Results of Feeding Experiments with Metacercariae

A total of 14 fish belonging to eight different species were fed on one or more occasions with specimens of Tripterygion sp. and Acanthoclinus quadridactylus that had been experimentally infected with metacercariae.

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The experimental hosts were examined between five days and six weeks after feeding for the presence of juvenile or adult worms. Two gravid specimens of Bucephalus longicornutus were recovered from the intestine of one specimen of Scorpaena cardinalis Richardson, the scarpee or red rock cod. The posterior half of a bucephalid, considered to be B. longicornutus by the nature of the structure of the cirrus sac and genital atrium, was recovered from another specimen of S. cardinalis. This latter specimen was damaged during examination of the host's intestinal contents and despite a careful examination, its anterior half could not be found. Details of all experiments conducted are summarised in Table IV.

It was stated elsewhere (p. 26) that metacercariae do not reach maximum development until after approximately 80 days. Accordingly, the feeding of sub-mature metacercariae of only 25 to 75 days development in the above experiments might be questioned assuming that metacercariae are not infective until they have reached maximum development. The reason for feeding with sub-mature metacercariae was that some experiments were conducted before a satisfactory indication was obtained of when maximum development of the metacercariae was reached. Furthermore, the feeding of metacercariae of different ages to the one experimental host in some experiments might be questioned. Again, without the necessary knowledge of when metacercariae reached maximum development it was hoped to establish an infection and establish the age limits within which the metacercariae were infective. This was not successful. Only two experiments were performed, with host species 3 and 14, in which the metacercariae used were known beforehand to have reached maximum development.

It is of interest to note that successful results were obtained only with Scorpaena cardinalis and that this species (apart from Trachelochismus sp. which, however, was fed sub-mature metacercariae) was the only species used that was free from intestinal parasites other than the specimens of Bucephalus longicornutus. Whether there is any correlation between the successful experimental establishment of the bucephalid and the absence of other intestinal parasites can, as yet, only be conjectured.

It should be pointed out that S. plumieri from Florida which is related to S. cardinalis is infected with Bucephalus scorpaenae Manter, 1940. Therefore, it is possible that S. cardinalis could harbour a natural Bucephalus infection which would invalidate the experimental results. However, Manter (1954) examined nine specimens of S. cardinalis from Wellington and Portobello, and the author has, during the course of this study, examined 15 from Island Bay, Wellington, and no natural bucephalid infections have yet been found in this species. This supports the view accepted here, that the specimens of B. longicornutus recovered from the intestine of S. cardinalis were established experimentally.