Freshwater snails of the subgenus Hinkleyia (Lymnaeidae: Stagnicola) from the western United States

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The subgenus Hinkleyia has heretofore included only Stagnicola caperata (Say), but is herein increased by addition of S. montanensis (Baker) on various conchological and anatomical criteria, and of S. pilsbryi (Hemphill) on shell features only. New information permits a revised diagnosis of Hinkleyia, which has several distinctive characters but is worthy of only subgeneric rank. Sixteen specimens of a preserved series of S. montanensis from Idaho were dissected. Anatomically they resemble S. caperata so closely that no distinguishing features could be identified. The two species can be distinguished by their shells. In shape, S. montanensis is usually more narrowly elongate and S. caperata more swollen, but these characters overlap and surface texture and sculpture are the only consistently reliable criteria for distinction. Relatively conspicuous raised spiral ridges of periostracum are diagnostic of S. caperata and a shiny surface without such sculpture but with spirally arranged series of tiny crescents of S. montanensis. The anatomical characters shared by these 2 species and typical American Stagnicola and, as far as is known, exclusive to them, are: a well-developed vaginal sphincter muscle; a large diverticulum appended to the proximal end of the upper prostate; a relatively stout and comparatively short penis sheath; a relatively short, bilaterally symmetrical penis having a muscular "knot" near its midlength; and a well-developed series of clearly bicuspid lateral radular teeth. In comparison with typical Stagnicola, Hinkleyia (i.e., S. montanensis and S. caperata) shows a somewhat different and less prominent development of the vaginal sphincter, a notably stout vas deferens and a shorter penis that has a less prominent, somewhat more distally placed "knot”. Of special note is the tapered portion of the penis that is distal to the "knot", which is short and stout and much less distinctly demarcated from the thick proximal portion than in Stagnicola proper. Both species also have an unusually short and thick penis sheath, a prostate of a particular conformation, and an uncommonly robust relative development of the lower part of the duct of the spermatheca. Additionally, in texture and pigmentation of their overall anatomy, they show similarities which, further serve to set them apart from other sufficiently known lymnaeid species. Their bodies are well pigmented and evidently their foot and tentacles are relatively slender in life. It was also found that S. montanensis had become sexually mature upon reaching a shell length of 6 to 7 mm. It is conjectured that the unknown egg mass of this species will prove to be similar to that of S. caperata, which among known masses is unique in the relative thickness of the individual egg envelopes and the thin inconspicuous outer tunic of the egg mass. Anatomical evidence indicates that a species formerly assigned to Stagnicola under the subgenus Nasonia is closely related to Fossaria and therefore not allied to S. montanensis and S. caperata. A possible special affinity of S. arcitica with Hinkleyia rather than the subgenus Stagnicola was considered but rejected on new anatomical data. The haploid chromosome numbers of S. montanensis and S. caperata are 18; the demonstrated diploid number of S. montanensis is 36. These numbers are characteristic of basommatophoran snails in general. Details of gametogenesis, including the monocentric nature of the chromosomes, seem to be the same as in other lymnaeids. During spermatogenesis there are normally 6 spermatogonial divisions, followed by 2 meiotic divisions. Therefore, spermatogonial cells occur in clusters of 2, 4, 8, 16 and 32, primary spermatocytes in clusters of 64, secondary spermatocytes in clusters of 128 and spermatids and sperm in clusters of 256. Of special cytological interest in the gametogenesis of both S. montanensis and S. caperata, and not reported previously for other lymnaeid snails, are 5 to 7 large chromatin bodies in each nucleus during early prophase of the first meiotic division of spermatogenesis. These chromatin bodies are perhaps another character demonstrating a close relationship between the two species. S. montanensis has a wide distribution in the western United States in the eastern Columbia River and northern Great Basin drainages, from western Montana and Utah to southern Idaho and central Nevada. It has the unusual habitat for Lymnaeidae of springs or clear mountain streams. S. caperata is found over most of northern North America and includes the area of S. montanensis within its distribution. It is found in such situations as irrigation ditches and muddy, seasonal waters where S. montanensis does not occur. Descriptions of specific habitats and maps of geographic distribution (living and fossil) of the two species provide data concerning non-morphological differences between the species. The observed geographic separation is due, in part, to differences in ecologic requirements, but is probably, in part, due to the geologic history of the region. S. pilsbryi is known only from the original material from one locality in western Utah, and no ecologic data are available.

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Reference Code: A63TAY02IDUS

Full Citation: Taylor, D. W., H. J. Walter, and J. B. Burch. 1963. Freshwater snails of the subgenus Hinkleyia (Lymnaeidae: Stagnicola) from the western United States. Malacologia 1(2): 237-281.

Location: ANIMAL EF: MOLLUSCS