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Discover the amazing world of Mollusca, the second largest animal phylum on earth with over 200,000 species. Explore the unique features such as shells, mantle, muscular foot, and radula. Dive into Gastropoda, Bivalvia, and Cephalopoda, each with distinct characteristics and habitats. From snails and slugs to octopuses and squids, learn about their feeding habits, locomotion, and specialized adaptations. Delve into the fascinating world of sea creatures like nudibranchs, limpets, abalones, and more. Uncover the secrets of pearl formation, scallop swimming, and jet propulsion in cephalopods. Immerse yourself in the beauty and diversity of Mollusca!
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Phylum - Mollusca The soft-bodied animals with a calcium carbonate shell
Mollusca is the second largest animal phylum on earth. There are as many as 200,000 species. • A shell (or shells) is often a feature of many mollusks. The shell provides protection for the body and a solid structure for muscle attachment. • All mollusk shells are lined with and secreted by a fleshy fold of tissue called the mantle. • Mollusks also have a muscular foot used for locomotion and burrowing. • Most mollusks have a head that includes eyes and other sensory organs. • Mollusks have efficient excretory organs, nephridia. They are small tubules that collect waste from body fluids and discharge them from the body.
Gastropoda- meaning “stomach foot”, • Includes slugs and snails. • Found on land, in salt water and in fresh water. • These organisms move by creeping along slowly in a muscular foot. • The shell, if present, is often brightly colored and usually coiled. • Gastropods have a specialized feeding organ called a radula. The radula is a flexible structure that is covered by rows of teeth made of chitin. It is used to scrape free small particles of food. In some mollusks, the radula is a sharp fang that is used to stab prey and inject venom. • Gas exchange is through paired gills. • Sea slugs often retain noxious chemicals or undischarged nematocysts taken undigested from prey. • Examples are limpets, abalones, periwinkles, whelks, oyster drills, cone snails, sea hares, and nudibranchs.
Bivalvia – Bivalves are laterally compressed and have two shells. Clams, mussels, oysters, and shipworms. • Strong muscles close the valves. • Bivalves feed by filtering small particles from the water using siphons. • Most bivalves are sedentary; oysters and mussels permanently attach themselves to hard surfaces by secreting byssal threads as adults. • Their foot is wedged-shaped and is used for digging in sand or mud or for secreting tough attachment threads. • Pearls are formed when the oyster secretes shiny layers of calcium carbonate to coat irritating particles or parasites lodged between the mantle and the iridescent inner surface of the shell, which is called mother-of-pearl. • Scallops can swim for short distances by rapidly ejecting water from the mantle cavity and clapping the valves.
Cephalopoda – “head-footed” Examples: octopuses, squids and cuttlefishes. All living species are marine. • Cephalopods are active, voracious predators that can be fast swimmers. • The foot is modified into arms and tentacles, usually equipped with suckers that are used to capture prey. • Except for nautiluses, cephalopods have reduced or no shell at all. • The body of the cephalopod includes a head, tentacles, and a large fleshy mantle. • A tube-like siphon derived from the foot serves as an outlet for water leaving the mantle cavity. When the mantle muscles contract, water is rapidly forced out of the siphon, allowing the animals to move by jet propulsion.
Squid • Squid are better adapted for swimming that octopuses. • The body is elongated and covered by mantle. It also forms two triangular fins. • Squids can remain motionless in one place or move backward or forward just by changing the direction of the siphon. • Eight arms and two tentacles surround the mouth. The arms and the broad tips of the tentacles are covered with suckers. • The tentacles are long and retractable and can be swiftly shot out to catch prey. • The shell is reduced to a chitinouspen that is embedded in the upper surface of the mantle. • The adult size varies from tiny individuals to 66 feet in the giant squid, the largest living invertebrate (a deep water species).
Thousands of jumbo flying squid (Dosidicusgigas) or Humboldt squid, were found stranded on southern California beaches over the summer of 2002. The normal range of these squid, also known as Humboldt squid, is from Peru north to Baja California. However, warm water events such as El Niño can extend the range of these squid to Oregon.
They attack WHALES for food: here is a picture of whale skin with tentacle suction scars on it
Other Mollusca • Chitons- About 800 species, all of them marine, they have 8 overlapping shell plates that cover their slightly arched dorsal surface. Most live on the rocky shores and use a radula to graze on algae on the rocks. Their internal organs are not coiled like a snail. • Tusk shells, or scaphlopods- have an elongated shell, open at both ends and tapered like an elephant tusk. They live in sandy or muddy bottoms. The narrow end of the shell protrudes from the bottom.
The Gumboot Chiton (Cryptochiton stelleri) is the largest chiton in the world and can grow up to 13" long. It resembles a football in size, shape, color and texture. It doesn't appear to have the signature eight articulated plates of a chiton, but upon closer examination of the photo below, you can distinguish the outline of the plates under the leathery "girdle" that has completely overgrown them.
