Shell: The species has two solid valves. The left valve is slightly convex and the right valve is deep and cup-shaped. One valve is usually entirely cemented to the substrate by a small area. The species varies in shape. Normally it reaches lengths of 80-200mm, but large individuals can grow up to 450mm (Coan et al., 2000: 217; Nehring, 2006: 2).
Its shell is sculptured with radial ribs and concentric frills. The hard frills are frequently abraded, but in some habitats they are very prolonged. On hard substrates, the shell is rounded in shape with extensive fluting. On soft substrates, it is ovate and smooth. On mini-reefs, the margins of the shell are irregular. The color is grey to purplish on the outside. On the inside, it is white, generally with a light-colored adductor muscle scar (Coan et al., 2000: 217; Nehring, 2006: 2).
Soft body: The soft body of the organism can be divided into four sections: the visceral mass (containing most major organ systems), the adductor muscle (closes the shell), the gills, and the two asymmetric mantle lobes. The mantle lobes are partially connected to the visceral mass and were divided in three parts by Evseev and Yakovlev (1996: 240): thick, thin, and marginal mantle. The labial palps are of different sizes. They connect to the visceral mass and mantle. The portions of the palps facing each other are covered with ridges (Evseev and Yakovlev, 1996: 242). The organs of the visceral mass are described below:
Pericardial cavity – Includes two accessory hearts, located on the mantle lobes of the epibranchial chamber (Evseev and Yakovlev, 1996: 245).
Alimentary system - Includes the oesophagus, stomach, crystalline style sac, and intestine. The stomach is shaped like a “dumb-bell” (Evseev and Yakovlev, 1996: 245). Following the stomach is the intestine, which curves around the stomach and eventually ends in a simple anus (Evseev and Yakovlev, 1996: 250).
Interestingly, magnetic resonance imaging (MRI) has been used to examine the soft body of the organism in vivo (Pouvreau et al., 2006).
Embryo/Larva: Fertilized eggs are spherical, 45-62 µm in diameter. The egg is multilayered, with membranes that divide the jellylike outer coat from the small nucleus. The germinal vesicle is located within the nucleolus. Once a length of 125 µm is reached, the organism is considered an oyster larva. It develops an eyespot, along with a foot containing a byssal gland, which allows for attachment to the substrate (Pauley et al., 1988: 5).
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