Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts
Lesions of Unknown Cause on Geoduck Clams
Surface 'Warts'
'Warts' are regions of smooth, raised, gray-pink or creamy coloured protuberances observed on both the siphon and mantle (Figs. 1 - 8). They ranged in size from a few millimeters to 7 cm long and consist of swellings of the periostracum which are packed with dead and dying cells.
Figure 1. Ventral surface of a geoduck clam collected from the west coast of Vancouver Island (Fisheries Management Area 23) with typical 'warts' on the siphon.
Figure 2. Close-up of 'warts' (smooth, raised, gray-pink or creamy coloured swellings) in Fig. 1.
Figure 3. Ventral surface of a geoduck clam collected from the west coast of Vancouver Island (Fisheries Management Area 23) with large, fused 'warts' on the mantle and siphon.
Figure 4. Close-up of Fig. 3. Note the flap of periostracum indicating an emptied 'wart'.
Figure 5. Ventral view of a geoduck clam collected from the east coast of Vancouver Island (Fisheries Management Area 14) with 'warts' on the mantle.
Figure 6. Close-up of Fig. 5. The large 'wart' (white area) has been eroded on the surface and the black material consists of layers of periostracum.
Figure 7. Unusual appearance of 'warts' on the siphon of a geoduck clam collected from the east coast of Vancouver Island (Fisheries Management Area 14).
Figure 8. Close-up of Fig. 7. The ridge appearance of the periostracum did not appear unusual when examined histologically.
'Warts' on both the siphon and mantle were observed in geoduck clams from all of the sampling areas but occurred more frequently on the siphon than on the mantle. Unusual looking warts such as those in Figs. 7 and 8 were rarely observed, and their histological appearance resembled that of other warts. The formation of a wart is the result of an accumulation of haemocytes (clam blood cells) in the musculature of the siphon or the mantle causing purulence (extremely high numbers of haemocytes). This lesion appears to migrate (mechanism as yet unexplained) from the muscle tissue to the surface, where it was eventually walled off from the underlying tissues with periostracum. The occurrence of the 'wart' on the surface of siphon and mantle may be part of a healing mechanism of the geoduck clam.
Histologically, the 'warts' consisted of densely packed necrotic granular haemocytes (Fig. 10). Some warts were enmeshed in the musculature with no evident demarcation and the necrotic material included the epithelium (Fig. 9). Occasionally the wart material was separated from the underlying tissues only by the epithelium and the wart extended deeply within the musculature of the siphon forming a pit (Fig. 11). In other cases, not only was the epithelial layer intact but the wart material was separated from living tissue and encapsulated by the eosinophilic acellular layer of the periostracum (Fig. 12).
Figures 9 to 12. Histological sections through 'warts' on geoduck clams. Haematoxylin and eosin stain.
Figure 9. High concentration of haemocytes (H) at the surface of the mantle under layers of periostracum. Note that the epithelial layer has been destroyed and the 'wart' material (H) is in direct contact with the siphon musculature.
Figure 10. Higher magnification of 'wart' contents consisting of haemocytes, many dead and dying with a picnotic nucleus and granular cytoplasm.
Figure 11. The 'wart' material (W) is within a pit in the musculature of the siphon but is separated from normal tissue by an epithelial layer.
Figure 12. The 'wart' material (W) is separated from normal tissue by an epithelial layer and is surrounded by the acellular periostracum (arrows) which is difficult to see at this low magnification.
To date, no identifiable etiological agent has been observed in association with these lesions. In nine clams, the tissues underlying the 'wart' were purulent. The purulent material (Fig. 10) and the tissue surrounding it from one clam were preserved for examination under the electron microscope and used in experimental transmission studies as described on the Wart Transmission Experiments Page. Etiological agents were not detected in the 'wart' material or in the surrounding tissues examined by light and electron microscopy.
Blisters
Blisters are swellings containing a clear fluid (Figs. 13 and 14) that were found on the surface of the siphon or mantle. They were observed much less frequently than the 'warts', and occurred in samples from all areas except for one area on the east coast of Vancouver Island (Fisheries Management Area 17) and in one area on the west coast of Vancouver Island (Fisheries Management Area 24). Histologically, several acellular layers (the outer periostracum and inner eosinophilic layers) were interspersed with a few necrotic cells. A few opportunistic bacteria were observed on the external surface of the clam.
Figure 13. Blisters on the siphon of a geoduck clam collected from the central coast of British Columbia (Fisheries Management Area 7) in July 1998.
Figure 14. Close-up of Fig. 13.
Scars
Scars (Figs. 16 and 17) consisted of layers of discoloured periostracum or conchiolin with no detectable effect on the living tissues. They are thought to be the remains of a healed lesion, or 'wart' and were observed in all samples except those collected from one location on the west coast of Vancouver Island (Fisheries Management Area 24).
Figure 15. Scars on the siphon of a geoduck clam collected from the central coast of British Columbia (Fisheries Management Area 7) in July 1998.
Figure 16. Close-up of Fig. 15.
Internal Orange Discolouration
Figure 17. Numerous 'warts' or orange tinted nodules in the mantle under the valve of a geoduck clam collected from the central coast of British Columbia (Fisheries Management Area 8) in April 1998.
Although 'warts', blisters and scars were not observed in the viscera of geoduck clams, anomalies of these organs or tissues were observed during dissection. In 11 of 112 geoduck clams with superficial anomalies, an orange tinted discolouration occurred on the conical papilla, located on the inner surface of the muscular mantle posterior to the pedal aperture. One geoduck clam collected from the east coast of Vancouver Island (Fisheries Management Area 14) had numerous orange tinted patches in several organs. Ten of the 112 geoduck clams had orange tinted nodules in the mantle tissue lining the valves (Fig. 17). This part of the mantle is normally thin with very few muscle fibers and lacks acellular layers (periostracum and eosinophilic layer) in contrast with that of the muscular mantle outside of the valves. In some cases the nodules were in the process of being coated with a nacreous layer and possibly would have developed into valve anomalies as described below.
Histologically, this orange colouration usually indicated an infiltration and aggregation of haemocytes (pustule formation) in the tissues. In the gill (Fig. 18) and in the orange tinted nodules on the mantle (Fig. 17), the reaction resembled the material observed in 'warts' and consisted of densely packed necrotic granular haemocytes. Large patches of orange tint in the digestive gland contained many cells with large eosinophilic inclusions (Fig. 19). Some of this material was preserved for transmission electron microscopy, and when examined, showed electron dense inclusions containing layers and whorls of membranes occupying the major portion of the affected cells. As with surface 'warts', no etiological agent was identified and the cause of this unusual cellular response was not evident in all cases.
Figures 18 and 19. Histological sections through patches of orange discolouration in the tissues of geoduck clams. Haematoxylin and eosin stain.
Figure 18. Pustules (Ps) in the gills of a geoduck clam, a reaction similar to that seen in the formation of 'warts' in the musculature of the siphon and mantle (e.g., Fig. 9).
Figure 19. Cells with large eosinophilic inclusions (Ec) in the connective tissue of the digestive gland of a clam collected in June from the east coast of Vancouver Island (Fisheries Management Area 14).
Valve Anomalies
Figure 20. Nacreous protrusions (arrowheads) on the inner surface of the valves of a geoduck clam collected from the central coast of British Columbia (Fisheries Management Area 7) in April 1998. Length of valves = 143 mm.
Infrequently, geoduck clams were observed with calcareous protrusions on the inner surface of their valves (Fig. 20). These are thought to be surface 'warts' that occurred in the thin mantle tissue under the valves (see above and Fig. 17). Over time these warts became coated with nacre and attached to the inner surface of the valve. A rough inner surface was observed on the valves of four clams; in two cases this corresponded with an orange tinted thin mantle that was in contact with this surface.
A few geoduck clams had freshly broken shells, probably incurred during harvesting. It was noted that the valve of one clam collected from the west coast of Vancouver Island (Fisheries Management Area 24) in early May 1998 had been broken in the past and then had successfully fused together resulting in valves of unequal length (Figs. 21 and 22).
Figures 21 and 22. The repaired left valve of a geoduck clam collected from the west coast of Vancouver Island (Fisheries Management Area 24). Note the dissimilar size of the two valves which is evident when viewed from the inner surface (Fig. 22). Apart from the damaged valve, this clam was in good health.
Figure 21. The repaired left valve of a geoduck clam. Apart from the damaged valve, this clam was in good health.
Figure 22. The repaired left valve of a geoduck clam. Note the dissimilar size of the two valves.
References
Fisheries Management Area locations as mentioned in above text can be viewed on the map.
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.
Simkiss, K. 1988. Molluscan Skin (excluding Cephalopods). The Mollusca Vol. 11 Form and Function. E.T. Truman and M.R. Clarke. Academic Press Inc. pp. 11 - 35.
Citation Information
Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Lesions of Unknown Cause on Geoduck Clams.
Date last revised: August 2020
Comments to: Susan Bower
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