Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts
Normal Histology - Outer Surfaces
The geoduck clam has a periostracum (a horny organic material containing conchiolin) consisting of two acellular layers that form a continuous, tough, flexible integument over the surface of the epithelium of the siphons and mantle (Fig. 1a). In juvenile clams, this integument is continuous with the periostracum on the outermost layer of the shell. Histologically the inner (proximal) layer of the periostracum is eosinophilic, generally homogeneous and appears to be secreted by the underlying epithelium. The outer (distal) layer appears fairly uniform in thickness and is occasionally basophilic in irregular patches. In juvenile geoduck clams of up to two years of age, a modification of the distal layer of the acellular integument was observed in the form of thorn-shaped structures on the surface of the posterior third of the muscular mantle (Fig. 1b). In contrast, external surfaces of the siphons of the Pacific littleneck clam (Protothaca staminea, Fig. 1c) and Manila (Japanese littleneck) clam (Venerupis philippinarum, Fig. 1d) lack this covering and consist of a layer of simple, columnar epithelial cells possessing microvilli.
Figures 1a to 1d. Histological sections through the external surface of clam siphons. Haematoxylin and eosin stain.
The viscera are surrounded by the mantle cavity, which is defined by the mantle tissues. A thin mantle lies against the inner surface of the shell extending from the dorsal surface to the pallial line. It is constructed of loose vesicular connective tissue and a few scattered muscle fibres lying between a columnar epithelium on the distal surface (against the shell) (Fig. 2), and a simple cuboidal epithelium lining the mantle cavity. The columnar epithelium which covers the entire distal surface of the thin mantle has a brush border (or microvilli) on its surface.
A dark brown, thickened glandular area (Fig. 4 label bg on the Anatomy Page) of the thin mantle covers the dorsal ends of the left and right gills and labial palps and is ventral to the hinge and may function as a secretory gland. Histologically, this area has the appearance of an holocrine gland (Fig. 3a and b). Two, thin, eosinophilic, acellular layers lie between the shell and the simple columnar epithelial cells of this brown gland (Fig. 3a). The gland is composed of epithelial cells adjacent to each other which are sometimes arranged in follicles that are surrounded by muscle fibres interspersed with vesicular connective tissue cells. These epithelial cells contain a single, large vacuole full of a granular material that is brown-red when stained with haematoxylin and eosin. In some areas, the cells appear to have sloughed into a lumen, their nuclei becoming compact and pycnotic, and the contents of the vacuoles more dense and uniform (Fig. 3b).
Figures 3a and 3b. Glandular tissue in the mantle adjacent to the hinge of geoduck clams. Haematoxylin and eosin stain.
The thick, fused muscular mantle forms the ventral and anterior surface of the mantle cavity, this surface is composed of ciliated simple columnar epithelium with goblet cells. Two raised pads of glandular tissue extend the length of the muscular mantle on either side of the midline (Fig. 4). A small (about 2x4 mm) conical papilla projects into the infrabranchial chamber at the dorsal end of the pedal aperture.
Morse, M.P. and Zardus, J.D. 1997. Bivalva. Microscopic Anatomy of Invertebrates Vol. 6A Mollusca II. F.W. Harrison and A.J. Kohn. Wiley-Liss. pp. 7-118.
Simkiss, K. 1988. Molluscan Skin (excluding Cephalopods). The Mollusca Vol. 11 Form and Function. E.T. Truman and M.R. Clarke. Academic Press Inc. pp. 11 - 35.
Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Normal Histology - Outer Surfaces.
Date last revised: August 2020
Comments to: Susan Bower
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