How to Identify Terrestrial Gastropods

Introduction

It may be difficult, even for malacologists, to identify molluscs, simply because they do not usually possess many characters that are consistently useful for distinguishing among related species. This section of the tool was designed to assist the user in becoming familiar with the common characters that are used in the identification of terrestrial snails and slugs.

Shell Classification

How do you know if you truly have a snail or a slug?

Gastropods that possess an obvious shell are termed snails whereas gastropods that appear to lack an obvious shell are termed slugs. In the case of semi-slugs it may be debatable whether the animal should be considered a snail or a slug. The entities included in this tool are divided into two major categories (snails and slugs) to reduce ambiguity and to allow users to quickly and more efficiently navigate through the key.

  • Shell Present:

Shell obvious with definite coiling and animal may be able to retract into it. shells Figure 1. Typical snails

  • Shell Absent:

Shell very reduced or internal and if present, it has no definite coiling. If the shell is partially external, it is usually small and is located on the posterior end of the mantle (see image below, far right). shells absent Figure 2. Typical slugs

Slug Characters

Several morphological characters can be used to identify slugs. A few of these include:

Mantle Characters:
  • Body covered by mantle (partly or wholly)
  • Location of breathing pore on mantle (or on the body of the animal)
  • Mantle groove
Body Characters:
  • Length (preserved specimens may shrink to approximately 70-80 % the length of living specimens)
  • Body color
  • Body markings (spots, blotches, stripes, bands)
  • Mucus pore
  • Length of the slug (fully extended at maturity)
  • Sole color
  • Tail constriction at the point of amputation (this is a faint groove that can be observed on the dorsal surface of the tail; behind the mantle. A narrow dark-colored band on the sole of the animal can also represent the point of amputation.) It should be noted that the point of amputation might not always be visible in species that typically possess one.
Mucus Color:
  • White, yellow, orange, clear
Keel Characters:
  • Presence or absence of the keel
  • Length of the keel

Shell Characters

Shells generally have a large number of characters that can be used to distinguish between groups of snails. Shell sculpturing is one such character.

Common shell sculpturing include:

  • Hairs/Bristles – projections on the shell that resemble mammalian hair
  • Pits – regularly shaped indentation in the shell
  • Dents- irregularly shaped indentations in a shell
  • Striae – groove-like indentations that follow the whorls
  • Lirae – raised ridges that follow the whorls
  • Ribs – raised ridges that run at an angle (usually transversely) to the whorls
  • Pleats/ Wrinkles – any type of ridging or creasing that appears to have been formed by folding or crumpling

Parts of a Shell Shell Terminology Figure 3. Shell terminology.

Common Shell Types

Parts of a Shell Parts of a Shell Figure 4. Common shell types.

How to Measure a Terrestrial Gastropod

Measurement can be a useful character in the identification of a terrestrial gastropod. In snails, the length is taken from the apex of the shell to the base of the aperture (mouth). The width should be taken at the widest part of the shell when the shell is oriented so that aperture faces the observer; the width is measured from the side of the body whorl to the outermost side of the aperture (mouth). Terrestrial slugs are measured from the head, excluding the tentacles to the tip of the tail (Figure. 5). It is important that the animal is fully extended to in order to obtain an accurate measurement. Measurement Diagram Measurement Image Measurement Image 2

Figure 5. Measuring terrestrial gastropods

Umbilicus

The umbilicus may be used as a diagnostic character when classifying snails. The umbilicus may be open or closed. The width of the open umbilicus is taken at the widest part of the inner surfaces of the body whorl (Figure. 6). Umbilicus Characters Umbilicus Measurements Figure 6. Types of umbilicus commonly observed in terrestrial snails.

Counting Whorls

There are several ways to count the number of whorls on the shell of a snail. The most commonly used method described by Pilsbry (1939) will be discussed here. Before counting the whorls, an imaginary line should be drawn across the shell as demonstrated in figure 7 below. The whorls are then counted following the direction of the coils. A complete turn indicates a whorl (i.e., every time the line is intersected when following the whorls). The body whorl may not be complete, meaning that it may end in quarters or thirds (Figure 7).

Counting Whorls Figure 7. Counting shell whorls.

Genitalia

The genitalia (formed by the fusion of both male and female structure) are one of the most diagnostic characters used to distinguish between mollusc species. In many groups (e.g., Veronicellids), a positive identification cannot be obtained without the use of the genitalia characters.  A generalized diagram of the genitalia can be found in Figure 8. There may also be reproductive structures that are present in some species and not others. Additional information on the genitalia (structure and function) can be found in the biology section of this tool. Generalized genitalia diagram Figure 8. Generalized diagram of a terrestrial mollusc’s reproductive system.