Haplosporidian Infection of Clams


Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Haplosporidian infection of clams and cockles.

Scientific name or taxonomic affiliation

  1. Minchinia (=Haplosporidium) tapetis (Azevedo 2001), Haplosporidium sp.
  2. Haplosporidium edule (Azevedo et al. 2003).
  3. Unnamed haplosporidian (Armstrong and Armstrong 1974).
  4. A second unnamed haplosporidian of unknown relationship to the parasites indicated above (Itoh et al. 2005).

Geographic distribution

  1. West coasts of France, Portugal and Spain.
  2. Galicia, Spain.
  3. Yaquina Bay, Oregon, USA.
  4. Iwakuni in Yamaguchi Prefecture, Japan.

Host species

  1. Tapes (=Ruditapes) decussatus, Venerupis aureus and cultured Venerupis (=Tapes, =Ruditapes) philippinarum.
  2. Cerastoderma edule
  3. Tresus capax.
  4. Venerupis (=Tapes) philippinarum.

Impact on the host

Prevalence of infection usually low (about 4%). However, M. tapetis occurred in up to 59% of T. decussatus and 25% of V. aureus in Spain. The pathogenicity of the plasmodial stage in clams is minimal but, the sporulation stage in the connective tissue causes important lesions in the digestive gland and gills. Haplosporidium edule in cockles from Spain was detected in a low prevalence at only 2 of 34 locations. Infections were heaviest in the digestive gland and induced a haemocytic response especially when the plasmodial stage was dominant (Carballal et al. 2001). In Japan, a haplosporidian infection was detected in May 2002 in one of 40 Manila clams experimentally deployed in April 2002 (from Oita Prefecture, Japan) to investigate the drastic decrease in commercial V. philippinarum stocks in the area of Yamaguchi Prefecture. This is the only known case of this parasite in Japan. The gills and connective tissue of the visceral mass of the infected clam was almost completely replaced by plasmodia and spores with a few plasmodia also occurring in the foot tissues (Itoh et al. 2005). Although no mortalities have been attributed to these parasites, the effect on clam populations is unknown but, related species are highly pathogenic to oysters on the east coast of North America.

Diagnostic techniques

Wet Mount:

  1. Fresh immature spores exhibit one or two (and rarely three) epispore cytoplasmic extensions. However, after 3 to 5 days of incubation in filtered seawater the extensions were lost.
  2. Fresh mature spores examined under phase contrast microscopy had two long spore extensions (25 to 45 µm in length), one at each end of the ovoid spore.

Figure 1. Wet mount squash of the digestive gland from Venerupis philippinarum from Japan heavily infected with an unidentified species of haplosporidian. Several plasmodia containing developing spores (arrows) and a few free spores (arrow heads) were liberated from the infected tissue. Image provided by Kazuo Momoyama PhD, Inland Sea Division, Yamaguchi Prefectural Fisheries Research Center, Aio-Futashima, Yamaguchi 754-0893, Japan; e-mail: momoyama.kazuo@pref.yamaguchi.lg.jp.

Figure 2. Phase contrast micrograph of a spore of an unidentified species of haplosporidian from Venerupis philippinarum from Japan. Image provided by Kazuo Momoyama PhD, Inland Sea Division, Yamaguchi Prefectural Fisheries Research Center, Aio-Futashima, Yamaguchi 754-0893, Japan; e-mail: momoyama.kazuo@pref.yamaguchi.lg.jp.

  1. First signs of infection are multinucleate plasmodia (5-16 µm in diameter with 3-16 nuclei with endosomes (nucleoli)) usually in the epithelia of the digestive tract. Intense sporulation occurs in the interstitial connective tissue underlying the digestive gland and gills. The ovoid operculate spores are 5-6 µm long and 4-6 µm wide.
  2. Plasmodia (5 to 6 µm in diameter), sporonts, sporoblasts (sporocysts, 11 to 15 µm in diameter) and numerous ovoid to ellipsoidal spores (3.2 by 2.2 µm) were most abundant in the connective tissue of the digestive gland but also occurred in the connective tissue of the gill, gonad and mantle (Carballal et al. 2001, Azevedo et al. 2003).
  3. The cysts consist of massive aggregations of host haemocytes circumscribed by fibroblast-like cells. Within the cysts, numerous oval to spherical plasmodia, ranging from 10-35 µm in length and containing 2 to more than 60 spherical nuclei. Development and multiplication of the plasmodia, and sporogony have not been observed thereby precluding positive identification.
  4. The connective tissue of the gills and visceral mass were almost completely replaced by parasite cells including spores. A few parasite cells were also observed in the foot tissues.

Figure 3. Heavy infection of an unidentified species of haplosporidian in the connective tissue of Venerupis philippinarum from Japan. Note the numerous developing spores (arrows) within plasmodia adjacent to the digestive gland tubules (D). Image provided by Naoki Itoh PhD; e-mail: nitoh@lsu.edu.

Electron Microscopy:

  1. In scanning and transmission electron microscopy, the spores were ellipsoid (about 4.8 µm long and 3.9 µm wide, n=30) with a typical hinged operculum (about 3.9 µm wide) at the apical end. The epispore cytoplasmic extensions of immature spores were formed by plasmalemma with a small amount of exosporoplasmic material (epispore cytoplasm) that lacked internal cytoskeletal structures and were not in continuity with the spore wall. The walls of mature spores had three layers (a 25 nm thick electron-dense outer layer, a 10 nm electron-light middle layer and a 15 nm thick electron-dense layer that was in contact with the endosporoplasmic membrane) and lacked ornamentation and attached cytoskeletal structures (Azevedo 2001).
  2. Like other members of the genus Haplosporidium, the spores of H. edule had outer ornamentation composed of spore-wall material. In scanning and transmission electron microscopy, the distinctive feature of these spores is the ornamentation of slender projections (up to 1.5 µm long) with bifurcated tips as well as numerous parallel latitudinal, circumferential folds up to 0.35 µm high (Azevedo et al. 2003).
  3. Not described.
  4. In transmission electron microscopy, immature spores were ovoid (3.98 ± 0.37 µm by >2.83 ± 0.18 µm, n=10) and had an apical operculum, haplosporosomes and many granules. The spore wall was comprised of three layers and no epispore cytoplasm was observed possibly because of poor sample preservation (Itoh et al. 2005).

Figure 4. Section through a spore of an unidentified species of haplosporidian from Venerupis philippinarum from Japan. OP, opericulum; H, haplosporosome; N, nucleus; and w, wall of spore. Image provided by Naoki Itoh PhD; e-mail: nitoh@lsu.edu.

Methods of control

No known methods of prevention or control.


Armstrong, D.A. and J.L. Armstrong. 1974. A haplosporidan infection in gaper clams, Tresus capax (Gould), from Yaquina Bay, Oregon. Proceedings of the National Shellfisheries Association 64: 68-72.

Azevedo, C. 2001. Ultrastructural description of the spore maturation stages of the clam parasite Minchinia tapetis (Vilela, 1951) (Haplosporida: Haplosporidiidae). Systematic Parasitology 49: 189-194.

Azevedo, C., R.F. Conchas and J. Montes. 2003. Description of Haplosporidium edule n. sp. (Phylum Haplosporidia), a parasite of Cerastoderma edule (Mollusca, Bivalvia) with complex spore ornamentation. European Journal of Protistology 39: 161-167.

Carballal, M.J., D. Iglesias, J. Santamarina, B. Ferro-Soto and A. Villalba. 2001. Parasites and pathologic conditions of the cockle Cerastoderma edule populations of the coast of Galicia (NW Spain). Journal of Invertebrate Pathology 78: 87-97.

Chagot, D., Bachère, E., Ruano, F., Comps, M. and Grizel, H. 1987. Ultrastructural study of sporulated instars of a haplosporidian parasitizing the clam Ruditapes decussatus. Aquaculture 67: 262-263.

Figueras, A., J.A.F. Robledo and B. Novoa. 1992. Occurrence of haplosporidian and Perkinsus-like infections in carpet-shell clams, Ruditapes decussatus (Linnaeus, 1758), of the Ria de Vigo (Galicia, NW Spain). Journal of Shellfish Research 11: 377-382.

Itoh, N., K. Momoyama and K. Ogawa. 2005. First report of three protozoan parasites (a haplosporidian, Marteilia sp. and Marteilioides sp.) from the Manila clam, Venerupis (=Ruditapes) philippinarum in Japan. Journal of Invertebrate Pathology 88: 201-206.

Navas, J.I., M.C. Castillo, P. Vera and M. Ruiz-Rico. 1992. Principal parasites observed in clams, Ruditapes decussatus (L.), Ruditapes philippinarum (Adam et Reeve), Venerupis pullastra (Montagu) and Venerupis aureus (Gmelin), from the Huelva coast (S.W. Spain). Aquaculture 107: 193-199.

Villalba, A. and J.I. Navas. 1988. Occurrence of Minchinia tapetis and a Perkinsus-like parasite in cultured clams, Ruditapes decussatus and R. philippinarum, from south Atlantic coast of Spain. Preliminary results. In: F.O. Perkins and T.C. Cheng (eds.). Abstracts, Third International Colloquium on Pathology in Marine Aquaculture, 2-6 Oct. 1988, Gloucester Point, VA. Virginia Institute of Marine Science, Gloucester Point, p. 57-58.

Citation Information

Bower, S.M. (2007): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Haplosporidian Infection of Clams.

Date last revised: October 2007
Comments to Susan Bower

Date modified: