Four-sided Ark

Overview

The Four-sided Ark (Tetrarca tetragona) is a solid, bivalve mollusk recognized by its distinctive, boat-like, trapezoidal shell. It is commonly found attached to hard substrates, such as rocks or within crevices, using a robust, greenish byssus that emerges through a wide ventral gape.

Appearance

This species typically reaches a length of about 5 cm. Its shell is thick and swollen, often featuring a prominent keel that runs from the umbo to the posterior-ventral angle. The exterior is generally a dull white, pale yellow, or brownish color, frequently covered by a dark brown periostracum and often encrusted with calcareous algae or tube worms. The interior of the shell is white with brown margins, sometimes displaying faint green or purple blotches. A key diagnostic feature is the straight hinge plate, which contains 40 to 50 small, uniform teeth.

Distribution & Habitat

Tetrarca tetragona is distributed throughout the Northeast Atlantic, ranging from West Africa northward to Norway, and is also present in the Mediterranean Sea. It inhabits marine environments from the lower shore down to depths of approximately 100 meters, where it secures itself to small rocks, dead shells, or within protective rock crevices.