Description:

Ark shells vary from small to large sizes and are mostly elongate or squarish in shape. About 200 species are found in this family, and the shells tend to be squared off and porcelaneous.  Their surface is sculptured by radial ribs and covered with a thick velvety periostracum. The beaks are hooked and usually apart from each other, forming a wide ligament between them. A wide, straight-edged hinge possesses numerous small teeth.

 

The animal frequently has a byssus by which it attaches itself to rocks or other substrata (see Ark Shell Photo Gallery, below). Interestingly, the 9 cm. Twisted Ark Shell shown at right has a nearly paper-thin, delicate shell, which is substantially larger than the shells of other ark clams, like species in the Anadara genus.

 

In the Arcidae, the presence of the hemoglobin pigment of vertebrates is most interesting. It is found simply dissolved (not in corpuscles) in both the blood and tissues and colors the muscle red (Morton, 1960). This is not typical of bivalves, which generally absorb oxygen from water directly into the tissues, without the aid of oxygen transport pigments like hemoglobin (or hemocyanin, which is found more typically in some gastropods). Where the water is very muddy, oxygen may be deficient, in which case any of these pigments facilitate oxygen transport into the tissue.

Classification:

Class: Bivalvia

Subclass: Pteriomorpha

Order: Arcoida

Superfamily: Arcacea

Family: Arcidae

Major Genera

• Genus: Arca
• Genus: Anadara (Mangrove Cockles)
• Genus: Barbatia
• Genus: Noetia
• Genus: Scapharca
• Genus: Senilia
• Genus: Trisidos

Arca tortuosa (Linnaeus, 1758)
Trisidos tortuosa (Linnaeus, 1758) [synonym]
Twisted Arc Shell, 9.0 cm
Japan

Arca ventricosa (Lamarck, 1819)
Ventricose Ark, 6.2 cm

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