early life dynamics are unknown. For example, the reproductive mode and timing of multiple coral species are still undocumented, preventing the sexual propagation of these species. In addition, early life-history traits (e.g., embryogenesis, larval ecology, settlement preferences, post-settlement development) are extremely variable among coral species, and the implications of this interspecific variability for larval culturing are unknown and potentially demand for specie-specific culturing approaches. During the larval stage, larvae of different coral species display different settlement behaviors and prefer different surfaces for settlement. Furthermore, the recruitment success of newly outplanted settlers not only differs widely among species, but also among reef habitats for the same species. Thus, by including these species specific differences in synergy with environmental factors, current coral larval propagation methods could likely be improved. We compiled new and existing data on species-specific traits associated with the reproductive biology and early life ecology of Caribbean corals to exemplify how natural history studies could help widen the number of coral species that can be reared for restoration purposes and increase the long-term survival and growth of recruits after outplanting. We then make species specific recommendations to improve various steps of the larval rearing process, including gamete collection, fertilization, culture maintenance, site selection for outplanting and outplanting techniques. This information will be made available to coral reef researchers and restoration practitioners in the form of Species Fact Sheets, which will be updated as new data becomes available.

Additional co-authors: Gabriel Juarez, Bruce Fouke, Forest Rower, and Linda Wegley. Materials scientists are engineers who study how synthetic and natural materials interact with their environment, whether that be mechanical, chemical, electrical, thermal, optical, magnetic, or even biological. They design materials to perform some intended function in those environments and are credited for major technological advances in the aerospace, automotive, electronic, defense, and even health care industries. Many of the resulting products we make use of everyday. A field to which materials science has not yet significantly contributed, but has the potential to help make significant advances, is in coral restoration science. Specifically, we propose that materials science and engineering can help to tackle the fundamental problem blocking robust coral reef recovery: recruitment and survival of coral juveniles. Corals suffer from widespread recruitment failure: >98% of juveniles raised for restoration die within two years of outplanting. In this talk, I will present work we have done the past 12 years in engineered scaffolds for bone replacement and repair as a particularly relevant example of how materials research might help coral restoration. I will describe how we design, fabricate, and evaluate materials with different compositions and combine structural features at multiple length scales, from microns to centimeters, that encourage a particular biological response, like bone regeneration. Our team aims to apply these principles not only to larval recruitment and settlement, but also to shifting reef systems back toward desirable organisms (corals, coralline algae, herbivores) and away from harmful competitors (pathogens, macroalgae, turf algae, cyanobacteria). By combining materials engineering and coral restoration science, we aim to describe how substrate characteristics affect critical biological outcomes on reefs, including the factors that promote: 1) growth of photosynthetic, non-pathogenic, and other beneficial microbes, 2) coral attraction, attachment, settlement, and calcification, and 3) coral-facilitating organisms such as coralline algae.

The Florida Aquarium has consistently increased success in larval propagation in several species of Western Atlantic stony corals with approximately 50% survival after one year in the most recent years. Culture conditions were optimized through construction of aquarium systems specifically for this purpose at the Center for Conservation, the use of herbivores, regulating light exposure, and routine feeding of recruits. Ongoing projects include rearing of additional species, construction of aquaria to induce spawning in the laboratory, and rearing of recruits produced via cryofertilization and assisted gene flow. Despite the increased cost of larval rearing ex-situ, the ability to maximize survival and early growth may outweigh these costs in some cases. The addition of land-based spawning will allow genetic crosses to be conducted that would not be possible in the wild in areas where populations are greatly reduced.

Acropora palmata is one of the major reef-building species in the Caribbean, however it is considered endangered due to global and local threats. Efforts to remedy its status have recently focused on restoration programs using sexual recruits to maintain or increase genetic diversity. Our objective is to develop low-cost, efficient techniques for upscaling sexual recruit production. We compared the survival, growth and production costs of A. palmata sexual recruits grown under short-term (< one-month) and long-term (1 to 3 years) nursery care. For short-term nursery care, we cultured three successive generations (2015-2017) in ex-situ nurseries, and out-planted them onto a severely degraded reef four-weeks post-settlement on SECORE Seeding Units. For long-term nursery care, we cultured three successive generations in ex-situ nurseries for 1 to 3 years. The colonies were then transferred to in-situ nurseries for one more year to finally be out-planted onto a reef damaged by a ship-grounding. Survival in short-term nursery care was much lower (0.01-0.02%) than for colonies produced under long-term nursery care (95-100%). Nevertheless, coral growth was three times higher after being out-planted onto the reef than under nursery conditions. Additionally, the maintenance costs of corals in long-term nursery care are considerably higher than for those maintained short-term. Upscaling the production of sexual recruits for early-stage seeding holds promise because costs are considerably reduced, however survival rates are extremely low, at least on a degraded reef in the Mexican Caribbean. A combination of ex situ cultivation during the early developmental stages coupled with seeding after the high sea surface temperatures and hurricane seasons are over may improve survival at a moderate cost. Using these techniques, seeding could then be applied at lower costs in remote areas and on a large scale.

To increase genetic diversity, coral restoration needs to use sexual reproduction. However, post-settlement mortality of sexually-produced corals in nature and ex situ nurseries is typically high due to macroalgal overgrowth and sensitivity to physical parameters that are optimal for adults, such as light levels. This study aims to optimize the grow-out of sexually-produced corals in land-based nurseries by testing the effect of three easily applicable techniques in large-scale land-based nurseries: cover of corals with coarse sediment to prevent algal overgrowth and provide light protection; shading; and downward orientation. This experiment is being performed in brooders and broadcast spawning coral species. For Porites astreoides, low light levels (20 µmol photons.cm-2s-1) allowed for significantly faster growth and higher coloration during the first 4-5 weeks. After 4-5 weeks, corals grew faster and survived more at higher light levels (180 µmol photons.cm-2s-1). Downward orientation can be used in the first 4-5 weeks to reduce light levels in tanks with light levels more adequate for adult corals. Sediment reduced algal overgrowth, but after 4 weeks it had a negative effect on growth, potentially because it reduced access to food. Results with broadcast spawning species, including Acropora and Orbicella, will be available at the time of the meeting.

Current outplanting methods for coral restoration require labor-intensive manual transplantation of each coral-substrate-unit using adhesives, nails or cable-ties. To conduct coral restoration at the needed scale, SECORE is co-leading a collaboration (with TNC and California Academy of Sciences; the Global Coral Restoration Project (GCRP)), to develop technologies and implement partnerships to reduce labor and costs, allow application of techniques in remote areas, and expand the spectrum of species restored. We primarily utilize larval propagules that are settled on designed, self-stabilizing substrates (“seeding units, SU”) that are ‘sowed’ on the reef, without artificial attachment. Based on initial tests with tetrapod-shaped substrates, we are currently testing seven new prototype substrates (3d-printed from ceramic) designed to improve survival of recruits and retention of SUs. Another approach to facilitate large-scale restoration at locations remote from land-based infrastructure aims at transferring the production process to an in situ environment. We have developed floating “pools” that are placed in sheltered sites (e.g. lagoon or dock) prior to a coral spawning event. Fertilized eggs resulting from in-situ gamete collection and in-vitro fertilization are placed directly into the pools containing SUs to complete larval development, settlement, and potentially a post-settlement nursery period with minimal labor. Upscaling of larval restoration is also facilitated by practitioner workshops centered around coral spawning events to provide hands-on experience with these developing tools and techniques and create a community of practice. We are cultivating partnerships with local organizations with commitment and capacity to implement larval restoration at expanded locations.

Short video on Project Coral from the Horniman Museum and Gardens.