Studies on a New Trypanorhynchan Larva, Callotetrarhynchus sp., Parasitic on Cultured Yellowtail-I

On the Distribution, Morphology and Classification

Abstract

A lot of bladder worms were found out in the abdominal cavities of emaciated cultured yellowtails in early spring, 1967, in a fish farm on the west coast of the island of Shikoku. The number of the worms per fish varied from 6 to over 20 and the emaciation of fish was attributed to this parasite. A succeeding survey revealed that not only cultured but also natural yellowtails along the whole stretch of the aforementioned coast mostly harvoured the same parasite at any season. This parasite, however, has not yet been noticed in any fish farms in the other regions.
Most of the blastocysts obtained were tadpole-shaped and ranged in body length from 5.8-28.0mm, but part of them had the shape of an egg (6.5-9.4mm) or the shape of a nematode (8.7-21.2mm) (Plate I, Fig. 1). Unencysted nematode-shaped blastocysts were occasionally noticed, particularly around the stomach of fish. Unexpectedly a few specimens in which the embryo partialy emerged from the cyst were discovered in the abdominal cavity of a fish (Plate I, Fig. 2). It was observed that in part of the egg-shaped or tadpole-shaped blastocysts examined an embryo had already been formed the receptaculm of the blastocyst. The rate of occurrence of blastocysts with embryo varied from 21.4-77.5% depending on the group of fish samples (Table 2).
Morphological observations were carried out of the 185 blastocysts obtained from 5 cultured and 1 natural yellowtails and of the 136 embryos collected from the blastocysts (Fig. 1 and Plate I, Fig. 3-7). The dimensions and proportions of the parts, or organs of the embryos were measured on fixed specimens (Table 3). Observations and measurements of the tentacle hooks were made on live embryos obtained from cultured yellowtails. The results are givenin Table 4, Fig. 2 and Plate II, Fig. 10-14.
On the basis of these observations the present worm was classified as belonging to the genus Callotetrarhynchus. It resembles in the arrangement of hooks three species of the genus; C. speciosum LINTON, C. gracillimum PINTNER, C. lepidum CHANDLER, and Tentacularia pseudodera SHULER, but it is definitely different from any of them in the size and proportions of the body parts and some morphological characteristics of the embryos. This strongly suggests the possibility that the present worm is a new species.