Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts
Normal Histology - Reproductive System
The gonad is an integral part of the visceral mass positioned ventral and lateral to the digestive gland and loops of the intestine. In immature (juvenile) geoduck clams, the gonad makes up a small part of the total visceral mass (see Fig. 3a in the Digestive System page). Small oval or flattened gonadal acini (follicles, tubules) containing primordial reproductive cells (Fig. 1) are infrequently distributed throughout the vesicular connective tissue. Until the development of gametes, the two sexes are indistinguishable histologically.
As the geoduck clam matures, the gonadal acini proliferate and expand in size. The lipids or glycogen stored in vesicular connective tissue cells surrounding the acini provide nutrition to the developing gametes. In mature (adult) geoduck clams, the gonad occupies a significant proportion of the visceral mass (see Fig. 3b (adult female) and Fig. 3c (adult male) in Digestive System page). Gametogenesis is described by Anderson (1971) and closely resembles that of other bivalves. Hermaphrodites were not observed. As for many bivalves, geoduck clams are broadcast spawners.
Sperm production was observed in geoduck clams as young as two years old (Fig. 2a). In male geoduck clams, gametogenesis involves the production of spermatogonia by mitotic division of the undifferentiated germ cells (primary gonial cells) located along the inner periphery of the tubular acini in the connective tissue of the viscera. The spermatogonia are the largest male germinal cells which divide by mitosis and become smaller as they move towards the center of the lumen and become spermatocytes. Spermatocytes in turn divide by meiosis and become spermatids. Spermiogenesis involves the differentiation of spermatids into spermatozoa without further cell division.
Figures 2a and 2b. Gonadal development in male geoduck clams. Haematoxylin and eosin stain.
In female geoduck clams, the undifferentiated (primordial) germ cells transform into primary oogonia that undergo meiosis to become the developing oocytes. The oocytes usually remain attached by a stalk to the internal surface of the acinus throughout development and have associated auxiliary cells that develop from the acinar wall and provide nutrition to the developing oocytes. As they mature, the oocytes project into the lumen of the acinus and are eventually released into the lumen and exit the clam via the ciliated gonadal ducts. Mature oocytes have a germinal vesicle within the nucleus that contains two nucleoli, extensive cytoplasm containing granules and a thin outer vitelline layer.
Figures 3a to 3c. Gonadal development in female geoduck clams. Haematoxylin and eosin stain.
Anderson, A.V. Jr. 1971. Spawning, growth, and spatial distribution of the geoduck clam, Panope generosa Gould, in Hood Canal, Washington. Ph.D. thesis submitted to University of Washington.
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 generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Normal Histology - Reproductive System.
Date last revised: August 2020
Comments to: Susan Bower
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