Vertical distribution and settlement competencies in broadcast spawning coral larvae: Implications for dispersal models

Successful larval dispersal between coral reef populations is one of the key determinants of coral reef connectivity. Dispersal patterns are generally predicted using hydrodynamic models, but these are frequently constrained by a lack of information on larval biology. This study examines some of the life history traits that may affect coral larval transport and hence be useful for dispersal models, in particular their settlement competency periods and vertical distribution in the water column, which we hypothesized would occur synchronously. The settlement competencies of the larvae of two species of broadcast spawning scleractinian corals, Pectinia lactuca and Platygyra sinensis were followed for up to 16 d. Larvae of both species were able to settle within the first 2 d after the spawning event. Peak settlement of P. lactuca larvae was shortly after day three post-fertilization with 71.6 ± 4.8% of live larvae being able to settle, while peak settlement of P. sinensis larvae was delayed till the sixth day with 85.1 ± 4.0% of live larvae settling. Given their later peak settlement and extended settlement window, larvae of P. sinensis potentially have a larger dispersal potential than P. lactuca, which is consistent with the geographical ranges of the two species. Changes in the daytime depth distribution of larvae of three species of broadcast spawning scleractinian corals, P. lactuca, P. sinensis, and Acropora hyacinthus were followed using water-filled 2 m vertical tubes. A downward shift over time in the vertical distribution of all three species was observed, which could affect larval transport patterns due to the possible stratification of horizontal currents. Sinking rate, however, was not necessarily consistent with their peak settlement competency periods. Such inconsistencies between settlement and sinking suggest that larvae may not settle even when they are capable of doing so due to their vertical distribution in the water column. This has implications for dispersal models which traditionally rely on settlement competencies, as larval vertical position may alter the timing of settlement. ► Larval settlement competencies of 2 species of broadcast coral species were tested. ► Larval depth distributions were also quantified in 2.2 m vertical tubes. ► Downward shifts over time were inconsistent with peak settlement competencies. ► Larval dispersal models should not rely on settlement competency periods alone.