Larval settlement rates and gene flow of broadcast-spawning (Acropora tenuis) and planula-brooding (Stylophora pistillata) corals

摘要

Larval settlement rates, genetic structure, and gene flow of broadcast-spawning (Acropora tenuis) and planula-brooding (Stylophora pistillata) corals (Scleractinia) were compared within a 500 km range in the Ryukyu Archipelago. We conducted a laboratory experiment to investigate planula settlement rates, and a broad sampling survey to determine genetic variation in both species in the Archipelago. In the laboratory experiment, the planulae of S. pistillata settled a few hours after release, while those of A. tenuis started to settle at least 4 d after the release of gametes. The survival rates and competency periods of larvae were higher and longer for A, tenuis than for S. pistillata. These results suggest that broader dispersal is more likely for A. tenuis than for S. pistillata. In the population genetic analysis, we measured local (2 stations in a region) and regional (Okinawa, Kerama and Yaeyama) patterns of genetic variation with allozyme electrophoresis. We also inferred the levels of gene flow in the 2 species. In the study area, gene flow (N_em) and genetic distance (D) were, respectively, higher and smaller for the spawner A. tenuis (N_em = 3.5 to 16.4, D = 0.028 to 0.187) than for the brooder S. pistillata (N_em = 0.9 to 1.5, D = 0.026 to 0.309). Therefore, the planulae settlement rates were well in agreement with gene flow. In addition, for both species, N_em between the Okinawa and Kerama regions (30 to 150 km apart; N_em = 9.4 to 22.5 in A. tenuis and 1.4 to 3.3 in S. pistillata) was higher than that between the Okinawa-Kerama and Yaeyama regions (up to 500 km apart; N_em = 3.1 to 9.4 in A. tenuis and 0.5 to 1.4 in S. pistillata). The results suggest that coral populations in the Kerama Island are a major source of the coral planulae needed for the recovery of both brooding and spawning coral communities around the Okinawa Islands, after the mass-bleaching event in 1998.