No time to relax: Age-dependent infectivity of cercariae in marine coastal ecosystems

Age dynamics of the ability of cercariae of two digenean species, Himasthla elongata (Himasthlidae) and Renicola parvicaudatus (Renicolidae), to infect the second intermediate host (SIH), mussels (Mytilus edulis), was investigated experimentally. This is the first study of this kind made on cercariae transmitted in the intertidal of the northern seas. The larvae of all tested ages (from 0.5 to 6 hr) were equally successful in infecting mussels. This finding disagrees with the literature data on cercariae of several freshwater digeneans, which are practically incapable of infecting the SIH during the first 1-3 hr of life. The presence of a time delay before the attainment of the maximum infectivity (TDMI) may be associated with the need for physiological maturation of cercariae in the very beginning of their life in the environment, the need for their broad dispersion, and the prevention of superinfection of the downstream host. The absence of TDMI in the cercariae examined in our study could be associated with the instability of environmental factors in the marine intertidal (wave impact, tidal currents). These factors promote a broad dispersion of cercariae in the intertidal biotope and prevent superinfection of potential SIHs. Biological and behavioural features may also play a role. We hypothesize that the presence or absence of TDMI does not depend on the taxonomic affiliation of the cercariae but is determined by the transmission conditions.

[Clonal variability in longevity of the cercariae of Himasthla elongata (Trematoda: Echinostomatidae)]

The study was carried out on Himasthla elongata cercariae shed by infected Littorina littorea snails. The infected periwinkles were collected from the settlement with the low prevalence of H. elongata. As shown earlier with the use of AFLP (Amplified Fragment Length Polymorphisms) method, rediae groups in all the infected periwinkles of this settlement arise from the infection of a mollusc with a single miracidium. Therefore, the cercariae shed by an infected mollusc have the same genotype or, in other words, represent a clone. The LT50 (the time during which 50% of cercariae perish in the experimental dish) were measured experimentally for cercariae Himasthla elongata belong to different clones. The investigated parameter demonstrated a high level of interclonal variability. Two groups of cercarial clones were identified: one of them was characterized by the high level of intraclonal variability in LT50 and the second, by the low one. It is assumed that the observed heterogeneity may be stipulated by different degrees of mitotic recombinations during formation of different cercarial clones.

[A case study of singular spectrum analysis application in parasitology: dynamics of prevalence of Cryptocotyle concavum and Bunocotyle progenetica trematode parthenitae in Hydrobia ventrosa snails at the White Sea]

In this study parasitological data were analyzed by different methods of revealing the structure of time series, namely auto-correlation analysis (ACA), Fourier spectrum analysis (SA) and singular spectrum analysis (SSA), and the results of these analysis were compared (SSA makes it possible to present non-stationary time series as a sum of independent components and to determine the contribution of each component into the dispersion of the initial series--Golyandina et al., 2001). This case study was based on the result of 10-year-long monitoring of changes in the prevalence of Cryptocotyle concavum and Bunocotyle progenetica trematode parthenitae in intertidal snails Hydrobia ventrosa at the White Sea (in total, 45 observations). ACA did not reveal any statistically significant oscillations in the analyzed series. The application of SSA and SA allowed us to reveal at least two quasi-periodical components. In addition, SSA made it possible to reveal a significant dome-shaped trend in the prevalence of B. progenetica parthenitae, which were described by SA as an oscillation with a period equal to the duration of the study, as well as to give proof that there was no trend in the changes of C. concavum parthenitae prevalence. The components (modes) extracted by the SSA described the changes in the prevalence better that the harmonics extracted by the SA. In particular, SSA modes (contrary to SA harmonics) reflected that the amplitude of oscillations of the B. progenetica prevalence increased as the prevalence grew. The sums of SSA modes correlated more with initial prevalence series that the sums of SA harmonics. A possible interpretation of the trends and modes extracted by the SSA in the light of the transmission features of the investigated trematode species in the study area was proposed.

[Clonal variability in expression of geo- and photoorientation in cercariae of Himasthla elongata (Trematoda: Echinostomatidae)]

The study was carried out on Himasthla elongata, a digenean common in the coastal ecosystems of the northern European seas. This species utilises intertidal prosobranchs Littorina spp. as the first intermediate host, bivalves (in the White Sea, Mytilus edulis) as the second intermediate host and gulls as the final host. The periwinkles Littorina littorea infected with H. elongata rediae (parthenogenetic generations) were sampled in the intertidal of the White Sea (66 degrees 20' N, 33 degrees 38' E) and used as the source of cercariae. Periwinkles were collected from the settlement with the low prevalence of H. elongata. As shown earlier with the use of AFLP (Amplified Fragment Length Polymorphisms) method, rediae groups in all the infected periwinkles of this settlement arise from the infection of a mollusc with a single miracidium. Therefore, the cercariae shed by an infected mollusc have the same genotype or, in other words, represent a clone. Photo- and geoorientation of cercariae originating from different clones and aged 1 h and 6 h were analysed separately. It was shown that in general the larvae of each clone followed the behavioural pattern characteristics of the species (positive geoorientation and negative photoorientation). However, the degree of expression of this typical behaviour was different in different clones. An especially high variability was observed in the manifestation of geoorientation (in several clones, most larvae demonstrated negative geoorientation). Differences in the distribution of cercariae in the illumination gradient were almost equally associated with the interclonal variability and the age of the larvae. On the whole, as the age of cercariae increased, the positive geoorientation became more prominent, whereas the ratio of cercariae with the typical (negative) photoorientation decreased. Statistical analysis revealed significant differences between the cercarial clones both in the initial manifestation of geo- and photoorientation and in the changes in the character of these reactions with the larval age. Taking into account that each cercarial clone investigated had the same genotype, it seems very likely that the interclonal differences noted in this study are hereditary. Maintenance of a rather high level of genetic polymorphism by the character "expression of orientation reaction" in trematode cercariae may enhance the chances for successful transmission of these larvae. Such variability increases the scale of cercarial dispersion in space and promotes the successful infection of the hosts, whose behaviour is also subject to intra- and inter-population variability. Besides, cercariae whose behaviour deviates from the basic behaviour of the species may play the role of the population's potential for colonisation of new species of animal hosts.

[The influence of infections with trematodes Bunocotyle progenetica (Hemiuridae) and Cryptocotyle cancavum (Heterophyidae) onto mortality of littoral mollusks Hydrobia ulvae (Gastropoda: Prosobranchia) after freezing]

The mortality of prosobranch snails Hydrobia ulvae uninfected and infected with the parthenites of two species of Trematodes (Bunocotyle progenetica and Cryptocotyle cancavum) after freezing in seawater of different salinity with following thawing has been studied. Reduced salinity exceeds mortality of snails under freezing significant (p < 0.01). The mortality of mollusks increases significant (p < 0.01) with the increase of the exposure time. The mortality of females exceeds the mortality of male significant (p < 0.01) within the group of 2-years individuals. The negative effect of trematode invasion to the vitality of mollusks after freezing was not showed. Trematode infestation registered in March 2004 and in September 2003 is not significant differ (Levakin, 2004). Thus any important differential mortality of individuals H. ulvae infected with parthenites of this Trematodes is not present during a winter time. The conclusion is conformed by the results of our experiments and well accords to features of realization of life cycles of these Trematodes under conditions of the White Sea intertidial zone, where infection is possible only within the worm season.

[The influence of infections with trematodes Bunocotyle progenetica (Hemiuridae) and Cryptocotyle cancavum (Heterophiidae) onto mortality of littoral molluscs Hydrobia ulvae (Gastropoda: Prosobranchia) in condition of extremely high temperature]

The effect of invasion of two trematode species, Bunocotyle progenetica (Hemiuridae: Bunocotylinae) and Cryptocotyle cancavum (Heterophiidae) on the host vitality is estimated. The mortality of infected and uninfected individuals of Hydrobia ulvae after different exposure time (3, 6, 9 and 12 hours) under extremely high temperature (+42 degrees C) was compared. The total death of the host was observed after 12 hours of experiment. The death rate of molluscs infected with B. progenetica ranged from 60 to 100% and was significantly higher (p < 0.01) than mortality of individuals infected with C. cancavum and uninfected snails in each case of exposure (3, 6 and 9 hours). At the same time the mortality of individuals infected with C. cancavum was significantly higher (p < 0.05) than the mortality of uninfected snails only after 9 hours of exposure (13.3 and 3.1%, respectively). Difference in pathogen effects B. progenetica and C. cancavum is expressed in different temperature resistance of the host that may be explained by different localisation of redia, their motile activity and manner of feeding. Besides, the invasion by C. cancavum causes the sterilization and accelerated growth of hosts. Low decreasing of the resistance to overheating in snails infected with this trematode could be explained by a compensation effect of this infection. This compensation realizes due to the redistribution of some part of the reproductive energy after the sterilization. An absence of parasitic castration in the case of infection with B. progenetica is a key reason of such considerable declining of their vitality. High lethality of the host appears to be adaptive for the parasite with the monoxenous life cycle.

[The use of the probability model for an estimation of a infection rate of White Sea mussels (MYtilus edulis) by Metacercariae Himasthla elongata (Trematoda: Echinostomidae) ]

A comparison of simple probability analytic model of infection rate depending on host's age with natural infection rate of mussels (Bivalvia: Mythilidae) with metacercariae Himasthla elongata (Trematoda: Echinostomidae) was carried out. Data on the natural rate of infection were accumulated during 3 years; 1152 individuals M. edulis were collected in two horizons within fucoid zone of the Kruglaya inlet and the Chupa inlet of the Kandalaksha bay (White Sea). A size of shell and number of H. elongata metacercariae were defined for each mussel using compressive dissection technique. The infection of mussels per year within our model is considers as independent evens: Pn = 1 - (1 - p)n, where n is the age of mussels, Pn is the theoretical part of infected n-year mussels, p is the probability of infection within one year. The probability of infection within year is assumed equal for every age of host. The estimate of probability of infection per year on the basis of sample of n-year's mussels was calculated as Pn = 1 - n square root of 1 - I(n), where n is the age of mussels, I(n) is the part of infected n-year mussels. The retransformed weighted average value of aresine-transformed p'n was used as p in our model (p = 0.3476). Statistically significant differences between empirical and theoretical (calculated from our models on the basis this value) infection rates were not found (P > > 0.05 chi 2-test). Moreover, statistical significant differences were absent (P > 0.05 Fisher exact test) in pairwise comparisons between empirical and theoretical infection rates for each age of mussels. The model does not take into consideration an effect of such factors as host's resistance, host's migration and increase of mortality in infected hosts. The absence of significant differences between the empirical and theoretical infection rates allows to suggest, that mentioned factors under the conditions of the Kruglaya inlet do not influence essentially onto infection of mussels with metacercariae H. elongata. This conclusions is in certain inconsistency with essential differences in such characteristic as an individual resistance of mussels to the infection with metacercariae H. elongata, detected in experiments in vitro (Gorbushin, Levakin, unpublished data). Analysis of intensity of the invasion of metacercariae H. elongata into mussels allows to suggest the existence of differential death rate of the hosts, which is exhibited in individuals over 7 years old. Studied example of mussels infected with metacercariae H. elongata under conditions of the Kruglaya inlet shows that the simple probabilistic model of the natural infection rate is usable for this kind of investigation. Our study also allows to conclude that in this case the infection rate of hosts is mainly determined by stochastic reasons. However, in some cases the probability of infection rate may not depend on the age and size of the host. The study of infection rate can not be used for analyses of individual differences of hosts in a resistance to parasites and an infection ability of the parasite.

Hydrobia ulvae (Gastropoda: Prosobranchia): a new model for regeneration studies

Within 2 weeks of decapitation, Hydrobia ulvae was able to regenerate new head structures including buccal ganglia. It was also capable of regenerating propodial ganglia after anterior foot amputation. The functional regeneration of the buccal ganglia was demonstrated by behavioural observations and by electrophysiological experiments. The presence of the oesophagus was shown to be important for regeneration of the buccal complex. H. ulvae provides a new model for regeneration studies, so details of the topographic anatomy and biology of this species are described. To standardize experimental animals in future studies, the effects of age, sex and trematode infestation on the regeneration capacity of H. ulvae have been evaluated. The high capacity for regeneration together with the possibility of using electrophysiological techniques makes H. ulvae a favourable model in which to study neurogenesis in adult animals.