Extreme habitat specialisation and population structure of two gorgonian-associated pygmy seahorses

Pygmy seahorses are a group of little-known miniature hippocampid fish that differ considerably in biology and ecology from their larger congeners. We estimated the population density, sex ratio and habitat of 2 sympatric, obligate gorgonian-associated pygmy seahorses, Hippocampus bargibanti and H. denise, in a 20 km long coastal marine protected area in southeast Sulawesi, Indonesia. Belt transects covering 200 m(2) each were surveyed at 7 sites and 5 depth contours to record the density of seahorses and their host gorgonians. The population density (+/- SE) was 1.17 (+/- 0.27) per 200 m(2) for H. denise and 0.34 (+/- 0.20) per 200 m(2) for H. bargibanti, some of the lowest densities for unexploited seahorse populations studied thus far. Male-female pairs (43.9%) were the most common group composition for H. denise, with single, 3 or 4 individuals found on 19.5, 7.3 and 29.3% of inhabited gorgonians, respectively. H. denise inhabited 7.8% of Annella reticulata gorgonians within the survey area but were recorded from a total of 8 gorgonian genera during extensive opportunistic ad hoc searches. Annella spp. density was 10.7 times higher than that of Muricella spp. (the sole host of H. bargibanti), of which 20.0% were inhabited. The small population size, occurrence of pygmy seahorses in groups on their hosts with the resulting skew in sex ratios, and habitat specialisation likely all impact the species' population dynamics, and hence these need to be considered in conservation management strategies.