Abstract
Due to difficulties with surveying, cryptobenthic fishes are understudied, which limits assessments of their extinction risk. Using a novel survey method (a combination of underwater visual census and underwater biofluorescence census), we surveyed the densities of two gorgonian seafan-associated pygmy seahorses ( Hippocampus bargibanti and H. denise ) in 63 sites across Indonesia. Densities were low for both seahorses (≤ 0.4 seahorses/host) and their hosts (≤ 0.25 seafans/200 m 2 ). These low densities, combined with habitat specialisation, increase extinction risk. We failed to detect regional or depth patterns in seahorse abundance per host or group size in occupied hosts. Host occupancy rates were low (< 13.7%), suggesting that other factors than habitat availability constrain pygmy seahorse population size. The low densities found in this study highlight the high extinction vulnerability for habitat specialists on coral reefs.