International Council for the Exploration of the Sea CM 1984
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International Council for C.M. 1984/K:30 the Exploration of the Sea Shellfish Committee Ref.:Marine Environmental Quality Committee . ON A MICROALGAL INFECTION OF MYTILUS EDULIS =========================================== by R. Meixner Bundesforschungsanstalt für Fischerei Institut für Küsten-und Binnenfischerei Palmaille 9, 2000 Hamburg 50 ABSTRACT Wild"mussels, ,not destined for human consumption, were collected near the shore'of theFlensburg Fjord. They have been studied with regard to macroscopic "green spots" in the mantle and adductor tissues. The "greening" of the soft parts is caused by colonies of a m;croalga, and is to be observed in mussels with shell erosion only. The endobiotic alga (3 x 4 ~m) provisionally was determined as a blue-green alga belonging to the genus Microcystis KüTZ . • "., .) ) - 1 - 1. INTRODUCTION Since 1981 green spots in the soft parts of musseis from the Flensburg Fjord were noticed. Microscopic inspection revealed numerous algal colonies giving greenish marks to the affected tissues. The algal spots often were so densely crowded that the edible part as a whole became green. In such cases a decline in vitality took place. This suggested a study on the relationship between the mussei and ~he algal colonies. 2. INVESTIGATIONS AND RESULTS 2.1 I~~_91g9~~ Blue-green to olive-green spots (up to 3 rnm in diameter) e and pseudo-filaments (up to 10 mm length) are formed by algal colonies. The spots, more or less of round shape, concentrated in the adductor muscle and in the mantle. Pseudo-filamentous algal masses occurred within the gonadal tubules. The colonies are composed of non-motile 'cells' (3x4"um), which lie irreguiarly embedded in mucilage. Simple cell division was the onlyreproductive process observed. The algae were determined provisionally as genus Microcystis KüTZ., belonging to the Cyanophyta. 2.2 I~~_~~~~~!~ Infection rates increase with mussel size resp. age, as is exemplified in Table 1. Table 1: The occurrence of green discolourations in mussel flesh. This sample of 200 specimens (10-59 mm), of 28 May 1984, was collected at ~ Bockholm (Flensburg Fjord/Western Baltic) in a depth of 0.5 m. ,., length of mussels number of rate of infection (mm) mussels (N) (n) (%) 10 - 19 14 0 0 20 - 29 45 8 18 30 - 39 68 20 29 40 - 49 56 22 39 50 - 59 17 6 35 I. - 2 - The amount of mussels infected by algal colonies seems to follow a seasonal trend, in accordance with the natural period of vegetation (Table 2). Table 2: The average algal infection of mussel samples, collected at Bockholm (Flensburg Fjord/Western Baltic) in a depth of 0.5 m. Month size of sample infection (N) (%) April 1984 66 26 May 1984 100 28 June 1984 100 35 July 1981 40 68 • August 1982 31 65 October 1983 94 9 2.3 ~~~11_~~~~1~~~ Shell erosion is a general external feature of individuals lodging algal colonies. Mussels with an intact periostracum never had green spots in the flesh. Erosion of the dark periostracum and the blue calcite layer brings the translucent nacreous layer to the shell surface. The existance of a translucent shell area permits the sunlight to diffuse through to the soft parts. Sufficient solar energy is an important requirement for algae, living in a mussel. ~ 2.4 ~~~~l~~~~~I_~ff~~~~~ If mussels are infected heavily, a decline, in the physiological condition becomes evident. In such mussels a slippery appearance of the mantle and a stringy weakening of the adductor muscle is to be found. The terminal stage is characterized by atrophy of tissues. When such mussels are lifted above the sea level, they will loose water rapidly. There may also be a possible impact on the reproduction of mussels. This assumption is based on observations on algal masses, filling the entire lumen of the branching genital tubules. The passage of'gametes through the gonadal ducts, necessary for successful spawning, may thus be,prevented. • '. - 3 - 3. DISCUSSION The findings of this paper can be interpreted in the light of another case of algal infection. Severe symptoms s somewhat similar to the above mentioned s were s'een in Placopecten magellanicus by Naidu &South (2)s Naidu (3) and Stevenson &South (4). Their publications were reviewed by Lauckner (1). The algaes after this reviews were initially believed to be symbiotic zoochlorellae. Subsequentlys however s the relationship was recognized as being parasitic. The causative agent was identified as Coccomyxa parasiticas a new member of the Coccomyxaceae s Chlorococcales s ~ Chlorophycea. Further studies on this algal infection of Mytilus edulis go on. Focal points,are etiologys histopathologys associated environmental conditions s geographical distribution and ecological significance. 4. CONCLUSION The association of mussels Mytilus edulis and endobiotic algal coloniess found in the Flensburg Fjords has to be considered in terms of a host- , parasite relationship. 5. REFERENCES • (3) Naidu s K.S. Infection of the giant scallop Placopecten magellanicus 1971 from Newfoundland with an endozoic alga. J. Invertebr. Path. 17: 145-157 (4) Stevenson s R.N. and South, G.R.: Coccomyxa arasitica sp. nov. 1984 (Coccomyxaceae, Chlorococcales , a parasite of giant scallops in Newfoundland. Br. phycol. J. 9: 319-329.