Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts
Normal Histology - Excretory (Heart-Kidney) System
The excretory process in geoduck clams (as for most bivalves) occurs in the heart-kidney complex (Fig. 1a) which lies in the visceral mass ventral to the hinge and slightly posterior to the umbo (see anatomy sketch for location). The kidney (nephridial sacs or organs of Bajanus) can be visualised as two parallel tubes consisting of a proximal aglandular portion and distal glandular section that has collapsed into itself resulting in the two aglandular tubes being enveloped by a fused bilobed glandular section (Fig. 1b). Thus, the kidney forms a large, extensively folded gland that is located ventral to the rectum and posterior and ventral to the pericardium (Figs. 1b and c). The pericardial gland (reddish brown organ or Keber's gland) located mainly on the surface of the auricles (Fig. 2a) filters the haemolymph. The filtrate accumulates in the pericardial space from which it is drained by two short renopericardial ducts, one on each side of the body, into the kidneys. Within the kidneys, substances within the filtrate are resorbed at the apical (internal) surfaces of the kidney cells and either stored in the kidney cells or returned to the haemolymph that passes on the external surfaces of the kidney epithelium. In addition, haemocytes within the surrounding haemolymph pass on wastes to the kidney cells and these wastes are discharged into the kidney lumen by apocrine secretion (Morse and Zardus 1997). The effluent leaves the kidneys through two renopores (nephropores) which empty in common with the gonads into the mantle cavity (Martin 1983).
Figures 1a to 1c. Histological sections of juvenile geoduck clams demonstrating the organs associated with the excretory system. The dorsal surface of each clam is at the top of each image. Haematoxylin and eosin stain.
The pericardial gland consisting of podocytes (brownish in colour) is mainly associated with the surface of the auricles (right and left). The podocytes filter the haemolymph and are located on membranes that separate the haemocoel from the pericardial cavity (Fig. 2a). In the geoduck clam, the renopericardial duct, which connects the pericardial cavity to the proximal part of the kidney, is short and lined with cells bearing long cilia, (Fig 2b). The kidney consists of two components, two proximal, thinner, aglandular tubes with highly convoluted surfaces and a fused distal deeply folded glandular mass (Figs 2c and d). Typically, the proximal part of the kidney is resorptive and the distal part excretory (Andrews 1988). Effluent in the lumen of the kidney empties into the suprabranchial chamber of the mantle cavity via two nephropores (renopores, ducts), one on each side of the body, in the lateral ventral region of the kidney (Fig. 2e) and exits to the environment by way of the excurrent channel of the siphon.
Figures 2a to 2e. Histological sections illustrating details of the heart-kidney complex. Haematoxylin and eosin stain.
Vacuoles within some cells of the glandular distal region of the kidney contain crystal-like inclusions. These inclusions are most evident when observed in a fresh tissue preparation (Fig. 3) because they are partially soluble in the histological fixative. Presumably, these are concretions (concrements) held within lysosomal vacuoles. Secretion of the accumulated concretions occurs by apocrine discharge into the kidney lumen. Large extracellular masses formed from the concretions are carried outside the body (Morse and Zardus 1997) through the kidney openings (nephropore) into the mantle cavity (Fig. 2e).
A pair of muscle bands that pass through the left and right ventral regions of the kidney (labeled rm in Fig. 1b), join and continue along the posterior curve of the visceral mass (labeled rm in Fig. 1a) to the base of the foot, forming the posterior retractor muscle for the foot.
Andrews, E.B. 1988. Excretory Systems of Molluscs. In: The Mollusca. Volume 11, Form and Function. Edited by E.R. Trueman and M.R. Clarke. Academic Press, San Diego. pp. 381-448.
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Morse, M.P. and Zardus, J.D. 1997. Bivalva. In: Microscopic Anatomy of Invertebrates, Volume 6A Mollusca II. Edited by F.W. Harrison and A.J. Kohn. Wiley-Liss, Inc, New York. pp. 7-118.
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Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Normal Histology - Excretory (Heart-Kidney) System.
Date last revised: August 2020
Comments to Susan Bower
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