A New Cotylocercous Cercaria from Melagraphia aethiops (Gm.) (Gastropoda).
[Received by the Editor, September 27, 1957]
Apart from the trematodes found in introduced domestic animals, the trematode fauna of New Zealand has not been extensively studied. Up to the present, 7 cercariae and about 70 adult digenetic trematodes have been described from this country; of these, the life histories of 5 species have been worked out (Macfarlane, 1939, 1945, 1951 and Dale, in preparation). In view of this dearth of information, it seems that any contribution to our knowledge of this group is of value.
Cercaria melagraphia n. sp. (Fig. 1 a–d.)
Host: Melagraphia aethiops (Gmelin, 1790); dark top-shell.
Location: Digestive gland and under gonadial epithelium.
Sporocyst simple, fusiform in shape, light yellow in colour when alive; length 1.4 mm, breadth 0.3 mm. No external features discernible. Each sporocyst contains a great number of cercariae. The exact number is difficult to determine, but counts ranged from about 100 to nearly 200. Sporocysts containing fewer cercariae (in the vicinity of 100) also contained a number of germinal balls. The cercariae are densely packed within the sporocyst, these are indicated in part of the sporocyst shown in Fig. la. In life, this sort of density obtains from end to end of the sporocyst.
Cercaria. The mature cercariae (Fig. 1, a–c) are active within the sporocyst Fully grown cercariae are 0.12–0.17 mm long by 0.03–0.025 mm at the widest part. The most characteristic feature of the cercaria is the stumpy tail (s t) 0.017–0.023 mm by 0.018–0.024 mm in size. It is so short that it cannot serve as a locomotory organ in the usual fashion, but by virtue of a terminal invagination which appears to serve as a terminal sucker the cercaria is enabled to crawl rapidly over the substratum by using all three suckers Macfarlane (1939) described a similar terminal sucker, and this type of locomotion in the cercaria of Coitocaecum anaspidis Hickman. The cuticle of the cercaria is smooth and without spines. The oral sucker (o.s.) is large, and takes on a variety of shapes, some of which are shown in the figures. The ventral sucker (v.s.) is similarly variable in shape, and is situated at about the mid-point of the animal. The neck region is very extensile and mobile. Associated with the oral sucker are the pear-shaped, single-pointed, stylet, and the openings of the penetration glands. The paued penetration glands (p.g.) appear to consist each of a single flask-shaped vesicle situated just anteriorly to the ventral sucker. The ducts from the glands run forward on either side of the mid-line, and open at the sides of the stylet A large excretory vesicle (e.v.) lies between the ventral sucker and the tail. The shape of the vesicle varies somewhat, due no doubt to the state of contraction of the animal and the contents of the vesicle. The cells forming the lining of the excretory vesicle are large, and their inner surfaces bulge into the lumen of the vesicle. The arrangement of the excretory ducts and pore was not observed. No pharynx or gut could be detected.
Type Material: Holotype and paratypes from Mclagraphia aethiops (Gmelin) collected at Menzies Bay, Banks Peninsula: August 9, 1956, W. C. Clark. The type material has been deposited in the Canterbury Museum.
Distribution: Infected snails have been collected at the following places:— McCormack's Bay, Heathcote Estuary, February, 1956; Menzies Bay (types) 9/8/56; Diamond Harbour, Lyttelton Harbour, 2/11/56, all coll. W. C. Clark; and Governor's Bay, Lyttelton Harbour, 19/10/56, coll. M. S. Scott.
Notes on Infection. Little can be said at present about the rate of infection, except that the four animals listed above are the only infected individuals that were found in the course of a study of the host, which extended over a year. Five hundred snails from Diamond Harbour were examined for parasites, but only one was found to be infected The site of infection was the same in all four specimens,
Fig. 1–Cercaria melagraphia n.sp a–c, Cercariae in different attitudes. d, Sporocyst, a few of the contained cercariae are shown to indicate the density of the cercariae. In life this sort of density obtains from end to end of the sporocyst. e.v., excretory vesicle; o.s., oral sucker; p. d, duct of penetration gland. p. g, penetration gland. s, stylet. s t, stumpy tail v.s, ventral sucker
the sporocysts were found in the vicinity of the digestive gland; some actually within the tissue of the gland, but the majority under the skin of the visceral hump, especially in the region of the gonad. At first it was thought that the gonad had been destroyed, and parasitic castration occurred, but dissection showed that the gonad was merely obscured by the dense packing of a superficial layer of sporocysts immediately under the skin. The number of sporocysts found in a single snail is enormous, and must run into many hundreds.
At present there seems little prospect of deciding the problem of the adult to which C. melagraphia belongs. For example, Manter (1954) has recently recorded 16 members of the Opecoelidae from marine fishes of New Zealand. The cotylocercous type of cercaria is commonly found in this family of the Digenea, but is by no means restricted to it. The only opecoehd life history known in New Zealand is that of the freshwater Coitocaecum anaspidis, which was worked out by Macfarlane (1939). It is possible, or even probable that the adult remains to be described.
Dale, P. J. The Life History of Gorgodera austrahensis Johnson, in New Zealand (in preparation).
Macfarlane, W. V., 1939. The Life Cycle of Coitocaecum anaspidis Hickman, a New Zealand Digenetic Trematode. Parasit. 31, 172–184.
—— 1945. The Life History of the Heterophyid Trematode Telogaster opisthorchis n g., n. sp. Trans. Roy. Soc. N. Z. 75, 218–230.
—— 1951. The Life Cycle of Stegodexamene anguillae n g, n.sp., an Allocreadid Trematode from New Zealand. Parasit. 41, 1–10.
Manter, H. W., 1954. Some Digenetic Trematodes from Fishes of New Zealand. Trans. Roy. Soc. N.Z 82, 475–568.
W. C. Clark, M.Sc.,
Entomology Division, D.S. I. R,
P. O. Box 223, Nelson