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Terrestrial
Gastropods of the Columbia Basin, British Columbia
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| Identification
of Species
To aid in identifying species, a good hand lens of ×10-20 magnification or ideally a low-power microscope for observing minute details and small specimens is required. A set of callipers or a ruler (in mm) is needed for measuring. If a microscope is available, then a scale in the eyepiece is ideal for measuring small species. Almost all of our species of snails can be identified by shell characters, but it is especially important to have fresh, mature shells. Worn shells without a periostracum, which can be diagnostic, make unsuitable material as they may be difficult or impossible to identify. Juvenile shells can be a problem, to the novice, because some juveniles look quite different from the adults, and often lack some diagnostic characters. With experience, recognition of adults and corresponding juveniles will become easier, and with practice on fresh material old worn shells may be recognisable. For a few species of snails, the colour of the body is useful for identification. For example, Zonitoides nitidus and Z. arboreus have similar shells but the animals are markedly different in colour. The former is nearly jet black, and the latter is greyish on the head, tentacles and back and lighter on the sides. Make a habit of noting the colour of all species you collect. The colour is given in most of the species descriptions. Slugs are generally more difficult to identify than snails. The shell of slugs is rudimentary and either internal or at least partially external. Little information about a slug's identification can be derived from a slug's shell, except perhaps that it belongs to a certain family or genus. For example, the presence of a partially exposed flat shell midway on the back of a slug indicates that it is a Hemphillia. Useful external characters in identifying slugs to genus are the: size of the animal, position of the pneumostome, texture of the mantle, extent to which the keel on the tail is developed and presence and placement of an external shell. Size, shape, banding and overall colour, tubercles, sole, and stickiness and colour of the mucus are other important characters. Internal anatomy (particularly of the distal genital system) is diagnostic, and dissection may be required to confirm identifications of some species. Important features of the reproductive system of slugs are given below. Variation in nature is commonplace and should
always be expected. Variation in colour and form will likely be
encountered, but in most cases subtleties in form or colour are
of no taxonomic significance. When known, common variants are described.
The two colloquial terms, snail-like and sluglike, refer to the outward appearance of the animal, but do not imply any phylogenetic relationships. Snail-like gastropods have a coiled shell into which the animal can completely or almost completely withdraw. Sluglike gastropods have the appearance of a typical slug, either with no shell, an internal shell, or with a small shell partially embedded in the posterior part of the mantle, but into which the animal cannot fully retract. For this report, snails are identified by external characters of the shell and the body. Ideally, slug species should be confirmed by internal anatomy -- external characters are sufficient for identification to genus. For many species, considerable variation in characters can complicate identification. The majority of terrestrial molluscs have a coiled, external calcareous shell. In most sluglike gastropods, the reduced shell is usually internal and platelike, but in Arion, the shell is very rudimentary and consists of only a few irregular, discrete calcareous granules. The following section introduces and defines many of the specialized terms for describing gastropod shells.
The platelike shell of a slug is convex above and concave below. The earliest part of a slug's shell is the nucleus; the nucleus is positioned near the midline or to the side. Fine incremental lines are present on the surface of the shell. As indicated by the many terms used to describe it, the coiled shell of a snail is much more complex in form than the shell of a slug. The revolutions or turns of the tube that makes up a coiled shell are called whorls. The whorls spiral around an imaginary central axis, and the axis runs through the central pillar of the shell known as the columella. Whorls may enlarge rapidly or slowly, and are separated from one another by a single, continuous line called the suture. Some thin-shelled species have a second line or a broad, more opaque band next to the suture giving the appearance of being a double suture. In almost all examples, the largest whorl of a shell is the last whorl (=body whorl), and the second to last is the penultimate whorl. The spire consists of all the whorls except the last. The tip of the spire is called the apex, and the opposite end is the base. The animal extends out of its shell through an opening called the aperture [=mouth].
Shells can be viewed in apical, basal, apertural and abapertural views. (The "usual" orientation of a shell is an apertural view, with the aperture toward the viewer and with the apex pointing up.)
Width (= breadth, diameter) is the maximum measurement of the shell at right angles to the central axis. Width does not include the outwardly flared lip at the edge of the aperture, if this condition is present. Height (=length) is the maximum measurement along the central axis. In the species accounts, the width is only given when it exceeds the height, and height is given when it exceeds the width. These two terms refer to the shell, not the orientation of the shell on the living animal.
In basal view, some coiled shells have a hole or depression, the umbilicus, which may be wide enough to show the basal surfaces of all or some whorls of the spire. In some shells, there is no umbilicus, but when it is present, its size is expressed as a percentage of the maximum shell width (e.g. umbilicus 25% of the shell width). Seen from the side (i.e. in apertural or abapertural view) the whorls may be flat or convex in profile. The outermost extent of a whorl is its periphery, which may be placed at, above, or less frequently below, the midline of the whorl. For example, if the periphery is above the midline, then the periphery is closer to the suture than if the periphery was below the midline of the whorl.
At the apex of the spire, the protoconch, formed at a very young age, typically has different surface sculpture from the other whorls which are termed the teleoconch. The general form of coiled shells may be described by the following terms (in each case the shell is in apertural view): heliciform (resembling the genus Helix, with the width and height approximately equal and the whole shell somewhat rounded); depressed-heliciform (heliciform, but shell is short and wide); conic-heliciform (heliciform but with the spire more cone-shaped); cylindric (with sides nearly parallel and ends blunt); subcylindric (close to cylindric in form, but with sides not nearly so parallel); elongate-tapering (with sides of the spire distinctly tapering to a point and apex pointed); subovate (more or less egg-shaped, with sides of the spire more or less convex); conic-ovate (basally ovate, apically conic); and elongate-ovate (more or less elliptical).
The shell surface is usually sculptured or coloured in a spiral or axial pattern. Spiral sculpture or colour patterns run parallel to the suture, and axial patterns are more or less parallel to the central axis of the shell (or perpendicular to the spiral direction). Colour patterns occur in some species of snails. Narrow or broad spiral bands of colour or axial streaks may be present. Shells without pattern are said to be unicolour. The shell may be opaque or translucent, and glossy or dull. Glassy indicates a combination of both translucent and glossy, like a piece of glass. Textural ornamentation of the surface of a shell is called sculpture, which may be either raised or incised. Even apparently smooth shells usually have very fine sculpture when viewed under magnification. Striae are fine, usually incised but sometimes raised, lines or ridges that may be regular in size and spacing or irregular and wrinklelike. Ribs are larger, raised, angular or rounded ridges that may be regular in size and spacing, or not. Riblike elements with very narrow, bladelike edges are lamellar ribs. Striae, lamellar ribs and ribs may be either spiral or axial. Beads may be formed where striae intersect with ribs or striae. In some species, shallow, irregular depressions are referred to as malleations.
The edge of the aperture is the lip, which is divided into four regions: outer lip, basal lip, columellar lip and parietal lip. In most shells, the parietal lip is indistinct, being more or less fused to the preceding whorl and with only a thin layer of shell or callus. The outer and basal lips in particular may be thickened, flared, or reflected (hooked back). The aperture may have one or more projections within called denticles (=teeth), which are designated by their position (e.g., a parietal denticle is situated on or adjacent to the parietal lip).
Lip of the aperture: outer lip (o), basal lip (b), columellar lip (c) and parietal lip (p) with denticle. The families Vertiginidae and Pupillidae possess
denticles and some unique features on the outer lip. Terms
for these are adapted from Pilsbry
(1948), but for brevity, two letter codes for identifying denticles
are utilised here. The plicae (singular, plica) are
denticles located on the outer lip; these have also been termed
folds. Lamellae (singular, lamella) are denticles
located on the columellar lip and parietal wall. The four major
denticles present in most of our Vertigo species, are the
columellar lamella (Lc), parietal lamella (Lp),
upper palatal plica (Pu) and lower palatal plica (Pl).
Accessory to these, and either present or wanting -- depending upon
the species -- are the angular lamella (La), infraparietal
lamella (Li), suprapalatal plica (Ps), infrapalatal
plica (Pi) and subcolumellar lamella (Ls).
The method used for counting whorls is that of Kerney & Cameron (1979), Gerber (1996) and others, but not Pilsbry (1936). With a hand lens or microscope, orient the shell in apical view. On the protoconch determine where the suture first begins, then imagine a line as in the figure below. Count the whorls from this line, estimating the number of whorls to the nearest one-quarter turn. (Each crossing of the line will be one-half whorl.)
In the descriptions, shells are described as very small, small, medium, large, or very large (refer to the table), which is then followed by a range or a maximum dimension in millimeters (either width or height, whichever is the greatest).
The external colour of snails and slugs can sometimes be helpful in identifying species. Many slugs, however, are quite variable in colour and patterns. Animals without spots or bands are said to be unicolour. Darker lateral bands may be present on the back of many slugs. The head is at the leading end of the body. In the basommatophorans, there is one pair of sensory tentacles with eyes near the bases. In the stylommatophorans, there are sensory tentacles and occular tentacles. The occular tentacles, bearing eyes, are longer than the sensory tentacles and situated above (or posterior) to them. The mouth is located on the underside of the head. The mantle is a conspicuous raised area on the dorsal surface behind the head of a slug. The mantle of snails is less conspicuous, usually only visible around the edge of the shell. The mantle of slugs may be smoothish, shagreened (granular) or concentrically wrinkled. On the right side of the mantle, a conspicuous hole, the pneumostome (=breathing pore), leads into a respiratory organ in the mantle cavity. The pneumostome of snails is less evident than in slugs; look for it next to the shell on the right side of the animal when it is extended. Some slugs have a mantle groove that can be horseshoe shaped, as in the genus Milax (European and not confirmed for B.C.) or not so curved, as in the genus Boettgerilla. The body is the dorsal surface of the animal exclusive of the head and mantle. The skin may be smooth or roughened by tubercules or wrinkles. The tail is the posterior end of the body and either tapers gradually to a point or is truncate (abruptly cut off). Many slugs show a sharp ridge, or keel, down the midline of the body. It may extend from the tip of the tail to the mantle or be incomplete and not reach the mantle. Some groups have a caudal mucus gland at the tip of the tail that secretes slime or mucus. The opening of this gland is termed the caudal mucous pore (=caudal mucous gland), which may bear a mucus plug. The genital pore, located on the right side of the animal behind the right occular tentacle, is smaller than the pneumostome and leads into the reproductive tract of the animal. The flattened ventral surface of the body is the sole of the foot. The sole may be white or pigmented. If it appears to be divided into three longitudinal bands, the sole is said to be tripartite. The edge of the foot may be expanded up the sides of the body and may be developed into a foot fringe (=skirt). In some slugs, the foot fringe may be brightly pigmented. Mucus, secreted from the sole and body, can be clear, milky, or pigmented (yellow or orange), and sticky, or thin and watery. The colour of the mucus can be useful for identification of some slugs. In the species accounts, a general statement of size (small, medium-sized, large and very large) for sluglike gastropods is given (refer to table). These and the actual length (in millimeters) are in reference to the length of the entire animal when it is fully extended.
 
The reproductive tract is a significant taxonomic trait, and cannot be avoided in identifying some of our species. Pulmonates are simultaneous hermaphrodites (both sexes in the same animal) and therefore possess a complex the reproductive tract. At the proximal end of the reproductive system is the ovotestis (=gonad, hermaphrodite gland) where gametes are produced. The ovotestis is connected to the hermaphrodite duct, which runs forward to connect, along with the albumen gland, to the common duct (=spermoviduct). The male and female gametes are separated before entering the common duct, which has various male and female glands associated with it. The reproductive system distal to the common duct is separated into male and female ducts. This is the portion of the reproductive system that will be considered more fully, and is particularly taxonomically significant. The male portion of the distal reproductive tract consists of the vas deferens, penis and structures associated with these, although the penis is absent in some genera (Arion, for example). The vas deferens is a long, narrow duct connecting the common duct to the penis or epiphallus. The epiphallus is a dilated distal portion of the vas deferens and is present in some genera and not in others. The vas deferens or epiphallus then usually connects to the penis, but in Arion the penis is absent. The penis is an eversible saclike organ. Attached to the inside of the penis in some groups is a stimulator (=sarcobellum). Also present in some is a penial flagellum (=appendage, appendix, caecum). The penis (or epiphallus when the penis is absent) opens into the genital atrium. The female portion of the reproductive tract consists of the oviduct, which transfers eggs from the common duct to the genital atrium. In Arion, the atrium is further subdivided into an upper and lower atrium. A stimulating organ called the ligula can be present in the atrium. Also entering the atrium is the spermathecal duct (=bursa copulatrix duct), which leads to the spermatheca (=bursa copulatrix), a sac for storing spermatozoa. Finally, the atrium opens to the outside of the animal through a single hole, the genital pore. The genital pore is located on the animal slightly behind and below the right upper tentacle on the head. A detailed account of the reproductive system of slugs can be found in Terrestrial Slugs by South (1992). Although the structure of the jaw, radula, digestive tract and other organ systems are considered useful in pulmonate systematics, they are not considered here. A complete list of anatomical and conchological
terns in the glossary.
Collecting, Preserving and Dissecting Terrestrial Gastropods Because so much of British Columbia is relatively unstudied, collections of specimens supplied with data, can be valuable to science. Basic specimen data, as given by Solem et al. (1981), consists of:
For how to dissect slugs and snails, refer to Kerney & Cameron (1979). |
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Copyright © Royal
BC Museum |
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