CLASS: GASTROPODA SLUGS AND SNAILS CHARACTERISTICS Most have a special shell and, since shape and texture vary widely from one species to the next, the shell provides a ready means of distinguishing the species. The visceral mass lies on top of the foot, giving the group their scientific name: gastropoda = gut-footed. The visceral mass contains the gut, reproductive organs, blood systems and kidneys. The foot and head can be withdrawn into the mantle cavity and in some is protected by an operculum or door. WINKLES Habitat From the littorina zone to the lower balanoid zone, on rocks, beneath rocks, among sea plants in rock pools. However, the alikreukel or giant periwinkle (see diagram below) can be found down to a depth of 8m. Description There are many different types. They have coiled shells with a round opening. Many species have an operculum, which is a trap door made of shell-like or horny material. When attacked the winkle withdraws into the shell and a perfectly fitting trapdoor slams shut. Feeding They are herbivores (plant eaters) and graze on algae. They scrape rocks and rasp sea plants with their tongues (radulae). Predators Fish and crustaceans. 44 Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES LIMPETS Habitat They form the dominant communities in the intertidal zones. Each limpet has a home scar where its shell grows to fit the shape of the rock. Description Flat molluscs with cap-shaped shells and a wide opening. They have a large foot and a head with feelers and eyes. Different species of limpets are adapted to different lifestyles. Feeding All limpets are herbivores. They have a great influence on the rocky shores by their grazing activities. They control seaweed growth and the settling of certain larvae. The limpet scrapes the rocks and rasps the seaweeds with its tongue (radula). Predators Fish, especially the Rocksucker fish, lobsters and whelks, eats them. Did you know? The radula of a limpet may be twice Keyhole Limpet as long as the body The gut of a limpet can be up to eight times the body length to cope with its sea plant diet. Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES 45 ABALONE - PERLEMOEN AND OTHER EAR SHELLS Habitat Juvenile perlemoen occur under intertidal boulders or beneath certain species of sea urchins. Adults have a very limited range and occur in kelp forests to depths of about 9m between Saldanha Bay to as far as East London in the lower intertidal and sub-tidal zones. Venus ear or siffie lives in rocky crevices and amongst red bait. Spiral - ridged siffie lives under stones in rock pools or on shallow reefs. Description It is a type of sea snail with a large muscular foot, which grips tightly to rocks. The perlemoen has a small head with two feelers and two eyes. The shell is shaped like an ear and forms a very flattened spiral with a row of small holes along the left side. As the shell grows, new holes are created and old ones filled in. Reaches up to 18 cm in length. Abalone is a much sought after seafood fetching high prizes on the overseas market. Why does the shell have holes? Water is circulated through the shell to supply oxygen to the gills, which lie beneath the row of holes, and to carry away waste products. The water enters at the front of the shell and leaves through the holes, ensuring that the intake water is separated from the wastewater. 46 Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES Siffie or Venus ear Spiral-ridged siffie Haliotis spadicea Haliotis parva Smooth shell with a red stain beneath the Has a conspicuous spiral ridge around the spire. shell. It reaches up to 8 cm long. Small - only up to 4,5 cm. Feeding It lifts its shell and part of its foot to trap kelp fronds and other seaweeds that are swept under it by the waves. It then uses the long ribbon-like tongue (radula) to rasp off the food.1 Predators Man and some reef fish feed on juveniles and adults. Did you know? The perlemoen is the largest ear shell in South Africa. It has beautiful mother of pearl lining on the inside of the shell. Shells of 18 cm are approximately 30 years old. The perlemoen resource is on the verge of collapse due to uncontrollable poaching and export to the Far East. It is easy to dive and retrieve these animals from the shallow waters where they live. Perlemoen farming is now in progress but it has high start-up costs and it is 5 years before they can be harvested. 1 Extract from Fact Sheet Mollusca by Margo Branch August 1996 Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES 47 WHELKS Habitat Great variety of habitats; under rocks, in shallow water, among redbait or mussels, in the holdfasts of kelp etc. Description Usually pointed spiral shells with an oval opening. Some shells have a notch or long groove. Most carnivorous species have a proboscis that can be extended along a notch or groove to catch prey. A siphon is present. It directs the water into the mantle cavity allowing the animal to sample the water from a different direction. It also aids in the location of its prey. Feeding Feed on dead or live animals. Some species use their hard tongue (radula) to drill a hole in the shell of live animals. Predators Fish, starfish, crabs, lobster and other predatory snails feed on whelks. Plough Shell 48 Two Oceans Aquarium Volunteer Manual – Module 3 – INVERTEBRATES .