The most generalized gastropod is the Subclass Prosobranchia, the shelled marine gastropods. Prosobranchs are the common snail-like animals you encounter in local tide pools. You can easily see a clear example of adaptive radiation within this group. For example:

(1) Limpets: retaining an ancestral life style

Zoologists suggest that the ancestral mollusc was probably a slow, bottom-dwelling animal that lived in shallow water and fed on fine, particulate material which it grazed off the rocks with its radula. Look at a modern gastropod called a limpet and you'll see an animal with essentially the same life style as the proposed ancestor. Limpets live along rocky shorelines and feed on microscopic algae that they scrape off the rocks with a strong, deeply cusped radula. Place a limpet on its back in sea water and observe its broad foot, head and mantle. You'll notice a large space behind its head, the mantle cavity, and careful observation will reveal a single, large ctenidium in this space. Watch the limpet's head and you may see it protrude its radula.

Next obtain a slipper limpet. Place it on its back in sea water and observe it as you did the limpet. This animal makes its living by filter-feeding. The slipper limpet uses its large ctenidium to trap food much like a bivalve mollusc does. Suspended food is trapped in mucous and pulled into the slipper limpet's mouth with its radula. You'll also notice a comb-like structure near the gill called the osphradium, which the animal uses to test the incoming water for potential food and suspended sediment. If your specimen is a male a large penis should also be evident on the right side of the head.

(2) The black turban snail

When you visit a rocky shoreline the abundance of seaweed is quite apparent. Seaweeds represent a nutritious plant food source and not surprisingly there are many gastropods that feed on these marine algae. The much-prized abalone is one such herbivore and the black turban snail is another. Place a black turban snail in a dish of sea water and observe. Chances are that handling the snail has caused it to retreat into its shell with its leathery operculum pulled up snug. If left undisturbed for a few minutes its should eventually emerge. Notice how it protrudes its foot from the shell .... how does it do this? Next notice its tentacles "feeling" the way ahead of it and special mantle tentacles extending from the snail's sides which it also uses to perceive its environment. The black turban feeds on large algae using a radula modified for feeding on plant tissue, not scraping across rocky substrata like the limpet's.

(3) Barnacle drills: carnivorous gastropods

With an abundance of herbivores and filter-feeders in the rocky intertidal, you'd expect to find carnivores using them for prey. We'd also expect adaptively-radiated prosobranch gastropods among these carnivores, and sure enough they are! Examine the group of snails known as "barnacle drills" in the lab. As the name implies these carnivorous snails use their specialized radulae to bore into shells of barnacles and other hard-shelled prey like clams and other gastropods. Observe a barnacle drill in a dish of sea water. Notice anything different about its shell or body? Carnivorous snails often have an elongated shell that extends anteriorly into a canal or groove. This canal harbors the snail's siphon, an elongated tube of mantle tissue, which the snail uses to draw water into its mantle cavity. The incoming water flows over the carnivore's osphradium allowing it to taste water for the presence of prey. You may notice your snail swinging the siphon from side to side attempting to pick up a promising scent. Another feature of these carnivores is the elaboration of their oral region into an extendable proboscis which contains the radula. The proboscis allows the snail to probe into areas otherwise not reachable, and tear the flesh of its victim with a radula equipped with dagger-like teeth. Prosobranch carnivores occur in many environments. Observe the shells of other prosobranch gastropods on display and locate the shell of the moon snail and the shells of clams with round, counter-sunk bore holes made by this predator.

(4) The cone snail: the bearer of flying venom-filled needle teeth

Find shells of cone snails, the deadliest carnivorous prosobranchs. These tropical snails use single radula teeth that function as harpoon-like hypodermic needles. The cone snail extends its large proboscis and fires the radula tooth into an unsuspecting prey, injecting poison from special glands into the wound through the hollow tooth. Toxin renders the prey immobile and the cone swallows the prey whole as seen in the diagram. There are also scavengers, parasites and prosobranchs that swim in plankton.

(B) Subclass Opisthobranchia

A second group of marine gastropod molluscs is Subclass Opisthobranchia. The gaudy sea slugs and nudibranchs belong to this subclass. Many diverse forms occur, but what characterizes them as a group is the lack of a protective shell. Furthermore, nudibranchs are typically brightly colored with warning coloration. Many nudibranchs also taste bad or have other noxious qualities that would teach a predator to avoid their bright images. Additionally, a number of nudibranchs feed on cnidarians such as sea anemones and hydroids, and are able to ingest the prey's stinging cells intact. These cells are transferred to the nudibranch's extremities where they are used for their own defense. Sponge-feeding nudibranchs incorporate the sponges's hard spicules into their own body wall. Thus, giving up a shell has been compensated for in a number of other protective syndromes. What are the advantages of not having a shell? Observe the nudibranchs available in the lab.

(C) Subclass Pulmonata

A third gastropod subclass is the Pulmonata (lung-bearers). These snails have evolved from marine ancestors and reside primarily in freshwater and terrestrial habitats. Many members of this subclass, such as the garden slug or the giant banana slug of redwood forests, have also lost their shells. The pulmonate snails have been so successful on land because their mantle cavity has been converted to an air-breathing lung. Through evolutionary time the gill has been lost and the mantle cavity has been closed except for an opening, the pneumostome. This opens and closes via a sphincter muscle allowing air to be trapped in the moist, highly-vascularized mantle cavity-lung. Because of this lung, pulmonate snails are able to carry moisture with them and live free of water. They have invaded even xeric habitats. Examine a garden snail or slug for the pneumostome opening and closing on the snail's right side. The pulmonate snails play many roles in aquatic and terrestrial habitats including scavenger, herbivore and carnivore; there are even pulmonate limpets!