Description

A sporocyst is milky-white, elongate, rounded, hollow, branching, and swollen at irregular intervals as brood chambers which contain developmental stages of cercariae (Text-fig. 1, B, E, F, G). Brood chambers range from 120μ to 750μ. in diameter, and are up to 3.5cm long. Narrower regions separating brood chambers, 45μ. to 110μ in diameter by 20μ to 3.5cm long, may exhibit a beaded appearance.

Longest portion of a single sporocyst isolated, 6.1cm long, branched dichotomously at an average of every 2.2cm. Total length and branchings of a single sporocyst not determined, due to its fragility and complex inter-twinings with its own branches and the branches of other sporocysts. No early developmental stages of sporocysts were recovered from dissection of 44 oysters, and sectioning 8 oysters, all of which were not obviously infected with sporocysts.

A sporocyst is hollow in transverse section (Text-fig. 1, A), covered by cuticle, 2μ to 4μ, thick, which is thinner over brood chambers (Text-fig. 1, D). Nucleated layer of sporocyst is internal to cuticle and individual cell membranes in this layer are incomplete and indistinct. This layer is 15μ to 22μ wide in narrower regions and 10μ to 12μ. wide in brood chamber regions. Cytoplasm is continuous suggesting a syncytium, and contains three types of nuclei. Ovoid to spherical vesicular nuclei, page 8

Text-fig. 1.—Bucephalus longicornutus. Morphology and anatomy of the sporocyst: Fig. A, T.S. of a sporocyst; Fig. B, whole mount of a portion of a sporocyst; Fig. C, T.S. of a brood chamber; Fig. D, portion of a T.S. through the sporocyst wall of a brood chamber; Fig. E, outline sketch of a branched portion of a sporocyst; Figs. F and G, variations in the morphology of the sporocyst. For abbreviations see p. 9.

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Right lateral view of a specimen of Ostrea lutaria Hutton, from Area B, Foveaux Strait, infected with the sporocysts of Bucephalus longicornutus (Manter, 1954). The visceral mass has been teased open to display the sporocyst tubules more clearly.

page 9 5μ to 6μ in diameter, usually with one distinct, eccentrically or centrally situated nucleolus and some peripheral chromatin; and ovoid nuclei, 3μ to 4μ in diameter, containing scattered chromatin granules which may resemble nucleoli, are considered as mesenchymal (somatic) nuclei. In narrower regions, larger of these nuclei lie, in general, nearer cuticle (Text-fig. 1, A). Both types are common throughout all regions of sporocyst; in representative cross section, between 28 and 38 mesenchymal nuclei are generally present. Germinal nuclei are found in syncytium of sporocysts from gill interribs, in and near terminal regions of sporocyst, usually numbering 1 to 2 per cross section. They are spherical, 5μ to 7μ in diameter, with a conspicuous, centrally situated nucleolus, and many small chromatin threads and granules dispersed throughout the nucleoplasm, and stain more intensely with haemotoxylin than mesenchymal nuclei (Text-fig. 2, A).

Germinal nuclei lie in clusters in syncytium after several divisions, and, after further divisions, bulge into sporocyst lumen as a germinal cyst (Text-fig. 2, B). Germinal cysts are analogous with ovaries. Mitotic figures only located in those parts of sporocyst wall where germinal nuclei occur.

Cytoplasm of syncytium essentially hyaline, faintly granular in irregular patches. Cytoplasm forming a distinct band approximately 1μ to 2μ wide around germinal nuclei, is more densely granular than granular patches of cytoplasm in other parts of syncytium (Text-fig. 2, A).

Terminal region of sporocyst is densely nucleated, containing both germinal and mesenchymal nuclei. Cell membranes are more distinct in this region.

No muscle cells found in sporocyst wall and no movements of sporocyst were observed. No indications of an excretory system, testes or specialised feeding or nutritive branches of the sporocyst were observed.

Discussion of Sporocyst

Published accounts of the sporocysts of bucephalids show that there are slight differences in form and structure among the different species.

The beaded appearance of narrow regions alternating with dilated regions or brood chambers has been noted for Bucephalus cuculus by Tennent (1906), Rhipidocotyle papillosum by Woodhead (1930), and for the present species. Narrower regions of more or less uniform diameter merging with brood chambers has been noted or figured for Bucephalopsis haimeanus by Lacaze-Duthiers (1854), Bucephalus elegans by Woodhead (1930), R. septpapillata by Kniskern (1952), and

R. papillosum by Ciordia (1956). It is notable that although Woodhead and Ciordia independently examined the same species they have described differences in form, so the external appearance of the sporocysts of the same species is not necessarily constant.

Text-fig. 2.—Bucephalus longicornutus. Germinal nuclei and cyst in the wall of the sporocyst: Fig. A, T.S. of a portion of the sporocyst wall near the growing point showing a germinal nucleus; Fig. B, T.S. of a portion of the sporocyst wall near the growing point showing a germinal cyst. For abbreviations see p. 9.

Both Woodhead (1930) and Kniskern (1952) have noted the presence of two types of sporocyst tubules in R. papillosum and R. septpapillata respectively. The former observed brownish, empty tubules which he interpreted as being spent sporocysts of the previous year. Kniskern observed tubules devoid of cellular content which he termed nutritive branches. However, polymorphism of the sporocyst tubules has not been noted in the present species.

The general structure of the sporocyst wall, namely, cuticle, cellular layer and lumen, is most similar to the sporocyst of R. septpapillata as described by Kniskern (1952). However, the present species differs in having two kinds of mesenchymal nuclei. Kniskern's figures do not permit any closer comparisons to be made and he fails to give any description of nuclear detail.

Muscle cells were not observed in the sporocyst wall of the present species. However, Tennent (1906) and Ciordia (1956) have noted "muscle fibres" immediately within the cuticle of the sporocysts of Bucephalus cuculus and R. papillosum respectively, but Ciordia's figures do not substantiate the presence of such fibres. Apart from Woodhead (1931), who noted undulations in the tips of the sporocysts of R. papillosum but did not show muscle cells in his figures, there seems to be general agreement that no appreciable movement of the sporocysts takes place and accordingly, the presence of "muscle fibres" in these two species is unusual. It seems probable that either Kniskern (1952) or Ciordia (1956) is in error since they were both dealing with the sporocysts of related species of Rhipidocotyle.

The various interpretations of the cellular layer of bucephalid sporocysts differ markedly. Haswell (1903) inferred that the entire cellular layer of the sporocyst of Bucephalus sp. was a "germinal epithelium" and any part of it was capable of proliferation to give rise to cercariae.

Tennent (1906) found two types of nuclei in the cellular layer of the sporocyst wall of B. cuculus. The larger of these, which he termed propagatory cells, occurred in the tip of the sporocyst and in parts of the wall nearer the tip. They resemble the germinal nuclei of the present species. These nuclei, as they appear in his page 11 figures, are surrounded by dense cytoplasm. The other type of nuclei in Tennent's figures resembles the larger mesenchymal nuclei of the present species. He stated (p. 649) "There is little doubt that the germ-cells arise in the wall of the sporocyst." However, he gives no evidence to substantiate this statement. The distinctly cellular nature of the terminal region, as described by Tennent, is comparable with the present species.

Woodhead (1931) described and figured germinal tissue, in the form of both ovaries and testes, projecting into the lumen as processes from the terminal regions of the branches of the sporocysts of B. pusillus and R. papillosum. He also described the occurrence of a redial generation in the sporocyst lumen in both species. However, he did not give any details or figures of the other types of nuclei found in the sporocyst wall. Woodhead's observations were not confirmed by Ciordia (1956) who repeated Woodhead's work on R. papillosum. No testes, true ovaries, or redial generation were found in the terminal regions of the sporocysts of the present species and, apart from Woodhead's observations, have not been found in other bucephalid species.

Kniskern (1952) stated (p. 322) "The true wall of the sporocyst was within the cuticle and was but one cell thick." His figures (1, 2 and 4, p. 323) do not support this statement but rather indicate that cell outlines are indistinct so that it is impossible to say whether or not the cellular layer is, in fact, one cell thick. He further stated that germinal tissue appeared only in the solid tips of the sporocysts and in isolated zones a short distance behind the tips, but his figures only show germinal tissue in the tips of the sporocyst.

Ciordia (1956) observed two types of nuclei in the cellular layer of the sporocyst wall of R. papillosum. These were stated to be mesenchymal and germinal nuclei. The latter were found in greatest number at the tips of branches and more sparsely, in clusters or singly, among mesenchymal nuclei throughout the remainder of the sporocyst wall. The germinal nuclei were characterised by the irregular and dense disposition of chromatin giving a pyknotic appearance, thus differing from the germinal nuclei of the present species. The mesenchymal nuclei were larger than the germinal nuclei and, in disposition of chromatin, resembled the larger mesenchymal nuclei of the present species. Ciordia stated that the mesenchymal nuclei had distinct cellular outlines but her figures show these outlines to be incomplete.

Analysis of the above observations coupled with those for the present species clearly show that the cellular layer of bucephalid sporocysts is a syncytium. Incomplete cell membranes are distinguishable in the syncytium which contains at least two types of nuclei (and three in the present species). The nuclei are mesenchymal and germinal.

Germinal nuclei have been located by Tennent (1906), Woodhead (1931), Kniskern (1952), and Ciordia (1956) in or near the terminal regions or growing points of the sporocyst, and more rarely in isolated regions of the sporocyst wall some distance from the growing point. It is therefore considered that the blind regions of the sporocysts of the present species on the gills of O. lutaria represent growing points, since the sporocyst wall in the vicinity of this region is the only region where germinal nuclei and mitotic figures have been located. Haswell's inference, that the whole cellular layer is a germinal epithelium, is probably incorrect. Characterisation of nuclei in descriptions and figures for many species is inadequate and does not permit close comparisons to be made with the present species, and it is evident that the sporocysts of most species need critical restudy.

Text-fig. 3.—Bucephalus longicornutus. Morphology and anatomy of the cercaria. I: Fig. A, ventral view of a living specimen; Fig. B, cuticular spines from the anterior region of the body; Fig. C, cuticular plates from the posterior region of the body; Fig. D, lateral view of a furca of a living specimen; Fig. E, ventral view of a furca of a living specimen; Fig. F, T.S. of the cercaria at pharynx level. For abbreviations see p. 9.

Text-fig. 4—Bucephalus longicornutus. Morphology and anatomy of the cercaria. II: Fig. A, lateral view of a fixed and stained extended specimen; Fig. B, diagrammatic representation of the excretory system; Fig. G, ventral view of a fixed and stained contracted specimen; Figs. D to F, body movements in a living specimen. For abbreviations see p. 9.