Monadenia fidelis

Monadenia fidelis (Gray, 1834)
Pacific Sideband

  • Helix fidelis Gray 1834: 67.
  • Helix nuttalliana I. Lea 1839: 88, pl. 23, fig. 74.
  • Helix oregonensis I. Lea 1839: 100, pl. 23, fig. 85.
  • Monadenia semialba Henderson 1929: 80.
  • ? Aglaia fidelis var. minor W.G. Binney 1885: 121, 493, fig. 91.
  • ? Monadenia fidelis celeuthia Berry 1927: 122, fig. 2.
  • ? Monadenia fidelis columbiana Pilsbry 1939: 43, fig. 16g, h.
Monadenia fidelis.

Identification. Shell depressed-heliciform. Spire conical, moderately elevated but rather variable, sides more-or-less straight. Whorls 5¼–6¾, convex. Periphery rounded in adults. Suture moderately indented. Last whorl descending before adult aperture. Protoconch initially smooth then microscopically granular. Teleoconch with irregularly sized and spaced, round-topped colabral wrinkles (strongest on the apical surface), and shallow, wavy, microscopic striae (most evident on base). Periostracum frequently showing zigzag pattern of “wrinkles”. Aperture subovate-lunate, edentulous, wider than high. Peristome incomplete. Lip a little thickened, slightly recurved; viewed from side, prosocline and slightly arched. Umbilicus ca 1/8 of shell width. Shell opaque, matte above rather glossy below; usually chestnut brown or rusty yellowish brown, with a narrow, pale-yellow band just below periphery, a slightly wider, dark-brown band above, and a dark-brown base (often slightly greenish towards lip); aperture with the exterior pigmentation clearly showing through a milky whitish enamel; lip pale brown or purplish brown; straw-yellow shells, with faint banding infrequent. Shell to 22.1–36.5 mm (usually averaging 32 mm; wider than high).

Monadenia fidelis; Mayne Island, BC (RGF 96.033.3503); width: 32.2 mm. Right: showing zigzag wrinkles in periostracum.
Monadenia fidelis, yellow colour form; Mayne Island, BC. The white streaks on the base are due to injury of the animal after a break about one-half whorl back and the absence of periostracum in streaks from that point onward as the shell was expanded.

Animal rosy brown, with the spaces between the tubercles greyish; behind the head are some sparse, black reticulations. Mantle with a bold rust-coloured band; sole of foot pale grey.

Monadenia fidelis; Koksilah River, Vancouver Island, BC.

Habitat. Forests generally, but also in open, Garry Oak (Quercus garryana) meadows, along sandy-grassy seashores with logs, and on rocky islets. Under logs and leaf litter. Often active in spring and then sometimes climbing trunks of trees. On a very small, rocky islet in the Gulf Islands, Monadenia fidelis was found under stunted ‘mats’ of oak (Forsyth 2004). Lord (1866) reported this species up to elevations of 6,000 ft [ca 1,800 m] although it is predominantly a species of the coastal lowlands. Adult snails are most often encountered in spring when crawling in the open on the ground or climbing up the trunks of shrubs and trees. Jackson (1923) observed M. fidelis as high as 22 ft (6.7 m) above the ground.

Biology. This species presumably feeds on fungi and plants. Animals readily ate a varied diet in captivity, but diet was not correlated to shell colour (Roth 1980). In captivity, individuals of Monadenia fidelis flava reached adult size in 2.5–2.75 years and survived up to eight years (Walton 1963, 1970, as M. fidelis beryllica. Monadenia fidelis is a host of the parasitic ciliate protozoan Tetrahymena limacis (Warren, 1932) (Kozloff 1956).

Global range. BC to Cape Mendocino, California. Although reported from Sitka, Alaska (Dall 1905), this is doubtful and there are no records of M. fidelis north of the Broughton Archipelago on the central British Columbia coast.

Canadian range. South-western British Columbia: all around the Georgia Basin, north to the Broughton Archipelago (Dall 1905); inland around Pemberton and in the Fraser Valley to near Hope.

La Rocque (1953) indicated this species occurs in Yukon; however, no specimens have been found in collections to verify this. Biogeographically, the presence of this species in Yukon is highly unlikely.

Etymology. Latin: fidelis, dependable or faithful.

Remarks. Subspecies and forms have been described for this variable and relatively widespread, large, and showy species (Pilsbry 1939), which shows the greatest expression of variation in the southern part of its range (Roth 1980). Some subspecies continue to be used in the modern literature, while studies of the genetics of other subspecific taxa have shown them to be synonyms of M. fidelis or another species, M. infumata (Gould, 1855) (Roth & Sadeghian 2003). The synonymy here includes the “remainders” which have not been investigated. Probably all Canadian populations belong to the nominotypical subspecies (if these were to be used). Eyerdam (1940) reported M. [fidelis] semialba from Vancouver Island, near Victoria. Monadenia semialba was described from Fidalgo Island, Washington, for a shell polymorphism in which the dark pigment on the base is much reduced and the pale, generally straw yellow subperipheral band is expanded onto the base. Pilsbry (1939) treated this taxon as a subspecies. However, at its type locality, semialba is mixed with normal fidelis (Eyerdam 1937; pers. observ.) and is treated as colour variant and a synonym.

In BC, the smallest shells have been found on open, seaside habitats, such as offshore islands and spits with no or minimal tree and shrub cover. The pale, yellow form is interspersed with normally coloured individuals, but generally quite rare, although in some places, individuals with yellow shells are rather more frequent.