Geoduck clam (Panopea abrupta): Anatomy, Histology, Development, Pathology, Parasites and Symbionts
Normal Histology - Organs Associated with the Visceral Mass
The foot of most clams is a muscular organ directed towards the anterior end as an adaptation for burrowing. It is a muscular outgrowth of the visceral mass or body of the geoduck clam. In adult geoduck clams, the foot is relatively small in comparison to the body size. In juvenile geoduck clams the foot is comparatively large and is roughly equivalent in portion to the body size as in most other clams. The relatively large size of the foot in juveniles aids digging in the substrate whereas the small foot of adults accounts for their incapability of reorienting in the substrate once they have been disturbed. The foot can be extended through the pedal aperture (see drawing, Fig. 4 on Anatomy Page) or may be entirely withdrawn into the mantle cavity (as illustrated in histological sagittal sections on the Histology Overview Page). In juvenile clams of up to two years in age, a functional byssal gland is located in the proximal region at the posterior end of the foot (Fig.1a). This byssal gland produces byssal fibers (Figs.1b and 1c) which aid the geoduck in attaching to the substrate.
Figures 1a to 1c. Longitudinal sections through the visceral mass of juvenile geoduck clams. Haematoxylin and eosin stain.
In addition to the anterior and posterior adductor muscles (labeled as am and pm, respectively, on Figs 1a and 1b of the Histology Overview Page), a paired band of muscle fibers pass through the left and right ventral regions of the kidney (Fig. 2). These muscle bands join and continue into the posterior curve of the visceral mass to the base of the foot, forming the retractor muscle of the foot.
The geoduck clam has a typical bivalve nervous system containing three pairs of ganglia (Fig. 3a): cerebral ganglia on the left and right sides of the esophagus (Fig. 3b), a fused pair of pedal ganglia at the base of the foot within the visceral mass (Fig. 3c) and a pair of closely adjacent visceral (posterior) ganglia ventral to the posterior adductor muscle (Figs. 3d and 3e). The visceral ganglia are the largest of the three pairs and extend laterally through the body from the vicinity of the kidneys to the region of the posterior adductor muscle. In geoduck clams, there appears to be a trend towards centralising nervous coordination in the visceral ganglia as indicated by their larger size. Characteristic of mollusc ganglia, the nerve cell bodies are concentrated in the outer layer (cortex) and the nervous system is contained in a perineural sheath formed of glial cell elements (Morse and Zardus 1997).
Figures 3a to 3e. Histological sections through the nervous system of juvenile geoduck clams. Haematoxylin and eosin stain.
A pair of statocysts are located anterior to the pedal ganglia. Although the statocysts are well developed in pediveliger larva (swimming stage late in larval development near the time of metamorphosis to benthic dwellers), they degenerate (Fig. 4) as the geoduck clam becomes established in the substrate.
The hinge complex is a structure that lies on the dorsal surface of the visceral mass and produces the hinge ligament (Fig. 5).
Barnes, Robert D., 1968. Invertebrate Zoology, 2nd ed. W. B. Saunders Company, Toronto, 743 pp.
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.
Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea abrupta): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Normal Histology - Organs Associated with the Visceral Mass.
Date last revised: May 2003
Comments to: Susan Bower
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