Reproduction and recruitment
Methods to help re-establish corals in degraded areas are being assessed, including the potential to distribute corals with greater survival traits onto degraded reefs. The techniques comprise various combinations of:
- larval seeding- enhancing the reproductive success, or the rate of coral larval settlement and growth, on reefs
- fragmentation - similar to plants, corals can be broken into small fragments, with each section growing into an adult coral. As coral fragments grow, they, in turn, can be repeatedly fragmented as required.
Larval seeding aims to speed up the return of coral cover to a disturbed or damaged reef by increasing the number of available coral larvae for natural settlement, particularly in cases where the reef has a low level of larval supply (eg following a large-scale bleaching event). This larvae could either come from wild populations – collected and relocated from other regions – or could be cultured through an aquaculture process, potentially selected to have desirable traits such as enhanced heat tolerance.
This technique has been successfully trialled on small scales, mostly using the larvae of a single species, spawned and reared in an aquarium setting. Natural wild coral larval slicks – collected from annual mass spawning – have also been successfully relocated onto reef surfaces, within fine mesh enclosures to prevent the slick from dispersing.
Harvesting natural slicks is likely to have minimal negative impact on Reef ecology, as the mortality rate of the larvae in a slick is naturally high. The method has the potential to capture a diverse suite of species, and allow the re-establishment of reef communities, compared with re-seeding just a few species. While coral spawn naturally travels long distances between reefs, relocation of both wild and cultured corals would require measures to ensure the introduced coral did not harm the local population. More research is needed into when and where slicks form, whether the wild larvae can be cultured, moved and re-distributed without negative effects on receiving populations. Such negative effects could occur if, for example, there is species bias within the slicks, or if pathogens and parasites are attracted to or retained by the moving process.
Further research is also needed to devise methods to scale-up larval seeding for dispersal over larger areas. It is likely this method would be used in conjunction with some form of reef surface conditioning, to increase the subsequent success of released larvae to settle onto the reef.
Photo by Marie Roman
Collecting coral fragments and redeploying
Coral fragments are harvested from donor reefs and transplanted onto degraded reefs; or coral colonies under threat (usually from dredging or construction activity) are relocated. For this method to be successful, the donor reefs need to be healthy enough to withstand harvesting, and the receiving reefs needs to have conditions favourable for coral growth and reef establishment.
Success is likely to depend on the size and health of the fragments, the methods of transport and attachment, and external factors such as the environmental conditions following the transplantation. Often fragments that have already been dislodged through a disturbance – ‘corals of opportunity’ – are used. Monitoring of the long-term results of coral fragment transplantation is required.
Fragmentation is an asexual propagation method. If used on a large scale, care is required to ensure genetic diversity is not unacceptably reduced.
Photo by Christian Miller
This newly-developed fragmentation method more rapidly creates adult corals through a process similar to skin grafting. Its use is currently focused on massive corals - with slow growth and thicker skeletons.
Only the outer layer of a massive coral is living tissue, and these species can spend up to 50 years building a skeleton before reaching a sufficient size to be able to reproduce. If coral fragments from the same parent are attached to a shaped structure, they can rapidly grow and fuse together to form a large colony - a ‘coral skin’- of a size to become sexually reproductive in as short a time as two years.
To date this method has been focused on growing massive corals, however it has potential to be adapted to a mass production aquaculture process.
Photo by Dr David Vaughan, MOTE Marine Laboratory
This method involves farming corals on a large scale using either sexual or asexual reproduction. In the sexual reproduction method, larvae are produced from captive coral ‘broodstock’, settled and grown, then transferred to a reef. This method has good prospects to be scaled-up through automation and mass production, and is a significant RRAP focus.
Options to combine sexual reproduction with asexual micro-fragmentation, and novel deployment devices, to increase coral production rates and reduce costs, are also under active investigation.
Photo by Christian Miller