Closing the circle: is it feasible to rehabilitate reefs with sexually propagated corals?

Abstract

Sexual propagation of corals specifically for reef rehabilitation remains largely experimental. In this study, we refined low technology culture and transplantation approaches and assessed the role of colony size and age, at time of transfer from nursery to reef, on subsequent survival. Larvae from Acropora millepora were reared from gametes and settled on engineered substrates, called coral plug-ins, that were designed to simplify transplantation to areas of degraded reef. Plug-ins, with laboratory spawned and settled coral recruits attached, were maintained in nurseries until they were at least 7 months old before being transplanted to replicate coral limestone outcrops within a marine protected area until they were 31 months old. Survival rates of transplanted corals that remained at the protected in situ nursery the longest were 3.9-5.6 times higher than corals transplanted to the reef earlier, demonstrating that an intermediate ocean nursery stage is critical in the sexual propagation of corals for reef rehabilitation. 3 years post-settlement, colonies were reproductively mature, making this one of few published studies to date to rear a broadcasting scleractinian from eggs to spawning adults. While our data show that it is technically feasible to transplant sexually propagated corals and rear them until maturity, producing a single 2.5-year-old coral on the reef cost at least US$60. 'What if' scenarios indicate that the cost per transplantable coral could be reduced by almost 80 %, nevertheless, it is likely that the high cost per coral using sexual propagation methods would constrain delivery of new corals to relatively small scales in many countries with coral reefs.

The cost and feasibility of marine coastal restoration

“…For oyster reefs, if survival of restored organisms was not reported, it was estimated based on percent cover of oysters on the reef (Konisky et al 2013) or the percentage of successful reefs out of the number of total reefs restored (Powers et al 2009). For the coral reefs records, where only cost per coral colony was provided (Shafir et al 2006, Garrison and Ward 2008, Shaish et al 2008, Ferse 2010, Levy et al 2010, Nakamura et al 2011, Guest et al 2014, calculations for the restoration cost per area were based on a transplanting schedule with four coral colonies out-planted per m 2 or 40 000 coral transplants/ha (Edwards and Gomez 2007). Accounting for a median survival of 64.5% (averaged over the reported pre-transplant, transplant, and post-transplant survival in the coral reef restoration database section), a total of 54 200 coral transplants would be required to populate one hectare.…”

Building coral reef resilience through assisted evolution

“…The primary purpose of coral nurseries is to grow coral colonies to a size that reduces mortality after transplantation onto damaged or degraded reefs. Coral transplants have a greater chance of survival the larger they are (60). The nurseries offer the advantage of decreased competition for resources (space, light), decreased predation, and suspension above sea-floor sediments.…”

Interspecific Hybridization May Provide Novel Opportunities for Coral Reef Restoration

“…If hybrids are comparatively resilient, interspecific hybridization may be combined with methods being developed for deploying coral larvae or recruits onto reefs requiring restoration (e.g., Nakamura et al, 2011;Omori, 2011;Villanueva et al, 2012;Guest et al, 2014;dela Cruz and Harrison, 2017). Overall, maternal effects were observed in hybrids of the A. tenuis × A. loripes cross and overdominance in hybrids of the A. sarmentosa × A. florida cross, with some variations between traits and treatment conditions.…”

“…Reef restoration is still in its infancy and all of the few successful efforts so far occurred on a small spatial scale (e.g., Nakamura et al, 2011;Omori, 2011;Villanueva et al, 2012;Guest et al, 2014;dela Cruz and Harrison, 2017). Traditionally, locally sourced biological material is used for restoration based on the assumption that these populations are locally adapted and therefore most likely to survive (Breed et al, 2013).…”