Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts
Normal Histology - Gills (Ctenidium)
The gills or ctenidium of geoduck clams are attached on each side of the visceral mass as in other species of clams. They extend from the attachment site of the labial palps to the anterior edge of the siphon septum and divide the mantle cavity into two regions, the suprabranchial chamber and the infrabranchial chamber. The apparent double set of gills (see drawing, Fig. 4 on anatomy page) on each side of the body actually arise by the folding of a single gill. A transverse section through the folded gill is seen in the region of the heart in Fig. 1.
Figure 1. Illustration of the folded gill (g) with direct connection (large arrow) to the heart (h) on the right side of the body below the kidney (k) and adjacent to the stomach (s) and digestive gland (dg).
Figure 1a. Magnification of Fig. 1 to illustrate the direct connection (large arrow) for haemolymph flow between the heart (h) and gills (g).
Figures 1 and 1a. A near transverse histological section through the gills and heart region of a juvenile geoduck clam. Haematoxylin and eosin stain.
In addition to the function of gas exchange during respiration, the gills also trap and transport food particles to the labial palps. A portion of the inner fold of the gill extends towards the mouth between each pair of palps (see Fig.1a of histology overview). The gill of adult geoduck clams differs from the gills of juvenile geoduck clams and of other local clam species (Fig. 2) by being a more robust muscular organ with thicker more closely adjoining filaments.
Figure 2a. The gill of the adult geoduck clam has clusters of basophilic cells (bc) on parts of adjacent filaments and numerous bundles of muscle fibers (mf). The water channels (wc) and haemal sinuses (hs) are evident in the centre of the gill.
Figure 2b. The gill of the juvenile geoduck clam has fewer muscle fibers (mf) and more closely resembles the gill of the littleneck clams which also have fewer muscle fibers evident but can be contracted or extended. As in adults, the water channels (wc) and haemal sinuses (hs) are evident in the centre of the gill.
Figure 2c. A contracted gill of the Manila (Japanese littleneck) clam, Venerupis philippinarum. The water channels (wc) and haemal sinuses (hs) are labeled for orientation and comparison to the gills of geoduck clams.
Figure 2d. An extended gill of a Pacific littleneck clam, Protothaca staminea. As in the Manila clam, the water channels (wc) and haemal sinuses (hs) are labeled for orientation.
Figures 2a to 2d. Transverse sections through the gills of three species of clams. Haematoxylin and eosin stain.
The size of the gills of the adult geoduck clam is relatively small in comparison to its large body size. Possibly, the muscular nature of this organ facilitates the flow of water through the mantle cavity and long siphon by muscular contractions (pumping). The clusters of basophilic epithelial cells on the sides of adjacent filaments (Fig. 3) have microvilli on their surfaces rather than cilia and may possibly function as an exocrine gland. These cells may be producing a mucus substance that assists in entrapping food items that are then carried to the labial palps and mouth by ciliary action on other parts of the gill filaments.
Figure 3. Cluster of basophic epithelial cells lining the sides of two adjacent gill filaments of an adult geoduck clam. Haematoxylin and eosin stain.
Clusters of basophilic epithelial cells were also observed in the gill filaments of both species of littleneck clams but they are mainly confined to the most proximal and most distal filaments of the gill whereas in the geoduck clam they appear to be distributed randomly. There are fewer muscle fibers and patches of basophilic epithelial cells in the gills of juvenile geoduck clams. The smaller body size and comparatively shorter siphons of the juveniles suggests that the morphology of their gills can be more like that of the littleneck clams. Depending on their stage of development, juvenile geoduck clams also utilize pedal feeding as described by Reid (1991) and Reid et al. (1992) and illustrated in the sagittal section of the histological overview where the tip of the foot in Fig 1b is located between the labial palps.
Interlocking ciliary junctions that hold the gills securely in place within the mantle cavity were observed both in juvenile geoduck clams (Figs. 4 and 5) and in adult geoduck clams. Similar ciliary junctions supporting the gills were observed in adult littleneck clams (V. philippinarum and P. staminea).
Figure 4. Sagittal section through the posterior region of a juvenile geoduck clam showing five ciliary junctions (cj). Note the spatial relationships between the kidney (k), the neural ganglion (ng) and the gills (g).
Figure 5. Magnification of a ciliary junction (cj) consisting of two adjacent sheets of epithelium with interlocking cilia.
Figures 4 and 5. Histological sections through ciliary junctions in the gills of juvenile geoduck clams. Haematoxylin and eosin stain.
References
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.
Reid, R.G.B., R.F. McMahon, D.O. Foighil, R. Finnigan. 1992. Anterior inhalant currents and pedal feeding in bivalves. Veliger 35(2): 93-104.
Reid, R.G.B. 1991. Feeding behavior of early juvenile shellfish, with emphasis on the Manila clam. In: T.Y. Nosho and K.K. Chew (eds.). Remote Setting and Nursery Culture for Shellfish Growers: Workshop, Olympia, WA (USA), 19 Feb 1991. Washington Sea Grant Program, Seattle. pp. 50-54
Citation Information
Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Normal Histology - Gills (Ctenidium).
Date last revised: August 2020
Comments to Susan Bower
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