- Related Topics:
- prosobranch
- opisthobranch
- pulmonate
- Bellerophon
- limpet
- On the Web:
- Food and Agriculture Organization of the United Nations - Gastropods (PDF) (Mar. 10, 2025)
The foot is the organ of locomotion in land gastropods. In swimming and sessile forms, however, the foot is greatly reduced or greatly modified. The normal progression of a snail is by muscular action, with a series of contraction waves proceeding from the posterior to the anterior end of the gliding portion of the foot. A few groups have the foot divided into right and left halves, with separate waves moving on each side. When the foot is narrow, as in Strombus and Aporrhais, the animal moves in fits and starts, tumbling along by a digging action of the foot and the pointed operculum. Certain small gastropod species move by the beating action of cilia of the foot on the mucous sheet secreted by the anterior part of the foot. Most prosobranchs are slow-moving, with a speed of less than eight centimetres (about three inches) per minute, although Haliotis has been reported to move at almost 10 times that rate.
Many opisthobranchs use foot musculature to move, but some glide on the underside of water-surface films through ciliary action. Swimming has been achieved in a number of ways. Body undulations propel such large snails as Dendronotus and Melibe. Pteropods, Gastropteron, Akera, and others move foot flaps (parapodia) to provide motion, and some species swim by undulating their entire bodies.
Freshwater pulmonates use ciliary action on a bed of mucus secreted by the snail.
Land pulmonates depend upon a combination of muscular action and cilia for locomotion. In many of these species the foot is divided longitudinally into three parts, with locomotor activity being confined to the central section, which glides on a mucous track. An additional use of slime by slugs is in the act of mating. A slime rope is secreted from which the mating pair of slugs are able to suspend themselves. If irritated, slugs can secrete copious quantities of slime. This reaction is the basis for one of the most effective methods of controlling slugs: spreading enough ashes in slug-infested areas causes exhaustion and death of the animals through the overproduction of slime.
Some of the small, tropical, brightly coloured sluglike species will, when disturbed, travel at a very high rate of speed with the anterior half of the foot lifted off the ground. They can continue moving at this pace for a distance of almost a metre at a rate faster than one metre per minute in snails less than two to three centimetres (or about one inch) in body length. Large gastropods, such as Achatina or Strophocheilus, are much slower, although carnivores are usually relatively fast-moving.
Food and feeding
As in all molluscan groups except the bivalves, gastropods have a firm odontophore at the anterior end of the digestive tract. Generally, this organ supports a broad ribbon (radula) covered with a few to many thousand “teeth” (denticles). The radula is used in feeding: muscles extrude the radula from the mouth, spread it out, and then slide it over the supporting odontophore, carrying particles or pieces of food and debris into the esophagus. Although attached at both ends, the radula grows continuously during the gastropod’s life, with new rows of denticles being formed posteriorly to replace the worn denticles cast off at the anterior end. Both form and number of denticles vary greatly among species—the differences correlating with food and habitat changes. Radular morphology is an important tool for species identification.
Evidently, the most primitive type of gastropod feeding involved browsing and grazing of algae from rocks. Some species of the order Archaeogastropoda still retain the basic rhipidoglossan radula, in which many slender marginal teeth are arranged in transverse rows. During use, the outer, or marginal, denticles swing outward, and the radula is curled under the anterior end of the odontophore. The latter is pressed against the feeding surface, and, one row at a time, the denticles are erected and scrape across the surface, removing fine particles as the odontophore is withdrawn into the mouth. As the marginals swing inward, food particles are carried toward the midline of the radula and collected into a mucous mass. By folding the teeth inward, damage to the mouth lining is avoided and food particles are concentrated. Mucus-bound food particles are then passed through the esophagus and into the gut for sorting and digestion.
From this basic pattern, numerous specializations have developed, involving changes in the numbers, sizes, and shapes of radular teeth that correspond to dietary specializations. Prosobranch gastropods include herbivores, omnivores, parasites, and carnivores, some of which drill through the shells of bivalves, gastropods, or echinoderms to feed. Some gastropods, for example, possess a “toxoglossate” radula that has only two teeth, which are formed and used alternately. Most toxoglossate gastropods inject a poison via the functional tooth. Prey selection usually is highly specific. Although many cones hunt polychaete worms, others prey on gastropods or fishes, using the radular tooth as a harpoon, with poison being injected into the prey through the hollow shaft of the tooth. Several of the large fish-eating cones, which produce a variety of potent nerve poisons, have been known to kill humans.
Some other gastropods, such as the opisthobranch Dolabella, have as many as 460 teeth per row with a total of 25,000 denticles. In terms of feeding, opisthobranchs are extremely varied. Besides the algae-sucking sacoglossans, Aplysia cuts up strips of seaweed for swallowing, and a number of the more primitive species feed on algae encrusted on rocks. Perhaps the majority of opisthobranchs, including the sea slugs, are predators on sessile animals, ascidians and coelenterates being especially favoured. Pyramidellids are ectoparasites on a variety of organisms. Some of the pteropods are ciliary feeders on microorganisms.
Pulmonate gastropods are predominantly herbivores, with only a few scavenging and predatory species. Primitively, the pulmonate radular tooth has three raised points, or cusps (i.e., is tricuspid), but modifications involving splitting of cusps or reductions to one cusp are numerous. The modification of the radular tooth reflects dietary differences between species. In particular, with each successive appearance of a carnivorous type during evolution, the teeth have been reduced in number, each tooth usually having one long, sickle-shaped cusp.
Much of the diversity achieved by the gastropods relates to the evolutionary shifts in radular structure, which have led to exploitation of a variety of food sources. Predators capable of swimming, surface crawling, and burrowing to capture prey have evolved among the prosobranchs and opisthobranchs; predators that produce chemical substances for entering the shells of their prey have evolved among the mesogastropods (family Naticidae and superfamily Tonnacea), the neogastropods (family Muricidae), and a nudibranch opisthobranch (Okadaia); and, in the pulmonates, predation and thus a carnivorous diet have evolved at least 12 times.
Form and function
Gastropods present such a variety of structures and adaptations that few all-encompassing characteristics can be presented. The following survey focuses on variety in the external shell and the body.
