Trematode metacercariae in the extrapallial space of Macoma balthica (Bivalvia) in brackish water (southwestern Finland, Baltic Sea)

Metacercariae, mainly of Lacunovermis macomae (Lebour), were present in Macoma balthica (L.) taken from 6 collection sites, where the depth varied between 5 and 40 m and the salinity between 2 and 7 ‰. Metacercariae were present even in specimens of 4-6 mm shell length. The infection incidence (% of infected individuals from those studied) and degree (number of metacercariae/shell valve) usually increased in conjunction with shell length increase. To all intents and purposes the infection incidence attained 100 % in most collection sites when the shell length was over 12 mm, and coincidently heavily infected individuals (≥30 metacercariae/shell valve) began to appear. Heavily infected individuals could amount to over 50 % of specimens in the larger size categories. At one collection site, at a depth of 35 m, the infection incidence and degree of infection by metacercariae in M. balthica were, however, considerably smaller. Only 30-55 % of individuals of 12-22 mm shell length were infected, and they usually carried no more than 1-2 metacercariae per valve.