A guide to the species of the genus Aspidisca Irene C. H. Wu & Colin R. Curds Department of Zoology, British Museum (Natural History), Cromwell Road, London SW7 5BD Introduction As a sequel to 'A guide to the species of the genus Euplotes"" (Curds, 1975) the present paper is primarily a collection of diagrams and descriptions of the species of the genus Aspidisca Ehrenberg, 1830 (1832). Keys to what we consider to be distinct species are included, and are designed to enable workers to make specific identifications of Aspidisca without the need to search the literature. Previous attempts known to the present authors are those of Plough (1916) who devised a key to eight species (see Appendix 1) and Kahl (1932) whose key included 28 species (see Appendix 2). Borror (1972), in a revision of the order Hypotrichida Stein, 1859, listed 22 species of Aspidisca with their synonyms. While we sometimes do not agree with Borror (1972), it should be pointed out that some of the disagreements are only matters of opinion. Information is still required to substantiate these opinions even though some effort has recently (Tuffrau, 1964; Borror, 1972; Curds, 1975, 1977) been devoted to the taxonomy of the family Euplotidae Ehrenberg, 1838. Since Ehrenberg, 1830 (1832) established the genus Aspidisca with Aspidisca lynceus (Miiller, 1773) Ehrenberg, 1830 as the type species, over 50 nominal species have been transferred and added to the genus. Species have been distinguished by the size and shape of the body, the number of dorsal ribs, the presence of a thorn on the dorsal surface, the number of cirri on the ventral surface, nuclear features and the configuration of the peristome. Kahl (1932) applied all these criteria in devising his key (Appendix 2) and divided them into marine and freshwater species. Plough (1916), on the other hand, considered the shape of the 'cuirass' to be the most stable character and believed that the numbers and disposition of cirri to be variable. More recently, silver-impregnation techniques have been used to study the morphology and morphogenesis of ciliates. The diagnostic value of the silver-line systems of Aspidisca spp. and the other features mentioned above will be discussed under separate headings. Features of taxonomic importance (a) Habitat The freshwater species listed by Kahl (1932) include A. lynceus, A. costata, A. turrita, A. herbicola, A. marsupialis and A. sulcata. Earlier Plough (1916) had reported the occurrence of A. turrita and A. costata in both sea water and freshwater but stated the others, including A. lynceus, to be strictly marine species. In fact the original specimens of A. lynceus were found in freshwater and the species has since been reported in freshwater sites (Kahl, 1932; Bick, 1972). A. herbicola Kahl, 1932 appears to be the only species which is reported to occur in freshwater alone. All other species described to date inhabit the marine environment. (b) Size It is known that the size of a ciliate may vary with many factors including rate of growth, con-centration of food, kind of food and so on, and is therefore of limited taxonomic value (see Curds, 1975). In the case of Aspidisca, while their sizes range from 16 to 150 um long most species are between 50 and 100 urn long (Fig. 1). Therefore, the exceptionally large size (135-150 um) of A. magna Kahl, 1932 can perhaps justifiably be regarded as diagnostic. (c) Shape The typical shape of Aspidisca is oval although it generally tends to be more convex on the right than on the left. The outline may be smooth or jagged with spurs or dentations which mostly appear on the left border. The dorsal surface is commonly arched and it may be smooth Bull. Br. Mus. not. Hist. (Zool.) 36 (1): 1-34. Issued 28 June 1979
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