Carychium exile

Carychium exile H.C. Lea, 1842
Ice Thorn

  • Carychium exile H. C. Lea 1842: 109, pl. 1, fig. 5.
  • Carychium exile canadense G. H. Clapp 1906: 139, pl. 8, figs 1, 2, 6, 7.
Carychium exile; RGF 1343), height 2.45 mm.
Carychium exile; RGF 1343), height 2.45 mm.

Identification. Shell fusiform, comparatively more slender than other Carychium species. Spire elongate, with sides mostly straight. Apex bluntly pointed. Whorls ca 6, somewhat convex but usually noticeably flatter. Suture well indented. Protoconch smooth. Teleoconch with rather evenly spaced, fine but rather strong colabral riblets on most whorls. Aperture subovate; 1 parietal lamella continuing internally in an undulating spiral around columella where it forms a broad, nearly vertical plate. Adult lip expanded, a little recurved, thickened on inner side. Belly of last whorl not projecting beyond the plane of peristome. Chink-like “false umbilicus” against base of last whorl. Shell translucent, colourless when fresh, opaque, and white when weathered. Shell to 2.2 mm high (higher than wide).

Animal likely as in C. exiguum.

Carychium exile is recognized by its slender shell, with rather distinct axial riblets, thickened but not recurved peristome. The penultimate whorl does not bulge beyond the profile of the palatal lip when viewed from the side but is even with it. The internal lamella is undulate. The shell of C. nannodes similarly shaped but smaller, smooth, and with a thin adult lip.

Habitat. Floodplain woods, marshy edges of streams and bodies of water, swamps, wooded rocky outcrops, and alvars. A calciphile. In moist leaf litter and dead vegetation; under fallen bark and other woody debris. This species is often sympatric with C. exiguum in southern Ontario, where shells are common in stream drift.

Global range. Parts of Canada, south in the USA to Alabama, Mississippi, and Georgia; west to Oklahoma, Kansas, and the Dakotas (Hubricht 1985).

Canadian range. Manitoba east to the Maritime Provinces. Alberta?

In Ontario, this species occurs from the Lower Great Lakes north to James Bay and northwest to Schreiber, Thunder Bay District (Oughton 1948).

Records west of Manitoba need confirmation. Specimens from Alberta (CMN 86010), which were identified by Van Es & Boag (1981), are C. exiguum. A single specimen cited by Hanna (1923; CASIZ 135227) was said to have been collected in 1916 at Union Bay on Vancouver Island, but there are no subsequent collections of that species in BC or anywhere along the west coast, and although correctly identified, it seems likely that the specimen was mislabelled (Forsyth 2004).

Etymology. Latin exilus, “small”, “thin”, or “slender”.

Remarks. Pilsbry (1948) recognized Carychium exile canadense, which was said to be larger than the typical form. Oughton (1948) noted that almost all Ontario specimens seen by him belonged to the canadense form and that only a few specimens from some southern lots were typical C. exile. However, based on analyses of relative heights and widths of shells, C. exile canadense was synonymized with C. exile by Nekola & Barthel (2002).

Another purported subspecies (not in Canada), C. exile mexicanum Pilsbry, 1891, was generally treated as a separate species (e.g. Hubricht 1985), and now recent molecular studies have revealed C. mexicanum has been used as a “lumping bin” and is indeed separate from C. exile (Weigand et al. 2013, Jochum et al. 2017). Thus, no subspecies are currently recognized.

The author of C. exile is H. C. Lea (1842), not I. Lea as shown by Turgeon et al. (1998) and sometimes repeated elsewhere.