Transplanting Coral Fragments in Close Contact Enhances Their Survival and Growth on Seawalls

Accelerated urbanisation has replaced many natural shorelines with coastal defences, resulting in the loss of natural habitats. However, structures such as seawalls can support some biotic assemblages, albeit of lower species richness. Ecological engineering techniques such as coral transplantation can enhance biodiversity on these artificial structures, but its success is circumscribed by high costs. Little is known about the fusion of discrete coral colonies that could potentially improve coral transplantation success on seawalls, particularly for the slow-growing massive species that are generally well-adapted to living on seawalls. Here, we investigated the feasibility and cost-effectiveness of transplanting Platygyra sinensis on seawalls by comparing the survivability and growth of fragments transplanted adjoining with those transplanted further apart. Fragments (approximately 3 cm diameter; n = 24) derived from three individuals were randomly grouped into two treatments, transplanted at 0.5 cm and 5 cm apart. Fragments in the former treatment came into contact with each other after three months. We observed that in all cases, the contact zones were characterised by a border of raised skeletal ridges without tissue necrosis, often termed nonfusion (=histoincompatible fusion). The adjoining transplants showed better survival (75 vs. 43%) and grew at a rate that was significantly higher than fragments transplanted 5 cm apart (3.7 ± 1.6 vs. 0.6 ± 1.1 cm2 month−1). Our projections demonstrated the possibility of reducing transplantation cost (USD cm−2) by 48.3% through nonfusion. These findings present nonfusion as a possible strategy to increase the overall cost-effectiveness of transplanting slow-growing massive species on seawalls