Coral restoration projects are increasingly focusing on sexual reproduction to boost genetic variation, particularly after disturbances. Many restoration projects use ceramic settlement tiles as recruitment substrates, but the composition of these tiles is rarely investigated as a means of promoting the growth of early-settlers. To test whether different cementitious materials increase settlement or promote growth, we created tiles using nine cementitious mixes. We conducted a settlement choice assay with larvae of the Caribbean species Orbicella faveolata. Settlement varied across mixes, with 51.8% of total living settlers attaching to FA60 (28.5% Portland cement, 43% fly ash, 28.5% water) tiles while the next closest mixture (SL60: 28.5% Portland cement, 43% slag, 28.5% water) had only 13.5% of total settlers. The least successful tile type in the settlement experiment was AAS1 (67% slag, 7% sodium hydroxide, 26% water) with only 1% of total living settlers. Next, we tested whether chemical compositions of the tiles affected growth rates of recruits. Pictures were taken of individual polyps biweekly under a dissecting microscope and area measurements calculated using ImageJ software. Recruits on FA60 tiles grew fastest among all tile types (2.2x faster than the group mean of nine treatments) and recruits on the AAS1 tiles grew the slowest (18% reduction in size due to major skeleton and tissue loss). We hypothesize the FA60 mix (and potentially other related mixes) may change the water chemistry in the boundary layer surrounding these cementitious materials, therefore enhancing larval settlement and growth, with potential applications for reef restoration efforts.
