Introduction to Gastropods
The Class Gastropoda includes soft bodied animals, with a head, foot, visceral body mass and a mantle, often protected by a shell – all animals referred to as ‘gastropods.’ Gastropods are mollusks, and a extremely diverse group that includes over 40,000 species. Envision a sea shell, and you’re thinking about a gastropod, although this class contains many shell-less animals as well, such as the beautifully colored nudibranchs.
Many gastropods, such as snails and limpets, have one shell. Sea slugs, like nudibranchs and sea hares, do not have a shell, although they may have an internal shell made of protein. Gastropods come in a wide variety of colors, shapes and sizes.
Many gastropods have one shell in which the animal can withdraw. The shell is usually coiled, and may be ‘left-handed’ or sinistral (spiraled counter-clockwise) or ‘right-handed’ or dextral (clockwise).
Gastropods move using a muscular foot.
All young (larval stage) gastropods undergo a process called torsion, in which the entire top of their body twists 180 degrees on their foot. This results in the placement of the gills and anus above the head. Gastropods have adapted in a variety of ways to avoid polluting their breathing water with their own wastes.
Due to torsion, adult gastropods are asymmetrical in form.
Sea Slugs: sea slug, name for a marine gastropod mollusk that lacks a shell as an adult and is usually brightly colored. Sea slugs, or nudibranchs, are distributed throughout the world, with the greatest numbers and the largest kinds found in tropical waters. They creep along the bottom or cling to submerged vegetation, usually in water just below the low tide line. Members of a few species swim on the surface in open ocean. Most sea slugs are under 1 in. (2.5 cm) long, although the largest, found in the Great Barrier Reef of Australia, reaches 12 in. (30 cm). Regarded by many people as the most beautiful of marine animals, sea slugs display a great array of solid colors and patterns. Many have feathery structures (ceratia) on the back, often in a contrasting color. Most sea slugs have two pairs of tentacles on the head, used for tactile and chemosensory reception, with a small eye at the base of each tentacle. Sea slugs graze on small sessile animals such as coelenterates, sponges, and bryozoans. Certain sea slugs that feed on corals and sea anemones ingest the stinging cells of their prey without discharging them; these then pass from the slug’s digestive tract to the ceratia, where they are used by the slug for its own defense.