Aspects of the morphology and life cycle of Lecithocladium excisum (Digenea, Hemiuridae), a parasite of Scomber spp.

Previously undescribed motile cystophorous cercariae which develop in sporocyst-like germinal sacs in the bullomorph opisthobranch Philine aperta are experimentally shown to develop into Lecithocladium excisum (Digenea, Hemiuridae), a common stomach parasite of mackerel Scomber scombrus in the north-east Atlantic and adjacent seas. Between 3 and 8% of P. aperta from the northern Oresund, Denmark, were infected with cercariae of L. excisum. Ninety per cent were infected with Rhopalura sp. (Orthonectida). Copepods of the genera Acartia, Paracalanus, Pseudocalanus, Eurytemora and Oithona were experimentally infected. Pressure exerted by their mouth limbs caused delivery tube eversion and the injection of the cercarial body into the copepod haemocoel. The metacercariae did not grow in the above mentioned hosts at 12 degrees C, but 590 microns long metacercariae developed within 22 days in laboratory-reared A. tonsa at 18 degrees C. The ctenophore Pleurobrachia pileus and the holoplanktonic polychaete Tomopteris helgolandica, which were found to be naturally infected with metacercariae of L. excisum, may act as transport hosts.