Apicomplexa Parasite in Adductor Muscle of Scallops

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• Category
• Common, generally accepted names of the organism or disease agent
• Scientific name or taxonomic affiliation
• Geographic distribution
• Host species
• Impact on the host
• Diagnostic techniques
• Methods of control
• References
• Citation information

Category

Category 2  (in Canada and of regional concern)

Common, generally accepted names of the organism or disease agent

Grey meat disease, Scallop Apicomplexan or SAP.

Scientific name or taxonomic affiliation

Merocystis kathae is an apicomplexan (Phylum Apicomplexa, Family Aggregatidae) that was initially described from renal tissues of the common whelk, Buccinum undatum, in northern Europe (Dakin 1911). More recently, it has been identified as the parasite that causes grey meat disease (SAP) in scallops (Kristmundsson and Freeman 2018). Scallops reported to be infected with M. kathae include Chlamys islandica (Island scallop), Aequipecten opercularis (queen scallop) and Pecten maximus (king scallop) (Kristmundsson et al. 2011, Soares et al. 2021) and probably Placopecten magellanicus (sea scallop) (Inglis et al. 2016). Note that in P. magellanicus on the east coast of North America, SPA or meat of darker colour was also attributed to and/or linked with senescence, chronic infestation of boring sponges (Cliona sp.), prokaryotic infestation and the synergistic effect of all 3 (Stokesbury et al. 2007, Inglis et al. 2016).

Geographic distribution

The discovery of the conspecificity of Merocystis kathae and SAP has extended the known distribution of this parasite. In welks and/or scallops, M. kathae and/or SAP was reported from: UK waters of the Irish Sea, Scotland and the English Channel near Plymouth; the Belgian part of the North Sea; in Øresund, Denmark; Gullmarfjorden, Sweden; Faroe Islands; Bay Breidafjördur off the west coast of Iceland; on the western side of the Atlantic, off the northeast coast of the USA (Georges Bank to Narragansett Bay) and Nova Scotia, Canada; and on the north west coast of Alaska, USA especially Dutch Harbor and Southwest Kodiak (Dakin 1911, Patten 1935, Stokesbury and Inglis 2014, Inglis et al. 2016, Levesque et al. 2016, Ferguson et al. 2018, Kristmundsson and Freeman 2018, Soares et al. 2021). Although M. kathae was originally described from the common whelk Buccinum undatum, collected on the coast of the Isle of Man, Irish Sea (Dakin 1911), the present knowledge of the geographical distribution of M. kathae in the definitive host B. undatum is poorly understood, mostly due to limited research on parasites of the common whelk and related species (Kristmundsson and Freeman 2018).

Host species

The presence of Merocystis kathae in the scallop host (associated with SPA), was first reported as an unknown apicomplexan species in 2011, infecting 3 different scallop species, Chlamys islandica (Iceland scallop), Pecten maximus (king scallop) and Aequipecten opercularis (queen scallop), in Icelandic, Scottish and Faroese, and Scottish waters, respectively (Kristmundsson et al. 2011). SAP (possibly caused by M. kathae) was reported from Placopecten magellanicus (sea scallop) on the Atlantic coast of the USA and Canada (Stokesbury and Inglis 2014, Ingles et al. 2016, Kristmundsson and Freeman 2018) and from Patinopecten caurinus (weathervane scallop) on the north west coast of Alaska, USA (Ferguson et al. 2018). Also, Leibovitz et al. (1984) reported unidentified species of coccidia mainly in the kidney tissue of Argopecten irradians, from the Altantic coast of the United States, that also occurred in the adductor muscle as well as other organs of diseased scallops.

Merocystis kathae was initially described in Buccinum undatum (common whelk) from the Irish Sea (Dakin 1911) and English Channel (Patten 1935).

Note that none of the Pecten maximus, sampled from a scallop ranch in Scotland, where whelks are virtually absent, had clinical signs of disease and only a few SAP life forms were detected in 1 of 20 P. maximus examined (Kristmundsson and Freeman 2018). However, a more recent survey for this parasite in wild stocks of P. maximus from Scotland revealed a high prevalence but low intensity of infection from 3 separate areas (Soares et al. 2021).

Impact on the host

Merocystis kathae is a serious pathogen of scallops, playing a major role in the sudden 90% decline in populations of Chlamys islandica in the Bay of Breidafjördur, Iceland and SAP was largely responsible for the total collapse of the scallop fishery in Iceland (Eiríksson et al. 2010, Kristmundsson et al. 2015, Kristmundsson and Freeman 2018). Furthermore, strong indications exist that SAP causes regular (cyclical) adductor muscle abnormalities and/or mass mortalities in a number of commercial scallop species inhabiting different geographic areas (Kristmundsson and Freeman 2018). For example, SPA severely affected the adductor muscle of Aequipecten opercularis  populations around the Faroe Islands (Kristmundsson et al. 2011) and was the suspected cause of periodic (since at least 1936) mass mortality events associated with a condition called "grey meat" in Placopecten magellanicus on the east coast of North America (Inglis et al. 2016, Levesque et al. 2016). In P. magellanicus increased adductor muscle discoloration, from white to brown to gray, was correlated with increased muscle degeneration and parasite intensity. The parasite was found in high intensity in all gray meat samples, in moderate intensity in brown meat scallops and rarely detected in white meat scallops (Stokesbury and Inglis 2014). Laboratory studies using P. magellanicus collected from the field, indicated that gray-meat infection was swift and fatal (26 of the original 28 gray-meat scallops died) whereas only 1 of 28 white-meat scallops died during the same period in the laboratory. Also, shells exhibiting high levels of boring sponge and worm damage had significantly higher incidence of gray-meat scallops. (Levesque et al. 2016).

In addition to known high scallop mortalities associated with SPA, Kristmundsson et al. (2015) and Kristmundsson and Freeman (2018) speculate that SPA may have been involved in other cyclical, unresolved mass mortality events suffered by various scallop populations. Specifically, in unresolved mass mortality events in Placopecten magellanicus and Chlamys islandica along the shore of the Gulf of St. Lawrence, eastern Quebec, Canada (Giguère et al. 1995, Belvin et al. 2008), in populations of C. islandica in the Barents Sea, Russia (ICES 2006) and in these species of scallops from other North Atlantic locations (Kristmundsson and Freeman 2018). Furthermore, Inglis et al. (2016) speculated and Ferguson et al. (2018) indicated that the apicomplexan associated with the abnormal condition of adductor muscles ("weak meat" phenomenon), observed in Patinopecten caurinus from Alaska may be M. kathae. If the apicomplexan is M. kathae,the definitive host would then most likely be a different whelk species (Kristmundsson and Freeman 2018).

Although M. kathae has been shown to severely affect scallops, Kristmundsson et al. (2015) indicated that the disease only occurs when infections reach a high intensity, as low-level infections exist in scallop populations under normal conditions. Thus, scallops seem able to regulate low level infections of M. kathae as they exist in normal populations suggesting that the immune system of the scallops can to some extent suppress light infections (Kristmundsson and Freeman 2018, Soares et al. 2021). Kristmundsson et al. (2015) also reported that in a population of Chlamys islandica, with a high prevalence of the infections in all size groups, a great difference in the severity of infections was apparent between scallop sizes. Thus, Kristmundsson et al. (2015) speculated that C. islandica get infected at a young age but for some unknown reasons the infections did not seem to intensify in the younger individuals. Nevertheless, epizootics occur during high levels of exposure from nearby infected whelks (Kristmundsson and Freeman 2018). After an almost complete collapse, C. islandica populations in the Bay of Breidafjördur, Iceland have been slowly recovering and macroscopic disease signs were rarely detected and muscle condition appeared normal although low-level infections still remained in the stock Kristmundsson et al. (2015). Similarly, highly prevalent but low levels of infection of M. kathae were observed in both Pecten maximus (prevalence up to 90%) and Aequipecten opercularis  (prevalence of 40%) from Scottish waters but no abnormal clinical signs were reported (Kristmundsson et al. 2011, Soares et al. 2021).

Dakin (1911) and Patten (1935) described the sexual life cycle (including gamogony and sporogony with the formation of infective sporozoites) of M. kathae in the whelk Buccinum undatum but an asexual multiplication cycle called merogony (or schizogony) was absent and thought to develop in an unknown alternate host, which would result in a heteroxenous (2 hosts) life cycle for M. kathae. When the etiological agent of SPA was first describe, Kristmundsson et al. (2011) mistook the large meronts observed in the adductor muscle of scallops for macrogametes (female cells). These forms in addition to merogonic life stages in the adductor muscle lead to the suggestion of a monoxenous (1 host) life cycle (presence of both sexual and asexual stages) in scallops, even though the identity of microgametes (male cells) was not confirmed (Kristmundsson et al. 2011). Kristmundsson et al. (2015), also observed a great number of developmental forms in infected scallops, suggesting that the parasite could potentially have a monoxenous life cycle, but could not exclude the requirement of an obligate alternate host. Recently, Kristmundsson and Freeman (2018) revealed that only asexual merogonic life stages occurred in scallops. Applying tools of molecular analysis including in situ hybridisation, they concluded that the highly pathogenic apicomplexan parasite in various pectinid bivalve species had a dual mollusc heteroxenous life cycle with the common whelk as the definitive host (location of sexual development) where M. kathae was not pathogenic (Kristmundsson and Freeman 2018). Specifically, M. kathae does not negatively impact the whelk definitive host, as heavily infected B. undatum are usually in good condition and the histopathological effect of the parasite is minor; even in extreme infections, pathology seems limited to hypertrophy of infected cells (Dakin 1911). Also, the condition of high prevalence and high intensity of M. kathae in B. undatum was observed throughout the year in B. undatum from Port Erin, Isle of Man (Dakin 1911).

The sympatric distribution of Buccinum undatum and scallops in the North Atlantic makes transmission of M. kathae extremely effective with transmission occurring via the gastrointestinal tracts of both hosts, by scavenging and predation in whelks and unselective filter feeding in scallops. Infective sporozoites from whelks utilize the haemocytes of scallops to reach muscular tissue, where asexual reproduction (merogony) occurs (Kristmundsson and Freeman 2018).

The development of Merocystis kathae follows a seasonal pattern in Buccinum undatum, with the earliest developmental stages appearing between March and June while the first mature (infective) sporozoites form in January and become increasingly common up to May (Patten 1935). Consequently, the scallops are most extensively exposed to infective stages in late winter and spring. During an epizootic outbreak of SAP in the Chlamys islandica population in Bay Breidafjördur in Iceland in the early 2000s, the scallops caught in the spring were significantly more infected with SAP and associated macroscopic signs, than those caught in autumn (Kristmundsson et al. 2015, Kristmundsson and Freeman 2018). The apparent reduction in disease could possibly be associated with mortalities during the summer months as suggested by Eiríksson et al. (2010) who speculated that the rising temperature in Breidafjordur, western Iceland, in recent years probably brought the summer maximum temperature close to the apparent temperature tolerance of C. islandica making them more susceptible to the disease and associated mortalities.

Another factor associated with the SAP epizootic in the Chlamys islandica populations in Iceland was recruitment failure. In addition to causing mortality in C. islandica, the infections significantly hampered gonad development (Eiríksson et al. 2010). Also, the energy demanding gamete maturation process in scallops close to spawning was suggested to make the scallops more vulnerable to infections. These factors contributed further to the collapse of the stock in the form of lower larval production (Kristmundsson et al. 2015). An additional plausible factor influencing the SAP epizootic was the extensive influx of infective sporozoites into the vicinity of C. islandica. Whelks known to be predatory, are also scavengers, feeding on moribund and dead animals (Himmelman and Hamel 1993). Thus, during such mass mortality events, the availability of dead or moribund scallops would be plentiful, resulting in whelks with intensified infections (Kristmundsson et al. 2015). Subsequently, substantial numbers of infective sporozoites are released into the surroundings which infect the remaining naive filter feeding scallops. This situation is reflected in the very high prevalence of M. kathae in both whelks and C. islandica, with many heavily infected in both species during a SAP epizootic outbreak (Kristmundsson and Freeman 2018).

Kristmundsson and Freeman (2018) illustrated and described the life cycle of M. kathae as follows. Sporogonic stages (either in the form of mature sporozoites or immature sporoblasts excreted from the kidney of Buccinum undatum enter Chlamys islandica, via the gastrointestinal tract to invade the host through the intestinal epithelium and into adjacent connective tissues. The sporozoites are transmitted via haemolymph, commonly inside haemocytes, to muscular tissues. The sporoblasts are either transmitted directly to muscular tissues where they sporulate or they sporulate in the connective tissues surrounding the gastrointestinal tract prior to transportation to muscular tissues. In the adductor muscle, the merogonic phase starts when the sporozoites invade muscle cells. The muscle cells become hypertrophied as the premature meront increases in size, eventually leading to rupture of the muscle cell).

Further development of the meront is characterized by recurrent nuclei cleavage resulting in a multi-nucleated mature meront containing numerous merozoites. Free merozoites, which are released from the meronts, then infect new muscle cells starting a new merogonic cycle in the muscle of the scallop. After the formation of 2 or 3 generations of merozoites, the last generation of merozoites infect B. undatum where the gamogonic phase starts. Merozoites invade B. undatum through the intestinal tract when they scavenge moribund or dead scallops and the merozoites migrate to the kidney where they infect renal cells and gamogony starts. Some merozoites develop into macrogamonts (♀) while others become microgamonts (♂). The gamonts mature, eventually leading to fertilization and the formation of a zygote which starts nuclear division initiating the sporogony process. Subsequent recurrent nuclear cleavage occurs at the periphery of the zygote resulting in an oocyst with regularly arranged nuclei at the periphery. With further development the nuclei migrate into the oocyst and start forming uninucleate sporoblasts, each containing cytoplasm from the zygote. The nucleus of the sporoblast divides to form a sporocysts and within each sporocyst, 2 sporozoites develope. The mature sporozoites and immature sporoblasts are released into the water where they are filtered out by cohabiting scallops to continue the life cycle of M. kathae

Diagnostic techniques

Gross observations

The adductor muscle of Chlamys islandica heavily infected with M. kathae (SAP) are greatly reduced in size and have abnormal grey/brown colouration (Stokesbury and Inglis 2014, Inglis et al. 2016, Levesque et al. 2016, Kristmundsson and Freeman 2018). In Placopecten magellanicus, the adductor muscle of infected scallops was weak and described as dark brown to gray, flaccid, stringy, distasteful and was often torn or stretched away with the mantle from the shell attachment site when being harvested (Levesque et al. 2016). The examination of the adductor muscle of live scallops could be facilitated by aneasthesia (Heasman et al. 1995, Levesque et al. 2016). In the welk (B. undatum) heavily infected with M. kathae, the kidney is characterized by numerous small white cysts visible to the naked eye (Kristmundsson and Freeman 2018).

Squash preparations

Various types of large cysts at different developmental stages are present in the fresh mount squash preparations of SAP infected adductor muscle (see Kristmundsson et al. 2011 for images). Their size and outer and inner morphology is quite variable. Thin walled elongated cysts, (320 ± 50 µm by 75 ± 25 µm, n = 10), with granular cytoplasm are common. Sometimes, these have membranous protrusions at each end. In other cases, similar cysts contain a clearer centralized area. Another commonly found type of cysts are usually more slender and with pointed ends (285 ± 25 µm by 45 ± 10 µm, n = 10) and are filled with numerous round spheres or nuclei (3.5 to 4.0 µm in diameter). Yet another type (297 ± 40 µm by 98 ± 25 µm, n = 10), has a very thick wall (5 to7 µm) with regular villar protrusions. Due to the thick wall, its inner structure is not clearly seen in wet mounts. Merozoites (incorrectly identified as sporozoites by Kristmundsson et al. 2011), are abundant in all muscular tissue of moderately and heavily infected individuals. The adductor muscle is generally most heavily infected. Live merozoites measure 17.5 ± 2.0 by 6.5 ± 1.5 µm (n = 100), the size range most frequently encountered being 18 to 19 by 6.5 to 7.5 µm. They are slightly curved with a distinct and large nucleus (Kristmundsson et al. 2011).

Histology

Beautiful colour photographs of the developmental stages of M. kathae in histological sections of the adductor muscle tissue of Chlamys islandica and kidney of Buccinum undatum were published by Kristmundsson and Freeman (2018). They depicted initial infections in the gastrointestinal epithelium and adjacent connective tissues of the digestive gland of C. islandica where sporoblasts, sporocysts and sporozoites appear identical in morphology to sporoblasts, sporocysts and sporozoites in the kidney of B. undatum. In the connective tissues of C. islandica, the sporozoites are often seen inside haemocytes. In addition, sporoblasts are occasionally seen in the adductor muscle. The target organ of the infective sporozoites, is adductor muscular tissue (smooth muscle and the phasic muscle) where the sporozoites actively invade muscle cells which become hypertrophied and eventually rupture. The first indication of merogony is the presence of trophozoites (15–20 μm in diameter), developed from sporozoites, inside muscle cells. The trophozoites significantly increase in size and develop into early meronts. Subsequent development involves recurrent nuclear cleavage giving rise to multinucleated premature meronts, with regularly arranged nuclei, which eventually become mature meronts containing numerous merozoites. Two generations of merozoites are present originating from 2 types of meronts with morphologically different merozoites; type I being shorter and with both ends somewhat pointed, while type II is convex, more slender and sausage-shaped. Additional images of merogony were presented by Kristmundsson et al. (2011). SAP causes severe histopathological changes in C. islandica which was comprehensively described by Kristmundsson et al. (2015).

In the kidney of heavily infected Buccinum undatum all developmental stages, representing gamogony and sporogony, could be observed in 1 histological section. The smallest forms detected were trophozoites (about 10 μm in diameter), intracellular in renal cells. As the trophozoites grow in size, some focal pathological changes occur in infected cells, i.e. the renal epithelial cells increase in size with parasite growth and hence projects into the renal cavity or the underlying connective tissue. Nevertheless, the host cell retains its position in the renal epithelium with no signs of penetration of the parasite into other host cells (Kristmundsson and Freeman 2018). The histopathology caused by M. kathae in B. undatum is minor, even in specimens with extensive infections and regardless of infection status, B. undatum appear to be in good condition (Dakin 1911, Kristmundsson and Freeman 2018).

Although Gulka et al. (1983) associated intracellular procaryotes with mass mortalities of Placopecten magellanicus (East Passage of Narragansett Bay, Rhode Island, USA) that possessed greyish and flaccid adductor muscles, 1 of the histopathology images (Fig. 4 on pg. 358) contained structures reminiscent of M. kathae which were identified as amoebocytes.

Electron microscopy

Ultrastructural features of merozoites (incorrectly identified as sporozoites by Kristmundsson et al. 2011) in the adductor muscle of Chlamys islandica contained all major structures characterizing apicomplexan zoites. The pellicle (cell boundary), consists of an outer unit membrane and an inner membranous layer, which is composed of 2 closely attached unit membranes. The outer unit membrane and the inner membranous layer were separated by an intermediate osmiophobic space. The anterior end of the merozoite contains various organelles including the apical complex. Approximately 80–85 sub-pellicular microtubules extended from the outermost front of the cell to the anterior margin of the nucleus. The large, round nucleus located in the posterior half of the parasite occupied almost the whole width of the cell and almost half of its length. The apical complex (conoid) consisted of rhoptries and micronemes, but the polar rings were not detected. The micronemes spanned from the apical complex to near the anterior surface of the nucleus and occasionally posterior of the nucleus. The Golgi cisternae were observed near the anterior surface of the nucleus and the endoplasmic reticulum occurred between the nucleus and the apical complex. Thick-walled structures located in the anterior part of the cell were speculated to be apicoplasts. Mitochondria were seen at various placements in the cell and seemed to occupy a large space in the cytoplasm. Amylopectin granules were found in large numbers and almost exclusively in the anterior part of the merozoite (Kristmundsson et al. 2011).

DNA probes

Apicomplexan small subunit ribosomal DNA (SSU rDNA) was amplified from Merocystis kathae using various universal primers and primers developed for myxozoans and apicomplexans with PCR conditions as described by Kristmundsson et al. (2015). All infected Chlamys islandica (Iceland scallops) and Buccinum undatum (common whelks) tested positive using a diagnostic PCR, initially developed for the SAP (Kristmundsson et al. 2015) and DNA sequencing showed that the SSU rDNA of M. kathae and SAP was 100% identical (Kristmundsson and Freeman 2018). DNA analysis of the apicomplexan parasite found in the Placopecten magellanicus (Atlantic sea scallop) indicated that it is conspecific with the parasite found in C. islandica, Pecten maximus (king scallop) and Aequipecten opercularis (queen scallop) (Stokesbury and Inglis 2014, Inglis et al. 2016). Ferguson et al. (2018) also used PCR to indicate that the apicomplexan parasite from the adductor muscle of Placopecten caurcinus (weathervane scallop) in Alaska, USA appeared to be the same parasite as the parasite that infects and causes disease in other scallop species in the Atlantic Ocean.

In situ hybridization (ISH) further confirmed the conspecificity of Merocystis kathae and SAP and all the developmental forms detected in both B. undatum and C. islandica gave strong positive reactions to the specific probes (Kristmundsson and Freeman 2018). The ISH also stained small (5 to 6 μm in diameter) intracellular forms in the intestinal tract of the B. undatum, allowing Kristmundsson and Freeman (2018) to indicate that the transmission of the M. kathae, from scallops to whelk, occurs via the gastrointestinal tract.

Methods of control

Due to the total and unexpected collapse of the Chlamys islandica, stocks around Iceland during the early 2000s, a commercial fishing ban was imposed on this valuable resource in 2003. Following the initial identification of an apicomplexan parasite in the scallops (SPA), a long-term surveillance program was established to evaluate the effect of the parasite on the population (Kristmundsson et al. 2015). Kristmundsson and Freeman (2018) indicated that low-level infections do not appear to have a negative impact on the scallops and suggested that it should be possible to lower the infectious load with reasonable fisheries on both the whelk and scallop stocks. This would minimize the chance of epizootics caused by M. kathae and create an optimal host - parasite equilibrium. In support of this suggestion, Kristmundsson et al. (2011) indicated that infections of SPA were almost absent in Pecten maximus (king scallop) from a "whelk free" area. Furthermore, only light infections were reported in both P. maximus and Aequipecten opercularis (queen scallop) collected in 2007 from other UK locations. Kristmundsson and Freeman (2018) and Soares et al. (2021) suggested that the extensive fisheries for whelks in the UK might help to explain this phenomenon. Thus, Kristmundsson and Freeman (2018) speculated that a targeted removal of whelks from valuable scallop grounds would be advantageous to minimize the occurrence of M. kathae epizootics and prevent damaging economic losses.

References

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Citation information

Bower, S.M. (2021): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Apicomplexa Parasite in Adductor Muscle of Scallops

Date last revised: April 2021

Comments to Susan Bower