Results of Infection Experiments with Cercariae

Forty-three of forty-five specimens of Tripterygion sp., ranging in size from 30mm to 50mm total length, and 30 of 40 specimens of Acanthoclinus quadridactylus (Forster), ranging in size from 30mm to 60mm total length, were successfully infected with metacercariae. Those specimens of each of these species which were not infected were 20mm to 25mm greater in total length than the largest specimens successfully infected.

Attachment of cercariae to 143 specimens of Helcogramma medium (Gunther) was observed and encystment took place in 111. However, the metacercariae did not develop. None were found alive in 47 specimens 20 days after exposure to cercariae, and cysts had decreased in size and their contents had degenerated in 25 specimens after 35 days. Subsequently, cysts underwent a further decrease in size and it was assumed that they were in the process of being resorbed. The range in size of specimens from which cysts were recovered was 25mm to 50mm. Seven specimens greater than 60mm in total length, were found to be uninfected five days after exposure to cercariae. The remaining 25 specimens which were found to be uninfected fell within the size range of those in which encystment took place, but they were examined 60 days after exposure to cercariae. In these cases, complete resorption of the cysts was assumed to have taken place.

Attachment of cercariae to two specimens of Trachelochismus sp., both 40mm total length, was observed but metacercariae were not recovered from either specimen six days after exposure to cercariae.

Metacercariae were not recovered from any of the invertebrate species (see p. 5 although attachment to each took place. With these experimental hosts, examination of the soft parts was made three days after exposure to cercariae.

A total of 12 fish died at various intervals after exposure to cercariae but there was no evidence to suggest that this was due to the infection. In fact, fewer cysts were recovered from those that died than from the majority of those that remained page 25 alive. Kniskern (1952) considered that a heavy infection with the metacercariae of Rhipidocotyle septpapillata resulted in death of the host fish.

Three main conclusions can be drawn from the results of these experiments. First, the cercariae show some measure of host specificity in that encystment did not occur in Trachelochismus sp., and the metacercariae show some measure of specificity in that development did not proceed beyond 20 days in H. medium. Secondly, penetration and encystment did not take place in specimens of H. medium, A. quadridactylus and Tripterygion sp. greater than 50mm, 60mm and 50mm total length respectively. This suggests age immunity of the experimental hosts in these cases. Thirdly, invertebrate species are unlikely to be natural hosts of the metacercariae.

A. quadridactylus proved to be more susceptible to infection than Tripterygion sp. Specimens of the former always yielded more cysts than the latter after both had been exposed to similar numbers of cercariae. The maximum number of cysts recovered from a specimen of A. quadridactylus was approximately 325; from Tripterygion sp., approximately 250.