Geoduck clam (Panopea abrupta): 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.

Figure 1. Two of the gonadal acini within the connective tissue (c) of the viscera of a juvenile geoduck clam. At this stage of development, the internal surface of the acini is lined with strongly basophilic undifferentiated germ cells and the sex of the clam can not be determined 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.

Male Gametogenesis

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.

Figure 2a. Spermiogenesis resulting in the expansion and filling of the acini (ac) of a 2 year old male geoduck clam. The acini empty into a ciliated gonadal duct (gd). A bundle of nerve fibers (n) occurs adjacent to the acini.

Figure 2b. Spermatogonia (sg) on the periphery of the acinus undergoing mitotic division (md) to form spermatocytes (sy) which further divide by meiosis and eventually develop into spermatozoa (sz) with flagella (eosinophilic) oriented towards the lumen of the acinus (gl) and eventually the gonadal duct.

Figures 2a and 2b. Gonadal development in male geoduck clams. Haematoxylin and eosin stain.

Female Gametogenesis

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.

Figure 3a. Oogenesis resulting in the expansion of the acini of an adult female. Note that developing oocytes are attached to the internal surface of the acini (arrows).

Figure 3b. Mature oocytes entering into the ciliated gonadal duct (gd) from acini (arrows) in a ripe female during the spawning process.

Figure 3c. Mature oocytes with a germinal vesicle (gv) within the nucleus which contains two nucleoli (arrows), granular cytoplasm and surrounded by a basophilic vitelline layer (vl).

Figures 3a to 3c. Gonadal development in female geoduck clams. Haematoxylin and eosin stain.

References

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.

Citation Information

Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea abrupta): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Normal Histology - Reproductive System.

Date last revised: March 2010
Comments to: Susan Bower

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