Immigration history of amphidromous species on a Greater Antillean island

Aim To use molecular data to test for dispersal structuring in the immigration history of an amphidromous community on an island. Location The Caribbean island of Puerto Rico. Methods Mitochondrial DNA sequences were obtained from 11 amphidromous species, including shrimps, fish and a gastropod, sampled from throughout the island. The timing of population expansion (T(E)) in each species was calculated using nucleotide variation and molecular clock dating methods. The order of species accumulation was then reconstructed (oldest to most recent estimate for T(E)), and groups of species with non-overlapping estimates for T(E) were identified. The temporal span and average immigration rate for each group were calculated and compared with expectations of two previously published models of island immigration [the 'dispersal-structured model of island recolonization' (Whittaker & Jones, Oikos, 1994, 69, 524-529), which predicts short phases of rapid immigration followed by extended phases with relatively slow immigration rates; and the 'colonization window hypothesis' (Carine, Taxon, 2005, 54, 895-903), which suggests that opportunities for island colonization are temporally constrained to discrete waves of colonization]. Results The molecular data indicated the immigration history of Puerto Rican amphidromous fauna from the late Pleistocene through the Holocene and identified two groups of species with non-overlapping estimates for T(E) and one group that overlapped with the other two groups. The temporal span, average immigration rate and lack of discreteness between all three groups indicated a continuum of immigration rather than distinct phases of species arrivals. Main conclusions This study did not support the expectations of the immigration models and suggested that amphidromous species from Puerto Rico comprise a single class of marine-based dispersers. The immigration sequence we report probably reflects a recolonization chronology in this community, in keeping with the notion of species turnover through time. Four areas of future research into the immigration history of amphidromous species on islands are identified, and indicated the possibility that equilibrium processes govern long-term community change in amphidromous biota on islands.