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Various Coccidia of Scallops

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Category

Category 2 (in Canada and of regional concern)

Common, generally accepted names of the organism or disease agent

Scallop coccidia.

Scientific name or taxonomic affiliation

The Apicomplexa parasites of scallops described below are presented in 3 groups according to species with information pertaining to each group under each topic given the same letter code.

  1. Pseudoklossia pectinis was described from the kidney of Pecten maximus by Léger and Duboscq (1917). However, in addition to asexual reproductive stages, Léger and Duboscq (1917) described and illustrated gamonts in syzygy, a key feature that is not present in coccidia of the heteroxenous family Aggregatidae Labbé where species in the genus Pseudoklossia currently occur (Desser and Bower 1977, Kristmundsson et al. 2011). Duszynski et al. (1998) believed that all members of the genus Pseudoklossia will be found to be monoxenous (homoxenous). The phylogenetic analyses conducted by Kristmundsson and Freeman (2018) supports this theory and thus, Pseudoklossia spp. probably belong in another family, possibly Rhytidocystidae. Note that heteroxenous parasites require more than 1 host in order to complete their life cycle while monoxenous parasites require only 1 host where asexual and sexual reproduction occurs.
  2. Margolisiella islandica in the alveolate phylum Apicomplexa, family Eimeriidae and a sister clade to the rhytidocystid group was reported from Chlamys islandica (Kristmundsson et al. 2011). If the synonomy between Pseudoklossia and Margolisiella proposed by Duszynski et al. (1998) is upheld then this parasite will become known as Pseudoklossia islandica.
  3. Unidentified species of coccidian parasite was reported in the kidney (and other organs in severe infections) of Argopecten irridians (Leibovitz et al. 1984, Karlsson 1991, Whyte et al. 1994) and was temporarily assigned to the genus Pseudoklossia sp. (McGladdery 1990; Cawthorn et al. 1992; McGladdery et al. 1993 a, b). In light of the discussion presented by Desser and Bower (1997) concerning taxonomic affiliations of coccidia of molluscs, preliminary assignment to the genus Pseudoklossia must be reconsidered when developmental stages of this parasite have been described. However, the coccidian parasite, in A. irridians held in quarantine, were transmitted from infected broodstock to their offspring (McGladdery et al. 1993a) suggesting a monoxeneous life cycle.

Other species of coccidia have been described from the kidneys of oysters, mussels, clams and abalone from various locations around the world.

Geographic distribution

  1. Roscoff, France.
  2. The bay of Breidafjordur, western Iceland.
  3. Eastern U.S. and possibly imported into eastern Canada when A. irridans  was introduced during the late 1970s as a potential aquaculture species (McGladdery 1990).

Host species

  1. Pecten maximus
  2. Chlamys islandica
  3. Argopecten irradians

Impact on the host

  1. Although the various developmental stages of P. pectinis were described by Léger and Duboscq (1917), little information was provided on the effect of this parasite on its host, except for the hypertrophy of infected epithelial cells of the kidney.
  2. Intracellular infections of M. islandica were found only in the endothelial cells of the auricale area of the heart. Occasionally, macrogamonts were observed in other organs, and Kristmundsson et al. (2011) speculated that presence there was most likely due to transport in haemolymph. The infection prevalence and intensity of M. islandica was high in C. islandica from both sampling sites in Breidafjordur bay. However, long-term studies on the potential pathological effect of M. islandica on the health status of the Iceland scallop population are required to determine the impact of this parasite on its host. Nevertheless, Kristmundsson et al. (2015) indicated that infections of M. islandica in the heart auricles are equally prevalent and intense in all sizes of C. islandica and that they did not negatively affect the scallops.
  3. The coccidian parasite in A. irradians infected kidney epithelial cells, which become hypertrophied, and the kidney tubules fill with coccidia. Heavy infections may cause kidney damage (Karlsson 1991) and have been observed to spread to other tissues (mantle, digestive gland, intestine, gill and gonad) (McGladdery et al.1993a). However, little or no apparent host response was elicited against the coccidian regardless of the tissue invaded (McGladdery et al. 1993a, Whyte et al. 1994). Associated mortalities appear restricted to artificial growing conditions using warm (greater than 20 ºC) recirculating artificial seawater (Cawthorn et al. 1992; McGladdery et al. 1993a). Breeding A irradians in quarantine failed to prevent transmission of the coccidian from 1 generation to the next (McGladdery et al. 1993a, b). This confirms that alternate hosts are not required in the life cycle and indicates that both sexual and asexual stages occurred in the same host, confirming a monoxenous life cycle.

Diagnostic techniques

Squash preparations

  1. and c. Preliminary diagnosis can be made on the presence of large mature macrogamonts in squashes of kidney tissues.

Histology

  1. Various stages of P. pectinis (ranging in size from 10 to 11 µm in diameter to 36 to 38 µm in length and 24 µm in greatest width) were reported in the epithelial cells and lumen of the kidney of P. maximus (Léger and Duboscq 1917).
  2. All life stages in the reproduction cycle (i.e., merogony, gametogony and sporogony) of M. islandica were present in C. islandica (Kristmundsson et al. 2011). Spherical trophozoites and meronts (about 10 µm in diameter) were common within endothelial cells of the heart's auricle and 2 generations of free merozoites (of 2 different sizes (6.0–7.0 by 2.8–3.2 µm and 12.0–13.0 by 3.0–4.0 µm) frequently occurred in great numbers in the haemolymph. Gamonts at various developmental stages were also abundant, most frequently free in the haemolymph. Mature macrogamonts (40–50 by 30–40 µm usually pear or heart shaped with coarsely granular cytoplasm and a large nucleus and prominent nucleolus) were much more numerous than microgamonts (30–40 µm spheroidal or ellipsoidal in shape at maturity containing numerous microgametes). Oocysts (48–60 µm in length and 40–44 µm at the widest part) containing numerous (>500) densely packed sporocysts each with 2 closely bound curved sporozoites, were exclusively in the haemolymph (Kristmundsson et al. 2011).
  3. Various stages of the coccidia (6 µm to 30 µm in diameter) in A irradians can be observed in the cytoplasm of kidney epithelial cells and within the kidney lumens. The parasites vary in shape from spherical to oblong or pear-shaped and have a granular cytoplasm and a single spherical nucleus. Several life-history stages can be seen in 1 host (McGladdery et al. 1993a). In heavy infections, various stages may occur systemically in the connective tissue and epithelia of organs adjacent to the kidney.

DNA probes

  1. Kristmundsson and Freeman (2018) obtained Pecten maximus from the northwest coast of Scotland and samples for molecular analysis were taken from scallops where P. pectinis was identified in the kidney microscopically. They provided a partial sequence of the 18S ribosomal RNA gene and used this information to determine that P. pectinis and M. islandica do not form part of the clade of Aggregatidae, but form part of a group of apicomplexans that infect marine bivalves and polychaetes (Rhytidocystidae) (Kristmundsson and Freeman 2018).
  2. Molecular data is also available for M. islandica (Kristmundsson et al. 2011). The complete SSU rDNA of 1773 base pairs was amplified and sequenced, and a nucleotide BLAST search showed the highest sequence identity (97.7% over 1768 bases of comparable sequence data) with an unidentified apicomplexan parasite that infects the eosinophilic granular haemocytes of the giant clam Tridacna crocea from Okinawa, Japan (Nakayama et al. 1998, Kristmundsson et al. 2011).
  3. Not available.

Methods of control

No known methods of prevention or control of coccidian infections in wild stocks of scallops. In scallop hatcheries, McGladdery et al. (1993a) indicated that mortalities caused by the coccidian parasite in A. irridians were correlated to adverse conditions. Under hatchery conditions, pathogenicity in A. irridians was controlled by reducing stocking densities and lowering water temperature to less than 20 ºC (McGladdery et al. 1993a).

References

Cawthorn, R.J., R.J. MacMillan and S.E. MacGladdery. 1992. Epidemic of Pseudoklossia sp. (Apicomplexa) in bay scallops Argopecten irradians maintained in a warm water recirculating facility (Abstract).Fish Health Section of the American Fisheries Society Newsletter 20 (1): 2.

Desser, S.S. and S.M. Bower. 1997. Margolisiella kabatai gen. et sp. n. (Apicomplexa: Eimeriidae), a parasite of native littleneck clams, Protothaca staminea, from British Columbia, Canada, with a taxonomic revision of the coccidian parasites of bivalves (Mollusca: Bivalvia). Folia Parasitologica 44: 241-247.

Duszynski, D.W., L. Couch and S.J. Upton. 1998. The coccidian genus Pseudoklossia. In: NSF grant PEET DEB 9521687.

Getchell, R.G. 1991. Diseases and parasites of scallops. In: S.E. Shumway (ed.). Scallops: Biology, Ecology and Aquaculture. Developments in Aquaculture and Fisheries Science No. 21. Elsevier Press, p. 471-494.

Karlsson, J.D. 1991. Parasites of the bay scallop Argopected irradians (Lamarck, 1819). In: S.E. Shumway and P.A. Sandifer (eds.). An International Compendium of Scallop Biology and Culture. World Aquaculture Workshops, Number 1. The World Aquaculture Society. Baton Rouge, p. 180-190.

Kristmundsson, Á. and M.A. Freeman. 2018. Harmless sea snail parasite causes mass mortalities in numerous commercial scallop populations in the northern hemisphere. Scientific Reports - Nature 8: 1-11.

Kristmundsson, Á., S. Helgason, S.H. Bambir, M. Eydal and M.A. Freeman. 2011. Margolisiella islandica sp. nov. (Apicomplexa: Eimeridae) infecting Iceland scallop Chlamys islandica (Müller, 1776) in Icelandic waters. Journal of Invertebrate Pathology 108: 139-146.

Kristmundsson, Á., Á. Erlingsdóttir and M.A. Freeman. 2015. Is an apicomplexan parasite responsible for the collapse of the Iceland Scallop (Chlamys islandica) stock? PLoS ONE 10: e0144685, 12 pp.

Lauckner, G. 1983. Diseases of Mollusca: Bivalvia. In: O. Kinne (ed.). Diseases of Marine Animals. Volume II: Introduction, Bivalvia to Scaphopoda. Biologische Anstalt Helgoland, Hamburg, p. 548.

Leibovitz, L., E.F. Schott and R.C. Karney. 1984. Diseases of wild, captive and cultured scallops. Journal of the World Mariculture Society 15: 269-283.

Léger, L. and O. Duboscq. 1917. Pseudoklossia pectinis n. sp. et l'origine des adéleidées. Archives de Zoologie Expérimentale et Générale 56: 88-95.

McGladdery, S. 1990. Shellfish parasites and diseases on the east coast of Canada. Bulletin of the Aquaculture Association of Canada 90: 14--18.

McGladdery, S.E., R.E. Drinnan and M.F. Stephenson. 1993a. A manual of parasites, pests and diseases of Canadian Atlantic bivalves. Canadian Technical Report of Fisheries and Aquatic Sciences 1931: 1-121 (see pgs. 98-100).

McGladdery, S.E., B.C. Bradford and D.J. Scarratt. 1993. Investigations into the transmission of parasites of the bay scallop, Argopecten irradians (Lamarck, 1819), during quarantine introduction to Canadian waters. Journal of Shellfish Research 12: 49-58.

Nakayama, K., M. Nishijima and T. Maruyama. 1998. Parasitism by a protozoan in the hemolymph of the giant clam,Tridacna crocea. Journal of Invertebrate Pathology 71: 193-198.

Whyte, S.K., R.J. Cawthorn and S.E. McGladdery. 1994. Co-infection of bay scallops Argopecten irradians with Perkinsus karlssoni (Apicomplexa, Perkinsea) and an unidentified coccidian parasite. Diseases of Aquatic Organisms 18: 53-62.

Citation information

Bower, S.M. (2020): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Various Coccidia of Scallops.

Date last revised: December 2020

Comments to Susan Bower

Date modified: