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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.
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.
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.
click image for details
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).
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
Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea abrupta): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Normal Histology - Gills (Ctenidium).URL: http://dev-public.rhq.pac.dfo-mpo.gc.ca/science/species-especes/shellfish-coquillages/geopath/gills-eng.htm
Date last revised: March 2010
Comments to Susan Bower