Studies on the life history of Bucephalus longicornutus (Manter, 1954) (Trematoda: Bucephalidae) and the biological control of its sporocysts in Ostre

Abstract

Previous life history studies of the family Bucephalidae Poche, 1907 (Trematoda : Digenea) are reviewed and the major features of the life history of a marine member of the family are confirmed experimentally for the first time. The sporocyst, cercaria, metacercaria and adult of Bucephalus longicornutus (Manter, 1954) (synonym: Alcicornis longicornutus Manter, 1954) are described. No redial generation occurs in the life history. Attempts to obtain miracidia from the eggs of experimentally induced adults were unsuccessful. Sporocysts, which give rise to free-living cercariae, were recovered from the visceral mass, gills and pericardium of the New Zealand mud-oyster, Ostrea lutaria Hutton. Mature, encysted metacercariae were recovered only from the fin web, muscles, orbit and branchial chamber of experimentally infected specimens of Tripterygion sp. and Acanthoclinus quadridactylus (Forster). Adult flukes were recovered from the intestine of Scorpaena cruenta Richardson, 35 days after feeding nature metacercariae, and from the intestine and pyloric caeca of Kathetostoma giganteum Haast, the definitive host. Development of the cercaria is described. Germinal lineage was not traced into the cercarial generation and may not occur in this phase of the life history. All previous reports of bucephalid cercariae are listed and the erroneous practice of assigning these to adult genera without experimental evidence is corrected. The number of cercariae liberated from oysters show that peak liberations of up to 10,000 cercariae per oyster per day occur intermittently with lulls during which few or no cercariae are liberated. Behaviour of the furcae of the cercaria to mechanical shock is compared with other species and it is postulated that the type of behaviour exhibited in a given cercaria may reflect its generic identity. Attachment and penetration of cercariae to small fish depends initially on accidental contact of the furcae, attachment of the tail stem and penetration with the aid of acidic secretions of the cystogenous organ. The initial cyst wall is formed by the breakdown and exudation of the cystogenous granules governed by osmotic phenomena. Cercariae are host specific towards Trachelochismus sp. Age immunity of the experimental hosts was found. Metacercariae mature in 75 to 80 days. They are host specific towards Helcogramma medium (Forster) in which development does not proceed beyond 20 days. Considerable variation in total length, egg size and relative position of internal organs is exhibited by adults recovered from a number of specimens of K. giganteum. These variations parallel those exhibited by B. varicus Manter, 1940. Possible taxonomic implications are that many species of Bucephalus, hitherto differentiated by these variable characters, will be reduced to synonymy. Sporocysts cause parasitic castration of O. lutaria. The results of mortality experiments and seasonality in the intensity of the infection show that death of infected oysters occurs but it is considered that many deaths are caused by secondary factors. The incidence of infection from seven localities in New Zealand, with particular reference to Foveaux Strait, where upwards of a 40% incidence occurs in areas A and B, is given. A new species of haplosporidian hyperparasite causes a 100% mortality of the embryonic cercariae in the sporocysts of B. longicornutus from oysters obtained from north of Tasman Bay. Ecological factors and the difficulty of collecting large numbers of the hyperparasite may preclude its introduction to the Foveaux Strait oyster beds as a biological control for B. longicornutus.