Distribution of an asymmetrical copepod, Hatschekia plectropomi, on the gills of Plectropomus leopardus

Hatschekia plectropomi , an ectoparasitic copepod found on the gills, infected Plectropomus leopardus from Heron Island Reef with 100% prevalence ( n  = 32) and a mean ±  s . e . infection intensity of 131·9 ± 22·1. The distribution of 4222 adult female parasites across 32 individual host fish was investigated at several organizational levels ranging from the level of holobranch pairs to that of individual filaments. Parasites demonstrated a site preference for the two central holobranchs (2 and 3). Along the lengths of hemibranchs, filaments near the dorsal and ventral ends and those in the proximity of the bend region were rarely occupied. The probability of coming into contact with a suitable attachment site and the ability to withstand ventilation forces at that site were proposed as the major factors affecting distribution. Two H. plectropomi morphotypes were identified based on the direction of body curvature. Regardless of morphotype, 99·9% of individuals were attached such that the convex side of the body was oriented towards the oncoming ventilating water currents. Further, 93·3% of individuals attached to the posterior faces of filaments, leading to a predictable pattern of attachment for this species. It is suggested that the direction of body curvature develops in response to the direction of the ventilating water currents.