METAZOAN PARASITES OF THE SNAPPER, PAGRUS-AURATUS (BLOCH AND SCHNEIDER, 1801), IN NEW-ZEALAND .2. SITE-SPECIFICTY

The metazoan parasites of the snapper, Pagrus auratus, collected in th e Hauraki Gulf of New Zealand were surveyed and their distribution map ped and examined in relation to the age of the host. Nine species of e ctoparasites and seven species of endoparasites were recorded and all were found to exhibit some degree of site-specificity. The gill monoge neans Lamellodiscus pagrosomi and Bivagina pagrosomi were found in wel l-defined microhabitats on the gills. L. pagrosomi was abundant on the middle section of the second gill arch and was found in greater numbe rs on the distal zone of the gill filaments of the external hemibranch . In contrast, B. pagrosomi was particularly abundant on the first gil l arch and on the basal zone of gill filaments of the external hemibra nch. A third gill monogenean, Choricotyle australiensis, was found att ached to the gill filaments in snapper of 3 years and older but was la rgely restricted to the buccal cavity in younger host fish. This sugge sts that site-specificity may not be a static attribute but may alter along with other parameters of the host fish such as age and size. The gastro-intestinal parasites were also found to exhibit habitat partit ioning and segregation into specific niches with a highly site-specifi c digenean, Diphtherostomum sp., occurring almost exclusively in the l umen of the rectum.