Studies on Entobdella hippoglossi (Muller, 1776) (Monogenea) and Lepeophtheirus hippoglossi (Kroyer, 1837) (Copepoda) : ectoparasites of Atlantic halibut (Hippoglossus hippoglossus L., 1758).

Abstract

This study has elucidated aspects of the biology of two ectoparasites of Atlantic halibut (Hippoglossus hippoglossus). The monogenean Entobdella hippoglossi can live for up to 146 days as an adult with a complete life span of up to 250 days. It becomes mature and commences egg production at 9mm in length but can reach 21mm. Egg production increases with size. A mean of 186 eggs can be laid in vitro and 122 in vivo over 24hours. Eggs take 27-54 days to hatch at 12°C and 5°C, respectively. Juvenile Entobdella were observed on halibut 85-93 days (8-11°C) after infection with adult parasites. Juvenile (<10mm) Entobdella were found on the tail and dorsal surfaces of the halibut whilst adults (>10mm) were usually found on the anterior ventral portion of the body and head. They caused significant haemorrhagic lesions, particularly when congregated. A timing regime for formalin bath treatments was devised. The life-cycle of the caligid copepod Lepeophtheirus hippoglossi consists of 10 stages and is completed from attachment to egg production in 29 days at 13°C and 62 days at 7.5°C. The timing for each of the life stages was ascertained at 13.1, 9.7 and 7.5°C. The morphology of each of the 10 stages is described. Settlement of the copepodid stage occurred mainly on the fins and body of the halibut and the motile stages were found on the dorsal anterior portion of the fish, particularly around the abdominal cavity. The relationship between E. hippoglossi and L. hippoglossi and the structure of the skin they parasitise is discussed. Severe haemorrhagic lesions occurred with as few as 25 adult female L.hippoglossi on halibut of more than one kilogram. An in-feed treatment emamectin benzoate (SLICE) was trialed as a control method for L.hippogloss and up to a 96.5% reduction in lice numbers was obtained.