Euchemotrema hubrichti (Pilsbry, 1940)

Anderson, Frank E. & Smith, David A., 2005, A redescription of the carinate pillsnail, Euchemotrema hubrichti (Pilsbry, 1940) (Pulmonata: Polygyridae), with notes on habitat and genetics, Zootaxa 807 (1), pp. 1-11 : 4-8

E. hubrichti can be found in Figure 1 .

In the lab, E. hubrichti spend nearly all of their time sealed up and attached to the sides or the top of the container in which they are placed. In the field, they are often found in abundance attached to the undersides of large limestone slabs. Most individuals appear to reach sexual maturity (as inferred by the presence of a reflected apertural lip) at approximately 9 mm shell diameter ( Figure 2 ).

Shell

The shell was the only aspect of this species available to Pilsbry, and Pilsbry’s description of the shell needs little modification:

"The imperforate shell is depressed, lens­shaped, acutely carinate [emphasis added], the height not much exceeding half of the diameter. Spire low conic, or slowly increasing whorls, the first two convex, the rest nearly flat, impressed above the keel; last whorl shortly descending in front. The base is flattened below the keel, then convex, and slightly impressed around the central axis. The embryonic whorls are most minutely granulose. Later whorls are lightly striate, the striae low, very unequal. Base with fainter striae and densely, minutely granulose. The aperture resembles that of S. fraternum but is narrower, the parietal tooth nearer to the basal lip, approaching it slightly towards the outer end; nearly straight, rather low, connecting with the end of the collumellar lip by a low, curved ridge. The basal lip is reflected, rather strongly thickened within, the thickening narrowing at the passage of basal into columellar margin, forming a rounded sinus there; in the outer half of the basal margin it is wider, but narrows gradually at the position of the keel, not abruptly as in S. barbigerum ." ( Pilsbry, 1940, p. 687).

The only addition we can make to Pilsbry’s description is that there is sometimes a small fulcrum (using Pilsbry's terminology; see his description of S. hirsutum barbatum, Pilsbry, 1940, p. 665 ) positioned well within the ultimate whorl. This fulcrum can be seen in Figure 3 (bottom left photo). Pilsbry (1940) reported a range of 4.5–5.0 for whorl count; our specimens had whorl counts of 4.5. Hubricht (1943) described the periostracum as reddish brown and covered with short hairs on both surfaces. Many specimens are reddish brown, but shell coloration in this species ranges from reddish brown to light brown or amber ( Figure 1 ).

Reproductive tract

In general outline, the genitalia of this animal are similar to those of S. stenotrema (Pfeiffer, 1842) as described by Pilsbry (1940, p. 640 and Figure 400A) ( Figure 4 ). There are, however, several features that may be considered diagnostic. In particular, the penis is short, stout, and surrounded by a broad band of muscle that extends from the origin of the penis, at the genital atrium, for some distance; the band constricts the diameter of the penis such that the apex of the organ, below its communication with the epiphallus, is inflated. The band and the most adjacent penile walls are annulated. The epiphallus inserts on the summit of the penis at the origin of the penile retractor and is inflated for a short distance before narrowing to the vas deferens. The latter structure is elongate and uniform in width and widens slightly at its insertion with the prostate, which is elongate, granular in texture, and tightly appressed to the uterus along its length. The free oviduct is short and unelaborated; it diverges into a simple, short spermathecal duct and connects to the unelaborated, ovoid spermatheca. The spermatheca and its associated duct are firmly appressed to and twisted about the free oviduct and uterine walls in the intact animal. The free oviduct is twisted like a rope; the twists number approximately ten in mature animals. The albumen gland is of the usual form, not elongate. The hermaphrodite duct is typically convoluted and associated with a small seminal receptacle that appears like an appendix. The ovotestis is merged with the digestive gland.

Radula

The radular ribbon is simple, with a dentary formula of 1:10:10 with minor variation (in the marginal count especially) ( Figure 5 ). The central tooth is symmetrical. More centrally positioned lateral teeth are asymmetric and are composed of a robust mesocone, reduced endocone, and a well­developed ectocone. Toward the zone of marginal teeth the lateral teeth become smaller and exaggerated in their asymmetry. Marginal teeth are smaller than laterals and are elaborately denticulate toward the margin. Overall ribbon length of five specimens was 2.0–2.5 mm and comprised a row count of 91.4 (range 75 – 104) for a total of between 1500 and 2000 teeth.

Jaws

In mature specimens examined here the broad ribs number from 5–8 ( Figure 5 ); Pilsbry (1940, p. 640) reported 8–13 in S. stenotrema . The jaw is amber colored and translucent. The intervals between the ribs are much smaller than the ribs themselves.

Habitat and Species Range

Euchemotrema hubrichti is known only from limestone bluffs in the Larue–Pine Hills region of Union County in southwestern Illinois. Euchemotrema hubrichti is found in low numbers at several localities throughout this region, but the tops of three outcrops appear to harbor hundreds of E. hubrichti individuals (estimated via mark­recapture at two sites; Anderson, in prep), and from another outcrop at lower abundance. These sites share a few characteristics. First, all of them have one or more relatively flat shelves, often covered with thin, loose limestone slabs of various sizes; E. hubrichti can sometimes be found in abundance attached to the underside of such slabs. Second, all of the high abundance sites are shaded to some extent by surrounding trees, but none are especially damp (rainwater seems to drain from the tops of the outcroppings quite quickly). Third, two of the three sites (and others where smaller numbers of E. hubrichti have been found) are topped with one or more Eastern red cedars ( Juniperus virginiana Linnaeus ), emphasizing the comparative aridity of these sites. Finally, E. hubrichti is far more numerous at these sites than any other species of land snail.

Genetics

A 647­bp sequence from the mitochondrial COI gene was amplified and sequenced from four E. hubrichti individuals. The base sequence in this region was identical in all four individuals, but it has been deposited as four sequences in GenBank (www.ncbi.nlm.nih.gov; accession numbers AY769091 ­ AY769094 ). A nucleotide­nucleotide BLAST search (“blastn”) using this sequence returned several stylommatophoran COI nucleotide sequences at>80% identity as the highest matches, supporting the assertion that the sequence is from E. hubrichti and not a contaminant (prior to this work, no other stylommatophoran snails had been studied in FEA’s lab).

Figures

FIGURE 1. Photographs of two live E. hubrichti specimens.

FIGURE 2. Relationship among shell height, shell diameter and sexual maturity for 238 E. hubrichti individuals. Inferred mature snails (individuals with a reflected aperture lip) are denoted with solid circles, immature snails (no reflected lip) with open circles.

FIGURE 3. The shell of E. hubrichti. Top row: Shell figures from Pilsbry, 1940 (fig. 423, p. 687). Bottom row: Shell figures from present study for comparison (shell measurements: 4.3 mm X 9.4 mm X 4.75 whorls). The black number 2 on the shell indicates that this shell was taken from one of the four specimens used for sequencing of the mitochondrial COI gene.

FIGURE 4. Line drawing of the reproductive tract. Abbre­ viations: AG = albumen gland, EP = epiphallus, FO = free oviduct, GA = genital atrium, HD = hermaphroditic duct, P = penis, PR = penial retractor, PS = prostate, UT = uterus, VD = vas deferens.

FIGURE 5. The radula (left) and jaw (right) of E. hubrichti.