Mollusks

Kingdom – Animalia

Phylum – Mollusca
Malacology is the study of Mollusks. Malacology differs from Conchology which is the study of shells only. There are over 100,000 described living mollusk species and an estimated 100,000 undescribed species that await formal description. For more information visit the INHS Mollusk Collection website.

Mollusks have:

  • a mantle covering the foot and visceral mass
  • reduced coelom (body cavity) containing the kidneys, gonads and pericardium
  • a through gut with mouth and anus
  • a nervous system
  • an open circulatory system with a heart and aorta
  • ctenidial gills
  • a radula – “tongue” with thousands of teeth that can grab onto food

Mollusks are often covered by a hard exoskeleton secreted by the mantle, though slugs, octopuses, squids and cuttlefish have lost their mantle and shell.

Bivalvia – Clams

clam Includes: clams, oysters, mussels, scallops and cockles. In Illinois:
Bivalves have:
  • Two shell halves connected by a hinge and “teeth”
  • Shell composed of calcium carbonate secreted by mantle
  • No head region or radula
  • Separate males and females
  • Filter feed through siphons
  • Ctenidia (gills) for breathing and feeding
  • A foot to dig into substrate or move along
  • Larval stage (glochidia) are parasites on fish

Gastropoda – Snails and Slugs

snail Includes: snails, slugs, nudibranchs, whelks, conchs and abalone In Illinois:
Gastropods have:
  • a coiled shell – slugs and nudibranchs have lost their shells
  • a muscular foot secretes mucus on which to glide
  • a larval stage that undergoes torsion – a 180 degree twist
  • asymmetrical bodies
  • a pair of stalked eyes

Freshwater mussels rely on a host species, typically a fish, to complete their life cycle. Larval mussels, called glochidia, attach to the gills or fins or other parts of the host fish and eventually transform to juvenile mussels and drop off. Some species use lures that look like a potential prey item or even a mate.

Conglutinate full of freshwater mussel glochidia. A fish that eats this becomes the host.
Conglutinate magnified
  • life cycle of mussel

Learn more about our research on freshwater mussels:
Freshwater Mollusk Ecology and Conservation
Urban Biotic Assesment Program

To see some of the other ways that mussels get their larvae onto fish, visit the Freshwater Mollusk Conservation Society.

Marine Mollusk Groups

Polyplacophora – Chitons
  • Reduced head
  • Flattened foot
  • Eight dorsal valves
  • Marine – no Illinois species
Aplacophora – Solenogasters
  • No shell, mantle or foot
  • Marine – no Illinois species
Cephalopoda – Cephalopods
Includes: squid, octopus, cuttlefish and nautiluses
  • Large head attached to tentacles
  • Tentacles are the “foot” divided
  • Lack external shell (except nautilus)
  • Squid and cuttlefish have internal shell
  • Marine – no Illinois species
Scaphopoda – Tusk Shells
  • Tubular shell open on both ends
  • No head
  • Marine – no Illinois species
Monoplacophora – Deep-sea Limpets
  • Single shell
  • Flat foot
  • Marine – no Illinois species