Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts
Pathology, Parasites and Symbionts Overview
References | Citation
Geoduck larvae and juveniles being cultured experimentally were surveyed for infectious diseases. The survey included the histological examination of 795 juvenile geoduck clams from two different year classes (spawned in either 1995 or 1996) and planted at four locations in the Strait of Georgia. To date, no infectious diseases or pathogenic organisms have been observed in cultured geoduck clams in British Columbia. However, a protozoan parasite (an amoeboflagellate) was found associated with mortalities among cultured larval geoduck clams in Washington State during the early development of hatchery technology for geoduck clams (Kent et al. 1987, Elston 1990).
The disease survey of wild adult geoduck clams consisted of the examination of geoduck clams that appeared abnormal when harvested in the commercial fishery from various areas on the coast of British Columbia. These abnormal animals are either rejected by the market or receive considerable reduction in price. One hundred and forty six abnormal clams from 18 locations in six of the Fisheries Management Areas (Area 7, Subareas 9, 12, 22, 23, 25, 27; Area 8, Subareas 2, 4; Area 14, Subareas 10, 13, 15; Area 17, Subarea 18; Area 23, Subareas 5, 6, 8; Area 24, Subareas 4, 6; for locations see http://www.pac.dfo-mpo.gc.ca/fm-gp/maps-cartes/areas-secteurs/index-eng.html) along the coast of British Columbia were submitted for examination. Sixty-six geoduck clams were examined from the east coast of Vancouver Island, 37 from the west coast and 43 from the central coast of British Columbia. Geoduck clams were maintained in tanks with ambient flow through sea water until examined. In order to prevent the introduction of potential infectious disease agents from these clams into the local environment, all effluent was disinfected with chlorine prior to discharge to the Nanaimo city sewage system.
Valve length, width and the live weight of each geoduck clam was recorded; anomalies present on the external surface were noted and included warts, blisters and scars on the siphon (neck) and mantle (between the shells), pustules in the musculature of the siphon and mantle and a black/brown leathery surface. Representative samples of the anomalies as well as apparently normal samples of the viscera, muscular mantle and the siphon were preserved for histological examination. For histology, tissues were fixed in Davidsons solution, embedded in paraffin, sectioned at 5 µ m, stained with haematoxylin and eosin stain and examined under a compound microscope up to 1000x magnification. In addition to histological analysis, the geoduck clams were examined 'fresh'. Sub samples of the digestive gland, gonad and kidney were observed under the compound microscope. In addition, the gills of each clam and the visceral mass of five clams from each fisheries area were pressed between glass plates and examined under a dissecting microscope.
At least five histological slides were prepared for each of one hundred and twelve geoduck clams, including sections through the following organs; mantle, siphon, gills, palps, stomach, digestive gland, kidney, heart, gonad, intestine, rectum, and foot. Additional slides and stains, such as gram stain for bacteria or Periodic Acid Schiff stain (PAS) for fungus were also prepared if warranted.
For transmission electron microscopy (TEM), tissues were preserved in 2% gluteraldehyde in 0.1M Sorensen's phosphate buffer pH 7.2 for at least 24 hours, post fixed 1.5 hours in 1% OsO4, sectioned and stained with saturated (5%) uranyl acetate in ethanol and then in 0.4% lead citrate and examined on a Zeiss EM10 electron microscope.
Although preliminary work was initiated to address the health concerns of geoduck clams, many questions remain unanswered. To date geoduck clams obtained from broodstock in the Strait of Georgia and returned to the Strait of Georgia for grow-out have not encountered any infectious diseases. However, further work is required to ascertain if these juvenile geoduck clams could present a health risk to clams in other areas or if geoduck clams from other areas could be carriers of diseases for geoduck clams in the Strait of Georgia. The absence of disease during geoduck culture trials in the Strait of Georgia is most encouraging for the industry but also brings complacency. Nevertheless, we must be on our guard concerning indiscriminate transplantation of geoduck clams around the province. Repercussions could be devastating and irreparable. To help reduce the risk of inadvertent disease transfer into and within British Columbia, a Federal / Provincial Fish (including Shellfish) Introductions and Transfers Committee was established. This Committee is bound by existing government legislation and policies regarding movements of live fish into and within the Province of British Columbia (see https://www.dfo-mpo.gc.ca/aquaculture/management-gestion/licen-permi-eng.htm). All proposed shellfish transplants must be submitted to the Committee for prior approval. Further details including contact information for the Committee are available via the Internet at https://www.dfo-mpo.gc.ca/aquaculture/management-gestion/contact-intro-eng.htm.
More recently, surveys of symbionts, parasites and diseases were conducted on natural populations of P. generosa in Washington State (Dorfmeier et al. 2015) and in the Pacific coast of Baja California, Mexico (Cáceres-Martínez et al. 2015). Although the results of these surveys are included on appropriate pages in the specific pathology, parasites and symbionts pages listed in the Table of Contents, parasites not found in British Columbia were reported in each of these surveys. Specifically, only one (Rickettsia-like organism in the gills) of the five morphologically distinct endosymbionts reported by Dorfmeier et al. (2015) was detected in British Columbia. The other four (metazoan in the siphon epithelium, Steinhausia-like organism in the oocytes, microsporidia-like organism in the intestinal submucosa, and a microsporidia-like organism in the siphon musculature) were not encountered. The copepods, trematode larvae, and rickettsia-like organisms in the epithelium of the digestive diverticula reported by Cáceres-Martínez et al. (2015) were also not observed in British Columbia. However, the image of the trematode larvae in the lumen of the intestine presented by Cáceres-Martínez et al. (2015, Figure 4B) is reminiscent of the turbellarian in the intestinal lumen of Japanese littleneck clam, Venerupis (=Tapes) philippinarum (see Fig. 2 in Turbellaria of Clams web page on the Canadian Fisheries and Oceans shellfish disease web site). Unfortunately, the magnification of the image in Fig. 4B of Cáceres-Martínez et al. (2015) is not adequate to determine if an outer layer of cilia are present which is a feature that can be used to differentiate between trematodes and turbellaria in histological sections.
Cáceres-Martínez, J., R. Vásquez-Yeomans and R. Cruz-Flores. 2015. First Description of symbionts, parasites, and diseases of the Pacific geoduck Panopea generosa from the Pacific coast of Baja California, Mexico. Journal of Shellfish Research 34: 751-756.
Dorfmeier, E.M., B. Vadopalas, P. Frelier and C.S. Friedman. 2015. Temporal and spatial variability of native geoduck (Panopea generosa) endosymbionts in the Pacific Northwest. Journal of Shellfish Research 34: 81-90.
Elston, R.A. 1990. Mollusc diseases: guide for the shellfish farmer. University of Washington Press, Seattle, p. 37-39.
Kent, M.L., R.A. Elston, T.A. Nerad and T.K. Sawyer. 1987. An Isonema-like flagellate (Protozoa: Mastigophora) infection in larval geoduck clams, Panope abrupta. Journal of Invertebrate Pathology 50: 221229.
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.
Shaw, B. L., and Battle, H.I. 1957. The gross and microscopic anatomy of the digestive tract of the oyster Crassostrea virginica (Gmelin). Can. J. Zool. 35: 325-347.
Bower, S.M. and Blackbourn, J. (2003): Geoduck clam (Panopea generosa): Anatomy, Histology, Development, Pathology, Parasites and Symbionts: Pathology, Parasites and Symbionts Overview.
Date last revised: August 2020
Comments to: Susan Bower
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