On the morphology and life-history ofOpechona bacillaris(Molin, 1859) Looss, 1907 (Trematoda, Lepocreadiidae)

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Abstract
The life-cycle of the digenetic trematode Opechona bacillaris (Molin, 1859) Looss, 1907 (fam. Lepocreadiidae Nicoll, 1934, subfam. Lepocreadiinae Odhner, 1905) is described. It is shown experimentally that ophthalmotrichocercous cercariae which develop in the marine prosobranch Nassarius pygmaeus (Lamarck) penetrate the ctenophore, Pleurobrachia pileus, a chaetognath, Sagitta sp., and small medusae. Here they develop into the metacercaria of O. bacillaris, which previously has been described from a large number of planktonic invertebrates. The redia and cercaria of O. bacillaris are described. The cercaria and metacercaria are compared with immature O. bacillaris from a naturally infested lumpsucker, Cyclopterus lumpus, by means of the stereoscan electron microscope. The infestation percentage of the snail host population from the northern Øresund was determined at intervals over a period of nearly two years. During winter most infestations comprise only rediae, whereas during summer nearly all infested snails contain emerging cercariae. The mating organ of male N. pygmaeus harbouring mature cercariae is highly reduced in size, and both sexes are castrated by the parasite. The shells of N. pygmaeus are covered with the hydroid Podocoryne carnea Sars, which often ingests recently emerged cercariae. The life-history of O. bacillaris is compared with that of other trematodes of the subfamily Lepocreadiinae.