High diversity, but low abundance of cryptobenthic fishes on soft sediment habitats in Southeast Asia

Abstract

Cryptobenthic fishes play a crucial role in marine ecosystems as trophic links between the base of the food chain and higher-level consumers. Infralittoral soft sediments are the largest marine habitat, yet little is known about fish assemblages in these ecosystems. This study investigates the cryptobenthic fish abundance and diversity on soft sediment habitats in the centre of tropical marine biodiversity. We surveyed 20 sites across three regions in Indonesia (Bali, Lembeh Strait) and the Philippines (Dauin) using Underwater Visual Surveys (UVC) and roving diver surveys. We tested the effects of depth (6 m, 16 m), benthic cover and sediment grain size characteristics on fish assemblages. Our results showed a high diversity (112 species), but low abundances (mean: 93 individuals/500 m2 ± SE: 28 ind.). Benthic cover on surveyed sites consisted for 90.1% (±SE: 0.7%) of unconsolidated sediments, which were predominantly poorly sorted (s = 0.975 f), gravelly sand. PERMANOVA analyses showed that fish assemblages and fish diversity was significantly different between regions (p < 0.001) and depth (p = 0.002). Distance based linear Models (DistLM) explained respectively 25% and 33.5% of the variation in fish assemblages and fish diversity. The high cryptobenthic fish diversity found in this study exceeds that of many coral reefs and contradicts the current view of soft sediment fish communities as depauparate ones. Our results provide valuable insights in a poorly studied marine ecosystem and call for more research in these valuable habitats.