Social structure and space use in a wild population of the Australian short-headed seahorse Hippocampus breviceps Peters, 1869

This paper presents the first information available from repeated field observations of a wild Hippocampus breviceps population, and an uncommon example of mixed-sex social grouping in seahorses. At two study sites in Port Phillip Bay, Victoria, Australia, adult seahorses were found at a mean density of 0.21 seahorses per m2 over rocky reef–algal habitat. Each site had a sex ratio of 1 : 1. Seahorses showed varying degrees of site fidelity, with 12 of 38 adults present in the study area throughout the 5-week study period. Adults at the sites moved within small (1–12 m2) and overlapping (with 2–10 others) spatial areas, with most seahorses restricting their movements in the early morning to a smaller core area. Females used significantly larger spatial areas and core areas than did males. Core areas generally coincided with particular seaweeds at which stable groups of two to five seahorses would be found each morning. These core areas were commonly the site of social encounters. Adult seahorses engaged in displays and interactions with opposite-sex partners, although not every day, and not necessarily with the same partner at each encounter. Longer-term observations over a larger study area are necessary to characterise H. breviceps’ movement patterns and mating system more precisely.