"Varied spawning patterns of reef corals in Nha Trang Bay, Vietnam, Western South China Sea"

Abstract

Nha Trang Bay is located in the south-central waters of Vietnam and forms part of the most diverse region within the western South China Sea. Oceanographic studies indicate that this coastal area is not influenced by cold water masses from the north in winter but is subjected to summer upwelling. Coral spawning patterns of four species were monitored by collecting samples of colonies at Hon Mieu Island (12◦11’37.31"N and 109◦14’1.82"E) and two nearby sites during 2016–2019. Mature eggs were recorded inside polyps Acropora florida and A. robusta branches sampled prior to and after the full moon of late March to early April in 2018, and March in 2019. Spawning for both species was inferred to have occurred 6–8 nights after full moon in early April 2018, and 2–3 nights after full moon for A. robusta in March 2019. Colonies of the tabulate coral A. hyacinthus were inferred to have spawned between 6–11 nights after the full moon of March 2019, up to six nights after the spawning of A. robusta. In contrast, the spawning time of the massive coral of Dipsastraea pallida occurred one month later than that of branching Acropora corals, during the period before and after the full moon of late April to early May in 2018, and 4–5 nights after the full moon in April 2019. Monitoring of the ratio of pigmented mature eggs over time and inferred spawning periods of the three Acropora species and D.pallida indicated that spawning of corals in Nha Trang Bay occurs over an extended spawning period from March to May. Therefore, spawning in these species does not appear to be concentrated during a short mass spawning period over a few nights.