Population dynamics and growth rates of free-living mushroom corals (Scleractinia: Fungiidae) in the sediment-stressed reefs of Singapore

Abstract

The reefs of Singapore provide an excellent opportunity to study the population dynamics and growth rates of free-living mushroom corals (Fungiidae) under sediment stressed conditions. Transect surveys at four study sites revealed a total of 11 free-living mushroom coral species―the same 11 species as those found by local studies since the 1980s. The abundance of the four most common species ranged from 1.0 to 68.3 Ind. per 100 m2, while their population size-structure showed a common pattern of a higher proportion of small-sized corals than large-sized ones (i.e. positively skewed size-structure), although very few individuals of the smallest-size classes were recorded for any of the four species. A more positively skewed size-structure for each of the four most common species was observed at the reef slope (5-6 m depth) than at the reef crest (2-3 m depth), possibly due to a slower growth rate caused by light reduction with depth. All the mushroom corals studied exhibited a decline in growth rate with increasing size and weight, indicating determinate growth. Growth rate of each of the four most common species was similar among the study sites, despite variation in environmental conditions. Our results demonstrate species richness stability over the past three decades, suggesting that these free-living mushroom coral assemblages comprise species that are well-adapted to the chronic high sedimentation characteristic of Singapore’s reefs. However, if the paucity of individuals of the smallest-size classes reflects poor recruitment and/or early mortality, there may be some cause for concern. Our robust baseline data can contribute to a long-term monitoring strategy for determination of changes in mushroom coral population dynamics.