2-4. ANATOMICAL FEATURES OF OYSTERS
a. General. Shortly after spawning, the small oyster is hatched from the larvae,
and then swims until it attaches itself to a rocky surface where it remains for the rest of
its life. Oysters are bivalves; the body is enclosed in two shells or valves. The shells
are hinged at one end by a dark colored elastic ligament. This ligament is so placed
that it tends to throw the free ends of the shells slightly apart when the large closing
muscle is cut or relaxed. This narrow hinge end is the anterior (front) end of the oyster.
The broad opposite end is the posterior (rear) end. The main structure of the body is
shown in Figure 2-2. This figure shows an oyster lying in the left shell, which is deeper
than the right one. Oysters normally grow upright in the water, the hinged end is down,
and the broad posterior end up. The left shell is normally the one by which the oyster is
attached to rocks.
Figure 2-2. Internal Features of Oysters.
b. Mantle and Shell. Approximately 90% of the oyster's weight is shell. Each
shell is lined with a thin membrane called the "mantle," which is fringed on the edge.
The mantle is loosely attached to the shell and covers nearly all of its area. It is free
along the margin. The shell, which is produced by the mantle, consists of three layers:
(1) a dark, horny, outer layer which is often worn away; (2) a middle layer consisting
largely of calcium carbonate deposits arranged perpendicular to the surface of the shell;
and finally, (3) an innermost pearly layer made up chiefly of thin sheets of calcium
carbonate laid parallel to the surface of the shell. The first two layers are secreted only
by the edge of the mantle, and hence show the concentric markings of irregular growth
periods. The inner, pearly layer is laid down by the whole surface of the mantle and has
a smooth, lustrous surface. In most oysters, the fringed edge of the mantle is of a
darker color due to melanin, a normal black pigment. Certain varieties in the southern
coastal areas of the United States have a distinct black edge to the mantle and around
the adductor muscle, which is considered normal.
c. Adductor Muscle. The large adductor muscle is located in approximately
the center of the body. The contraction of this muscle closes the oyster shell. It
consists of two parts. The inner part merely closes the shell. The white outer part acts
as a locking mechanism, keeping the shells together. When the adductor muscle
relaxes, the ligament at the hinge forces the shells apart. The main body of the oyster