Gnathia aureamaculosa, a likely definitive host of Haemogregarina balistapi and potential vector for Haemogregarina bigemina between fishes of the Great Barrier Reef, Australia.

Investigations to determine whether juvenile gnathiid isopods are vectors of haemogregarines between coral reef fishes were undertaken at Lizard Island, Australia. Haemogregarina balistapi parasitaemias in triggerfish, Rhinecanthus aculeatus, decreased under gnathiid-free, laboratory conditions, compared with those in tagged R. aculeatus returned to the reef. Gnathia aureamaculosa juveniles were common ectoparasites of reef R. aculeatus and laboratory reared specimens of this gnathiid were fed on R. aculeatus infected with H. balistapi. Subsequent squashes of this gnathiid contained haemogregarine gamonts similar to those seen in blood films of R. aculeatus, and haemogregarine developmental stages, including oocysts, sporozoites, meronts and merozoites. Biological transmission of H. balistapi and a second haemogregarine species, Haemogregarina bigemina, using laboratory reared gnathiids to several species of triggerfishes and surgeonfishes raised from larvae was then attempted. Investigations involved recipient fish ingesting, or being bitten by, G. aureamaculosa juveniles fed on donor fish with haemogregarines; control fish were exposed to gnathiids fed on uninfected donor fish. Subsequently, no haemogregarines were detected in recipient triggerfishes and controls were negative. However, a recipient surgeonfish, Acanthurus xanthopterus, which had ingested gnathiids likely infected with donor fish H. balistapi, carried H. bigemina-like stages. A second recipient surgeonfish, which had ingested gnathiids presumed to be infected with H. bigemina, also carried haemogregarine stages. Finally, a third surgeonfish apparently carried haemogregarines after gnathiids presumed to be infected with H. bigemina had bitten this fish, although not all gnathiids were recovered during the trials and the third infected surgeonfish may have also ingested gnathiids. The study provides strong evidence that G. aureamaculosa is the definitive host of H. balistapi, to our knowledge the first such observation from a coral reef environment. Although transmission of H. balistapi has not yet been demonstrated, laboratory trials tend to support the view that G. aureamaculosa is also a potential vector of H. bigemina between surgeonfish.