Picoeucaryot alga infecting blue mussel Mytilus edulis in southern Norway

Abstract

During summer 2001, blue mussels Mytilus edulis with abnormal shell growth were collected near Kragerø, southern Norway. The mussels had green spots in their mantle tissues, mainly posteriorly and ventrally, and in the adductor muscle. Mussels from 4 sites had a prevalence of green spots varying from 2 to 71% that correlated well with shell deformities. Histological examination revealed the presence of round or ovoid algae, 0.9 to 1.5 × 1.2 to 2.4 µm, free within haemocytes and in the lesions, characterised by an inflammatory response and the presence of cellular debris. The alga contain a relatively large nucleus, 1 chloroplast and 1 mitochondrion. Size and morphology suggest that the alga might be a picoeucaryot green alga. Infection of mussel tissues appears to start in the posterior mantle edge, near the siphons, and spread anterior-ventrally in the mantle connective and storage tissues -occasionally spots were also found in the gonad follicles. Large infected areas were also observed in sinuses within the adductor muscle. Only mussels that were 3 yr old or more were infected. Deformations apparently resulted from years of continuous shell formation by a contracted, partly deformed mantle. Most deformed mussels had eroded shells, allowing some light penetration through the exposed, thin nacre. Young, thin-shelled mussels were not infected. The present work suggests that the alga has, at least partially, a parasitic relationship with the mussels, and is associated with pathological alterations in mussel tissues. MATERIALS AND METHODSField sampling. Mussels were sampled from 4 sites around Burøy on the island of Skåtøy, and Jomfruland, near Kragerø, southern Norway (58°50' N, 9°35' E) ( Fig. 1) in June 2002 and April 2003. Three samples (1a, 1b and 1c) were collected at Site 1, and 1 sample at each of Sites 2, 3 and 4. Each sample consisted of 150 randomly collected mussels, dredged or picked by hand. Sample 1c consisted of empty shells and shell fragments from approximately 300 mussels. Additional mussels were sampled for fixation of affected tissues. Geographical, hydrographical and biological conditions were noted at each sampling site.Gross morphology. During sampling, 20 mussels were steamed, aircooled, and opened to determine if green spots were apparent after processing. Samples 1a, 2, 3 and 4 were subsequently steamed and cooled separately. The length of each mussel shell was measured to the nearest 1 mm. Shell and tissue morphology was categorised as: shell morphology (normal/ deformed); infection site (adductor muscle/mantle edge/gonad area); and infection rate (no/low/moderate/high). Sample 1a had the highest prevalence of infected mussels, and was used for examination of the relationship between infection and shell length. Age of the mussels was estimated using Sample 1b, which was divided into mussel year-classes based on size, shell shape (fast versus slow growing specimens), presumed winter growth rings and erosion and fouling, partly according to Seed (1968). The length of each musse...

Phylogenetic and morphological characterisation of the green algae infesting blue musselMytilus edulisin the North and South Atlantic oceans

“…The observation of digested algae inside mussel haemocytes aggregating around algal colonies suggests that the mussel can counteract the infections (Gray et al 1999, Mortensen et al 2005. Mortensen et al (2005) reported the presence of green pustules in soft tissues of Mytilus edulis, resembling the earlier descriptions of Coccomyxa parasitica in M. edulis chilensis in the South Atlantic Ocean (Gray et al 1999). This and previous studies on bivalves infested by C. parasitica were, however, only supported by morphological features.…”

“…Bootstrap values for major clades are indicated above internodes and correspond to NJ and MP methods, respectively. Bootstrap values < 70% are indicated by hyphens the Gulf of San José, Argentinean Patagonia, Boraso de Zaixso & Zaixso 1979, Bala 1995) and 2 in the North Atlantic Ocean (Flensburg in the Baltic Sea, Meixner 1984; the southern Norwegian coast, Mortensen et al 2005). Algae isolated from the South Atlantic Ocean mussels were reported as Coccomyxa parasitica based on microscopical observations, while the taxonomic position of the others remained unclear.…”

“…Green pustules were located in the posterior end of the mantle and gonad tissues and the posterior adductor muscle, as described by Mortensen et al (2005). Mussels from the Falkland Islands differed from the others by additionally exhibiting infections in the dorsal connective tissue covering the pericardium.…”

“…Blue mussels showing shell deformations as described by Gray et al (1999) and Mortensen et al (2005) were collected at 3 sites. The Norwegian Mytilus edulis sample was collected from a shallow water site at Burøy, near Kragerø, southern Norway (58°50' N, 9°35' E) (see Mortensen et al 2005). The Danish M. edulis sample was collected from the lower part of the intertidal zone at Bockholm, Flensburg Fjord (54°51' N, 09°35' E), where mussels with green pustules were reported by Meixner (1984).…”

“…The parasitic relationship was previously reported as facultative, based upon the culture of Coccomyxa parasitica (Culture Collection of Algae and Protozoa [CCAP] strain 216/18) on an inorganic medium by Stevenson & South (1974). The observation of digested algae inside mussel haemocytes aggregating around algal colonies suggests that the mussel can counteract the infections (Gray et al 1999, Mortensen et al 2005. Mortensen et al (2005) reported the presence of green pustules in soft tissues of Mytilus edulis, resembling the earlier descriptions of Coccomyxa parasitica in M. edulis chilensis in the South Atlantic Ocean (Gray et al 1999).…”