Ventricose Ark

Associated Smithsonian Expert: M. G. (Jerry) Harasewych, Ph.D.

Dr. Jerry Harasewych showing drawer of marine mollusk specimens

Photo by Smithsonian Institution

Dr. Jerry Harasewych is a Research Zoologist and Curator of Marine Mollusks at the Smithsonian National Museum of Natural History. His research specialties include the systematics and biogeography of several groups of deep-sea snails. He conducts field work using a variety of research submersibles to sample and observe these animals. Other areas of research include Antarctic mollusks and a highly diverse group of land snails endemic to the tropical western Atlantic. Harasewych first started to work with shells at the age of ten, when he began as a volunteer at the Academy of Natural Sciences in Philadelphia. He was an active member and officer of the Philadelphia Shell Club while pursuing an undergraduate degree in chemistry at Drexel University in Philadelphia. After completing his doctorate in Biological Oceanography at the College of Marine Studies of the University of Delaware, he moved to the D.C. area and became a Research Fellow in Clinical Neurogenetics at the National Institute of Mental Health. Harasewych joined the Smithsonian in 1985.

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This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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California mussel
Courtesy of Josh Pederson, National Marine Sanctuaries Media Library, public domain


About Bivalves (Class Bivalvia): Defense

Bivalves have a hard shell made of minerals, typically layers of calcite and aragonite. The shell has two halves (valves) connected by a flexible ligament hinge. Powerful muscles contract to close the valves into a tight fit. Teeth along the edges of each valve interlock to keep them from sliding apart if the bivalve is attacked by a predator. The pattern of teeth, because it is often species-specific, is one feature used to identify bivalves. The shell grows over time, using calcium taken up from seawater or ingested food. Bivalves have no heads at all, and a flattened foot. The foot is shaped for wedging into sand, explaining why bivalves have also been called pelecypods (hatchet-feet). To keep from getting moved by water currents, bivalves tend to attach themselves to hard surfaces or burrow into sediments. They burrow by probing down with the foot (lengthening) and then retracting it (shortening) to tug the shell downward.

Queen scallop (Aequipecten opercularis) feeding
Courtesy of Kattegatcentret Gren†, via Biopix, CC-BY-NC


About Bivalves (Class Bivalvia): Feeding

Most bivalves filter food with their gills, then transport it to their mouths. While still used to get oxygen, their gills are modified to also act as food strainers. Typically, a special tube called a siphon serves as a straw to suck water through the bivalve's gills, and another siphon lets the water out after filtering has occurred. Tiny grooves in the gills are covered with hair-like cilia that trap particles passing by, such as microscopic plants (phytoplankton). Particles not accepted as food are sent to where they can be purged from the shell. Particles accepted as food are wiped off the gills by flaps (palps) and sent on to the bivalve's mouth. From there they travel to its complex stomach, where more cilia sort it out by size. Some bivalves are scavengers, scraping food from the sea bottom, or sucking in crustaceans or other prey with their siphons.

Related Resources
Common Pelican's Foot (Aporrhais pespelecani)
Courtesy of Isidro Martinez, via Flickr: EOL Images, CC-BY-NC-SA

About Mollusks (Phylum Mollusca): Body Plan

Mollusks have soft bodies (mollis = soft) with no internal skeleton. They hold their shape by internal water pressure (a hydrostatic skeleton). A muscular skin-like structure called the mantle covers the back of a mollusk, protecting its mass of internal body organs (viscera). Most mollusks also have a hard shell or at least some hard plates over the mantle. Shells are made of a protein matrix holding together crystals of calcium carbonate. Under those layers is a calcium-containing third layer that in some species is shiny mother-of-pearl. This layered structure makes for a strong shell that protects the soft parts from predators and provides a site for muscle attachment. Most mollusks move their bodies slowly using a muscular structure called the foot to creep along, stick to, or burrow into surfaces, although some mollusks (e.g. squid and scallops) swim.