Aspects of the ecology and pathology of Stephanochasmus baccatus (Nicoll, 1907) Digenea; Stephanochasmidae

Abstract

The digenean Stephanochasmus baccatus (Nicoll, 1907) is a parasite found in marine flatfish which are of importance to the mariculture industry in Scotland. In the area of study, Loch Ewe, on the North-west coast of Scotland, the first intermediate hosts are the gastropods Buccinum undatum (L. ) and Neptunea antiqua (L. ). Several species of teleost fish of the Order Heterosomata act as second intermediate host and the definitive host is Eutrigla gurnardus (L. ) or Myoxocephalus scorpius (L. ). Aspects of the host/parasite relationships at all the host levels were investigated. Samples of B. undatum and N. antiqua were collected from May to September and the incidence of the natural infections of the major digenean species were recorded. In some samples, height, weight and copulatory organ length were measured with a view to finding an indicator of parasitism. The histopathology of S. baccatus in the molluscan host was described and seasonal changes discussed in relation to observed variations in the infectivity of cercariae to second intermediate hosts. Groups of 1+ fish were experimentally infected with cercariae at intervals from May to September when no more S. baccatus infections were found in the molluscs. Differences in the infectivity of the cercariae during these months were demonstrated. The pattern of glycogen deposition in both the molluscan digestive gland-and the intra-molluscan larvae were studied in an attempt to explain the loss of infectivity in cercariae in July and August. A. reduction in infectivity of cercariae after ageing was demonstrated experimentally by infecting fish with cercariae which were freshly dissected out and with cercariae kept for 10 hours at 12°C. This loss of infectivity was correlated with loss of glycogen from cercarial tails. A variety of I-group intermediate host species were infected experimentally and the distribution of cysts in the body, fins, skin and muscle were compared with the turbot, Scophthalmus maximus (L. ) and the plaice, Pleuronectes lap tessa L. These were shown to differ significantly. Four species of intermediate host which are of culturable significance, S. maximus, P. platessa, the common. dab, Limanda limanda (L. ) and the Dover sole, Solea solea (L. ) were experimentally infected and the tissue responses to invasion and encystment of S. baccatus were examined sequentially. The inflammatory response and metacercarial growth in each species of fish were found to differ in some important aspects which affected the subsequent viability in the definitive host. The viability of metacercariae from P. platessa and S. maximus were tested by controlled infection of the definitive host M. scorpius. A number of wild caught definitive hosts were examined for natural infections. An attempt gras made to describe the seasonal cycle of events in the life history of S. baccatus based on the information from this and other studies. The relative significance of different host species in different geographic locations was discussed.