Bivalves Are Mollusks with Two Shells

Bivalves Are Mollusks with Two Shells


  • All mollusks have soft bodies composed of a head, foot, and internal organs, and most have either an external shell (like a clam ) or a beak (like an octopus or squid ).


  • Bivalves are mollusks with two shells
  • Four common types of bivalves are:
  • clams
  • oysters
  • scallops
  • mussels
  • Mollusk shells are hard because they are made of calcium carbonate (CaCO3) .
  • You can often tell the age of a bivalve by looking at its shell. For instance, the bands or lines on a clam shell represent about one year’s growth. A clam that has four bands would be about four years old . You may notice that some bands are wider than others. Wider bands indicate more growth . In some years, conditions for growth are more favorable , so the clam grows more.
  • A giant clam (Tridacna gigas) from the tropical waters of the South Pacific was found to be over 60 years old, and weighed over 1000 pounds!
  • In terms of feeding, all bivalves are filter feeders.
  • Some bivalves like the mussels are sessile as adults. Mussels use tough, fibrous threads called byssal threads to attach themselves to substrates.
  • Most clams can move by using their muscular foot to dig into the sand.
  • The fastest bivalve is the scallop, which can shoot across the seafloor, by quickly forcing water out of its shell .


  • Gastropods are mollusks with one shell, and are sometimes known as univalves because of this.
  • The term “gastropod” means “stomach-foot”.
  • The most commonly known gastropod is the snail. Snails move along the surface of a substrate on their large muscular foot .
  • When a snail is threatened, they can retract their soft bodies into their shells .
  • Most marine snails feed on algae and the microscopic animals that live in the algae . All gastropods have a ribbon-like toothed structure known as the radula , which they use to scrape algae off the substrates.
  • Some marine snails are predators, which actively hunt and kill their prey, and others are scavengers , which feed on dead or dying organisms.
  • One unusual gastropod is the nudibranch, which means “naked lungs”- in reference to this animal’s shell-less structure. The nudibranchs include the sea slugs , and the sea hares .
  • The California sea hare, which can grow to 30 inches in length and weigh up to 35 pounds , is the biggest gastropod in the ocean.

CephalopodsWhere's the Octopus?

  • The term “cephalopod” means head-foot. This refers to these animal’s most prominent features being the head and the tentacles (the “foot”).
  • Unlike the bivalves and gastropods, the cephalopods are excellent swimmers . They swim by a kind of jet propulsion . Water is drawn into the head or mantle cavity. When the mantle contracts, water is expelled through a structure known as the siphon . This movement is similar to how a balloon flies around when its air escapes.
  • Cephalopods have highly developed nervous systems.
  • Probably the most commonly known cephalopods are octopi (singular octopus ) and squid . Other cephalopods include the cuttlefish and the chambered nautilus .
  • In terms of their feeding, allcephalopods are predators ,capturing prey, such as fish or crabs, with their tentacles , and killing prey with a bite from its bird-like beak , inside its mouth.
  • Octopi have eight tentacles, while squid have ten tentacles. On their tentacles are hundreds or thousands of suction disks used for grasping and holding prey and for climbing or crawling along the seafloor .
  • In addition to being able to move quickly using jet propulsion , cephalopods use camouflage to avoid being detected. Special pigmented cells in the skin, called chromatophores , expand and contract causing changes in skin pattern and coloration that allow these animals to quickly match their background.
  • Many cephalopods can also discharge a thick cloud of ink

in an attempt to escape from threats.

  • The largest cephalopod (also the largest invertebrate) is the giant squid Architeuthis, which can grow to over 60 feet in length.


  1. Calculate the failure rate of the untrained octopus.
  1. Why is the octopus considered to be so smart?
  1. Why was electric shock used in this experiment?
  1. Why were fish used?