Mollusks

What is a Mollusk? Phylum Molluska Soft-bodied animals that usually have a shell – either internal or external. Snails, slugs, clams, squid, and octopi are all in their group. Many aquatic mollusks have free- swimming larvae called trochophore.

Form and Function in Mollusks Most mollusks have 4 parts: foot, mantle, shell, and visceral mass. The foot can look very different – it can be used for crawling, burrowing, or modified as tentacles. The mantle is thin layer that covers the body and is used for protection and to make the shell. The shell is for protection. In some mollusks it is small or not there at all. The visceral mass is all of the internal organs.

Mollusks feed differently – there are herbivores, carnivores, filter feeders, detritivores, and parasites. Snails and slugs have a radula – a tongue- like organ with rows of many sharp teeth. This helps scrape algae off rocks or eat plants. Carnivorous mollusks use their radula to drill through shells of other animals. Octopi and sea slugs have sharp jaws to eat prey. Clams, oysters, and scallops are filter feeders. They have a siphon where water enters and exits the body. They use gills to trap food.

Aquatic mollusks use gills for gas exchange. Land snails and slugs have a large mantle cavity that is lined with blood vessels for gas exchange.

Mollusks have an open circulatory system – this means that blood is pumped through vessels by a simple heart and through sinuses – a large saclike space – to the gills, and then back again.

Gastropods Class Gastropoda – include snails, slugs, sea butterflies, sea hares, limpets, and nudibranchs. These mollusks have single shells or no shell and use a muscular foot to move. Many gastropods can pull their body into their shell. Slugs will move at night and hide during the day. Nudibranchs are poisonous. They eat cnidarians and use the nematocysts from the cnidarians to sting predators.

Bivalves Class Bivalvia – clams, oysters, mussels, and scallops. Have 2 shells held together by 1 or 2 powerful muscles. They do not move much – they burrow in mud or sand. They are filter feeders.

Cephalopods Class Cephalopoda – octopi, squids, cuttlefish, and nautiluses. They are soft bodied mollusks with a head attached to a single foot, which is divided into tentacles or arms. They have a small internal shell or no shell at all. The only one with an external shell if the chambered nautilus. Cuttlefish have small shells in their bodies. Cephalopods have complex sense organs. They have eyes that are very complex.

Ecology of Mollusks Many mollusks feed on plants, prey on animals, and filter algae out of the water. Some are important for symbiotic algae. They are also a very important food source.