The phylum Mollusca is one of several invertebrate (animals without a spine) groups and comprises a wide array of animals including gastropods (snails and slugs), cephalopods (squids, octopuses) and bivalves (clams, oysters). Of this group, the primary focus of this tool will be the terrestrial gastropods. In general, snails are often described as those species that possess a shell into which they can retract partially or wholly. Slugs may or may not have shells and for those species that do have shells, it is much reduced and may be internal. Also, for those slug species that have external shells, the shell cannot host the body of the animal and no obvious coiling can be observed.
All terrestrial gastropods have sensory organs referred to as tentacles. There are often two pairs: the larger, upper pair (ocular tentacles) bears the eyes at their tips, and the lower pair (oral tentacles) is used as a sensory organ for detecting odors (Figure 1). Some snail species have only one pair of tentacles (i.e., they lack the ocular tentacles). In these species, the eyes are located at the base of the sensory tentacles.
The mouth of the animal is located below the tentacles. It contains a specialized structure known as a radula, which is comprised of a mass of chitinous teeth arranged in rows. The radula is used to scrape pieces of food into the mouth of the animal using a back and forth motion.
The reproductive opening (genital pore) of terrestrial gastropods is generally located anterior-laterally. In snails, the genital pore is located on the head of the animal, just behind the tentacles. Slugs, however, have their genital pore located between the breathing pore and the head, and in some cases this structure may conceal by the mantle. Slugs in the family Veronicellidae are a notable exception to this rule. The genital opening of this group is located ventrally and there are two openings: one that allows access to the female portion of the genitalia and another that allows for the eversion of the male portion of the genitalia.
In most terrestrial gastropods, both sex organs occur in the same organism; however, there are a few cases where aphallic (does not have a penis) specimens of normally hermaphroditic species (e.g., Deroceras laeve) do exist. However, there are a few species in which separate sexes occur (e.g., Marisa cornuarietis).
The mantle is a structure that is located on the dorsal surface of the animal, just behind the head, and it mainly functions to secrete compounds that are used to construction the shell. In snails, the mantle is not readily noticeable as it is often restricted to the shell. On the other hand, the mantle of slugs is readily visible and generally extends over the back of the animal, covering anywhere from 30-100% of the dorsal surface (Figure 2). The mantle may extend over the shell of a few species of semi-slugs (e.g., Helicarionidae) when they are active, and can be retracted voluntarily by the animal.
The pneumostome or breathing pore is an opening in the mantle of the animal that supports gas exchange, by serving as the entrance to the animal’s lung. The pneumostome is located on the right side of the animal (i.e., when the animal is positioned with the tail facing the observer, the pneumostome is on the right of the observer).
The ventral portion of the animal bears a muscular structure termed the foot, which is used in locomotion. The skin of the entire animal secretes mucus that aids in the movement of the animal and also serves to reduce dehydration. Many terrestrial gastropods will produce copious amounts of mucus in an attempt to evade potential predators or when irritated.
Snails and slugs display selective preference for moist, humid habitats (e.g., gardens, forests, wetlands, greenhouses). There are a few terrestrial species that are adapted to environments atypical of terrestrial gastropods (e.g., the snail Cernuella virgata is adapted to living in sand dunes). Snails may aestivate under unfavorable conditions, by retracting into the shell and producing a mucilaginous structure (epiphragm) in the aperture (mouth) of the shell. The epiphragm will desiccate and become papery, thus sealing the aperture to reduce moisture loss. Prior to aestivation, some species prefer to affix themselves to vertical structures such as the sides of buildings, grass blades, and fence posts.
Terrestrial slugs generally prefer to inhabit dark, humid places such as beneath rocks and logs on the forest floor, in leaf litter, and under tree bark during daylight. They are normally nocturnal, although they may be found wandering about during the day after it rains. Snails and slugs feed primarily on plant material (living or dead), mushrooms, and lichens. On occasion, terrestrial slugs and snails may feed on conspecifics, other species of molluscs and their eggs, and calcareous material (e.g., rocks, headstones).
Snails: Juvenile to Adult
It is sometimes difficult to determine if a snail of a given species is a juvenile based solely on its shell. In many cases observation of the genitalia, through dissection of the specimen, is required. As a general rule, the shell of juveniles tend to have brittle apertural lips, whereas the apertural lips of adult specimens are often thickened, rigid and may be reflected in some species (e.g., Otala spp. and Eobania vermiculata). Also, the base of the juvenile aperture curves downward, whereas in adult specimens the apertural lips generally curve outward, rather than downward (Figure 4).
The genitalia (formed by the fusion of both male and female structures) are one of the most diagnostic characters of molluscs. In many groups (e.g., Veronicellids), positive species identification cannot be made without the use of the genitalic characters. A generalized diagram of the genitalia can be found in Figure 5. There also may be genitalic structures present in some species and not others. Some of these structures are illustrated in Figure 6.
Parts of the Reproductive System and their Function
- Ovotestis/Gonad: Site of egg and sperm development in hermaphroditic species (i.e., it functions as an ovary and a testis).
- Hermaphroditic duct/Ovotestis duct: Allows for the passage of the gametes to the fertilization pocket.
- Seminal vesicle: Functions in sperm storage (sometimes allow for further sperm maturation), re-absorption and degeneration.
- Albumen gland: The function of the albumen gland is to produce albumen or perivitelline fluid for the egg.
- Fertilization pouch-spermatheca complex (FPSC)/Fertilization pocket (pouch)/Talon/Carrefour/Spermoviduct: As its name suggests, this is the place where fertilization occurs.
- Prostate gland: Functions to produce seminal fluid.
- Bursa copulatrix/Spermatheca/Gametolytic gland: Functions to receive sperm during copulation. It is also said to have a function in sperm degradation.
- Oviduct: Functions to separate the groups of oocytes coming from the ovary into a line in order to increase the chances of being fertilized.
- Vas deferens: Functions to accumulate sperm prior to copulation.
- Vagina/Upper atrium: Functions to receive sperm during copulation.
- Atrium: Allows entry to the reproductive system.
- Flagellum: Used in sperm transfer.
- Penis: Functions to transfer sperm during copulation.
Terrestrial gastropods have the ability to independently manipulate the movement of the eggs and sperm that originate in the ovotestis.
- Sperm cells are continuously produced by the ovotestis and released into the hermaphroditic duct. The sperm cells may be temporarily stored in the hermaphroditic duct in seminal vesicles. When the sperm cells are needed for fertilization, the sperm cells actively migrate from the hermaphroditic duct to the fertilization pocket. Inside the fertilization pocket is a structure called the sperm duct. The sperm duct forms a groove that can be voluntarily closed by the animal during copulation. This functions to prevent self-fertilization when not desired.
- The sperm then migrates to the prostate gland, which produces fluids that provide nourishment to the passing sperm cells. This fluid is very thick and immobilizes the sperm cells. The immobilized sperm cells are then transported towards the vas deferens by the peristaltic movement of the walls of the prostate gland.
- The sperm cells are then transferred from the vas deferens to the penis via the epiphallus. The penis is then everted and the sperm mass deposited into the recipient’s atrium.
- The sperm cells may be transferred directly into the mating partner’s bursa copulatrix.
- A small percentage of the sperm cells deposited into the bursa copulatrix will migrate into the oviduct.
- The sperm cells now migrate from the oviduct into the fertilization pouch-spermatheca complex.
- Eggs are voluntarily released from the ovotestis into the fertilization pouch-spermatheca complex where it will unite with sperms that have migrated there.
- The fertilized eggs (zygotes) are provided with a nutritious albumen coat that is produced by the albumen gland. The eggs are then transported from the fertilization pouch-spermatheca complex into the oviduct section of the common duct where they may be arranged in a line (resembling a pearl necklace). Several layers of material of rich in calcium are then deposited around each egg prior to being laid by the recipient.
- The recipient animal then deposits the fertilized eggs.
It should be noted that self-fertilization could occur in a similar manner as described above, except no donor is involved.