Effects of depth and microhabitat on growth and survivorship of juvenile corals in the Florida Keys

Despite the well known discrimination among substrata by settling larvae of benthic marine invertebrates, there is limited evidence showing that recruits benefit from the microhabitats selected. To explore the fitness consequences of substrate selection by scleractinian corals, this study was designed to test for selective advantages arising from the microhabitat location of juvenile corals on natural substrata on Conch Reef (Florida Keys). Juvenile corals (ca. 2 to 40 mm diameter) were censused for density and microhabitat at 14, 17, 20 and 26 m depth, and a subset were tagged to measure growth and survivorship. Microhabitats were scored as exposed (upward facing), vertical or cryptic, and the results were analyzed first for all taxa combined, and second by life-history strategy in order to contrast 2 ecologically distinct groups (i.e. brooders versus broadcasters). The majority (>53%) of juveniles occupied exposed microhabitats at all depths, with the remainder mostly in vertical instead of cryptic microhabitats at less than or equal to17 m, and in cryptic instead of vertical microhabitats at greater than or equal to20 m depth. Overall, juvenile distribution was unrelated to life history, and the abundance of brooders and broadcasters was largely similar across depths (regardless of microhabitat). Despite clear evidence of non-random distribution of juvenile corals, their growth and survivorship were statistically indistinguishable among the microhabitats or depths. The disparity between these results and evidence that coral larvae actively select specific microhabitats for settlement could reflect (1) a strong size-dependency of the fitness consequences of microhabitat, (2) a consequence of reef degradation such as a modification of the selective value of microhabitats, or (3) reduced exploitation competition for settlement locations caused by low rates of coral recruitment.