Abstract:Trematode sporocysts and rediae reproduce by parthenogenesis, forming clonal groups in the molluscan host. It has recently become popular to consider these groups as eusocial colonies, with division of labour between rediae morphs: small 'soldiers' incapable of reproduction defend the colony, while large rediae reproduce. Alternatively, clonal groups can be considered as self-sustaining infrapopulations. We tested these two hypotheses in the light of new data on rediae of Himasthla elongata (Mehlis, 1831) from snails Littorina littorea (Linnaeus) concerning ultrastructure, growth character and composition of their groups. Clonal groups under study contained rediae of different age and maturity stages: small (young) rediae, rediae with early cercarial embryos, rediae with late embryonic cercariae, rediae with fully formed motile cercariae, rediae with redial embryos and degenerating rediae. Small rediae had a reproductive organ, the germinal mass, whereas most large rediae with developing cercariae did not, which contradicts the eusocialconcept. Overall distribution of rediae by size and by gut to body length ratio was bimodal, which agrees with the eusocial concept ('soldiers' and 'reproductives' as modal size classes). On closer inspection, however, the bimodal size-frequency distributions (SFD) turned out to be the sum of unimodal SFD of rediae at various stages of maturity. The overall bimodality was determined by the character of redial growth resulting in a relatively low occurrence of intermediate morphs and by the developmental arrest in young rediae. The facts that small rediae can attack other rediae and concentrate in the anterior parts of the mollusc can be explained by age-related feeding preferences and niche segregation. They are unlikely to be associated with the 'colony' defence against invaders. To sum up, clonal groups of H. elongata rediae in our study represented self-sustaining infrapopulations. We failed to find any arguments in favour of their eusocial organisation.

abundance of avian final hosts) on trematode infection was examined. Infection levels correlated with the interaction between local bird abundance and mussel age and density. They increased with mussel age but their dependence on mussel density and bird abundance varied among parasite genera. Prevalence and intensity increased with SSTmax for Renicola spp., but no relationship was found for Gymnophallus and Himasthla spp. The ambiguous effect of SSTmax is likely explained by the broad range of optimal temperatures for the normal functioning of trematode larvae (cercariae) infective for mussels and by the high dependence of the level of mussel infection on a combination of local ecological factors. High infection levels were observed even in localities with a low SSTmax. No mussels were infected in the most northeastern population, probably due to extreme conditions in the Arctic intertidal preventing trematode transmission. Abstract Blue mussels, Mytilus edulis, serve as second intermediate hosts for several trematode species, but little is known about the mechanisms underlying parasite infections in mussels. To reveal these mechanisms, the prevalence and intensity of trematode larvae (metacercariae in species of Gymnophallus, Himasthla and Renicola) were examined in blue mussels at 19 intertidal sites from the North Atlantic (Ireland, Iceland, Norway) to the Arctic Ocean (north-western Russia). Mussel samples were taken in [2005][2006][2007][2008]. The impact of a number of environmental (maximal sea surface temperature, SSTmax) and biological factors (mussel age and density and the Communicated by J. Grassle. Electronic supplementary materialThe online version of this article (

Infection of mussels Mytilus edulis L. by 2 trematode species was studied in a natural intertidal population in the Chupa inlet of the White Sea. The prevalence of metacercariae of Himasthla elongata (Mehlis, 1831) and Cercaria parvicaudata (Stunkard & Shaw, 1931) in mussels reached 100% in 3 to 4 yr old molluscs and remained at this level in older individuals. Infection intensity increased evenly with the age of the molluscan host, showing a tendency to decrease only in the oldest (9 yr old) mussels. These patterns of age dynamics of prevalence and infection intensity were associated with accumulation of trematode larvae in the course of the molluscs' lives. Ability of metacercariae to exist in mussels for long periods (at least 2.5 yr) was verified in the course of an experiment, during which infected molluscs were kept in a subtidal net cage. Decrease of infection intensity in the oldest individuals may reflect selective mortality of the most severely infected molluscs. Among mussels of the same age, higher infection intensity values occurred in larger individuals. This may be due to an enhanced pumping rate in large molluscs, which increases the probability of cercariae, free-living trematode larvae, infecting them via water currents.KEY WORDS: Mussels · Metacercariae · Digenea · Age · Size · Himasthla elongata · Cercaria parvicaudata · White Sea Resale or republication not permitted without written consent of the publisherDis Aquat Org 71: [51][52][53][54][55][56][57][58] 2006 is a well-established concept of ecological parasitology (Dogiel 1964, Kennedy 1975. Coincidentally, size and age are closely correlated in animals with asymptotic growth, including bivalve molluscs (Alimov 1981). Therefore, a change in physiological functions in the ontogenesis and, consequently, a change in infection indices could be due to an increase in size (linear size, volume, weight) of the organism, or an increase in age (i.e. ageing), or a joint impact of these 2 factors. Usually it is almost impossible to determine how these factors interact. However, in the White Sea intertidal zone, Mytilus edulis are exceedingly variable with regards to their growth rate (Savilov 1953, Sukhotin & Maximovich 1994. Molluscs in the 4 to 6 age groups may fall into the same size class, whereas mussels of the same age may differ in size significantly. This makes it possible to perform a differential analysis of the influence of mussel size and age upon changes in indices of infection with trematode metacercariae. MATERIALS AND METHODSSampling. Blue mussels Mytilus edulis L. were collected in the intertidal zone of the cape dividing the Kruglaya and Seldyanaya creeks of the Chupa inlet (66°20' N, 33°40' E, Kandalaksha Bay, White Sea). This area of the White Sea is characterised by quite strong seasonal fluctuations in temperature and salinity. Throughout the year the temperature of the surface water fluctuates from -0.8 to 13.5°C, and the average salinity from 26.2 to 14.5 ‰ (Babkov 1998). Maximal tide amplitude is 2.0 to 2.1 m. ...

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