Loss of species and genetic diversity during colonization: Insights from acanthocephalan parasites in northern European seals

Studies on host-parasite systems that have experienced distributional shifts, range fragmentation, and population declines in the past can provide information regarding how parasite community richness and genetic diversity will change as a result of anthropogenic environmental changes in the future. Here, we studied how sequential postglacial colonization, shifts in habitat, and reduced host population sizes have influenced species richness and genetic diversity of Corynosoma (Acanthocephala: Polymorphidae) parasites in northern European marine, brackish, and freshwater seal populations. We collected Corynosoma population samples from Arctic, Baltic, Ladoga, and Saimaa ringed seal subspecies and Baltic gray seals, and then applied COI barcoding and triple-enzyme restriction-site associated DNA (3RAD) sequencing to delimit species, clarify their distributions and community structures, and elucidate patterns of intraspecific gene flow and genetic diversity. Our results showed that Corynosoma species diversity reflected host colonization histories and population sizes, with four species being present in the Arctic, three in the Baltic Sea, two in Lake Ladoga, and only one in Lake Saimaa. We found statistically significant population-genetic differentiation within all three Corynosoma species that occur in more than one seal (sub)species. Genetic diversity tended to be high in Corynosoma populations originating from Arctic ringed seals and low in the landlocked populations. Our results indicate that acanthocephalan communities in landlocked seal populations are impoverished with respect to both species and intraspecific genetic diversity. Interestingly, the loss of genetic diversity within Corynosoma species seems to have been less drastic than in their seal hosts, possibly due to their large local effective population sizes resulting from high infection intensities and effective intra-host population mixing. Our study highlights the utility of genomic methods in investigations of community composition and genetic diversity of understudied parasites.

DNA barcoding reveals different cestode helminth species in northern European marine and freshwater ringed seals

Three subspecies of the ringed seal (Pusa hispida) are found in northeastern Europe: P. h. botnica in the Baltic Sea, P. h saimensis in Lake Saimaa in Finland, and P. h. ladogensis in Lake Ladoga in Russia. We investigated the poorly-known cestode helminth communities of these closely related but ecologically divergent subspecies using COI barcode data. Our results show that, while cestodes from the Baltic Sea represent Schistocephalus solidus, all worms from the two lakes are identified as Ligula intestinalis, a species that has previously not been reported from seals. The observed shift in cestode communities appears to be driven by differential availability of intermediate fish host species in marine vs. freshwater environments. Both observed cestode species normally infect fish-eating birds, so further work is required to elucidate the health and conservation implications of cestode infections in European ringed seals, whether L. intestinalis occurs also in marine ringed seals, and whether the species is able to reproduce in seal hosts. In addition, a deep barcode divergence found within S. solidus suggests the presence of cryptic diversity under this species name.

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COI barcoding reveals different cestodes in marine and freshwater ringed seals.

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Ligula intestinalis is reported for the first time from seals.

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A deep barcode divergence is found within Schistocephalus solidus in the Baltic Sea.

Corynosomaacanthocephalans in their paratenic fish hosts in the northern Baltic Sea

Cystacanth stages of three Corynosoma (Acanthocephala) species, C. strumosum and C. semerme, and a new species in the Bothnian Bay, C. magdaleni, were studied in the fourhorn sculpin (Myoxocephalus quadricornis) in 1996-1997. The length of the trunk and proboscis differentiated the three species in the fish. The stability of the Corynosoma infection was studied by comparing the present results with those collected from the same areas (central and coastal) in 1977-1982 (Valtonen, 1983a). As C. magdaleni and C. strumosum were not separated at that time, the joint infections of these two species (called "C. strumosum") were compared. The stability of infection with C. semerme in the central Bothnian Bay was noticeable (prevalences were 82.9 in earlier period and 81.9% in later period), while the prevalences of "C. strumosum" in the same areas had decreased (21 and 13.5%, respectively). This is suggested to be due to the disappearance of the marine bull-rout, Myoxocephalus scorpius, from the Bothnian Bay during the 1990s due to an overall decline in salinity throughout the Baltic Sea. In the coastal area Corynosoma infection was clearly lower than in central area in both periods.

Population dependent effects of photoperiod on diapause related physiological traits in an invasive beetle (Leptinotarsa decemlineata)

Organisms undergoing latitudinal range expansion face a change in the photoperiod which can lead to a mismatch between the timing of seasonal changes in physiological and life history traits with seasonal environmental changes. This mismatch can lead to lowered survival, for example, due to unsynchronized diapause timing. Successful range expansion even in recent introductions requires that organisms which use the photoperiod for seasonal predictions should show interpopulational differences in photoperiodic responses at different latitudes, as the photoperiod is a function of latitude. We investigated among population differences in photoperiodic responses of life history and physiological traits linked to diapause in the invasive beetle Leptinotarsa decemlineata. Beetles from a northern marginal and a southern European population were reared under short day (12:12L:D) and long day (18:6L:D) photoperiods. Both populations reacted similarly to the short day photoperiod. Their abdominal total lipid content increased and water content decreased which suggests that the beetles prepared for diapause. This was also indicated by low mortality during diapause. In the long day photoperiod large interpopulational differences were found, the southern population ceased lipid accumulation after 5 days, while the northern population continued lipid accumulation as beetles in the short day photoperiod. This indicates that the northern population has a longer critical photoperiod than the southern one. Abdominal total lipid stores in 10 day old beetles were shown to be predominantly composed of neutral lipids (85%), most likely representing storage triacylglycerols. Fatty acid profiles of both the neutral lipids and the phospholipids showed large shifts during the first 10 day of adult life, predominantly in the fractions of 18:0, 18:1ω9, 18:2ω6 and 18:3ω3. Although the degree of unsaturation increased with age, it was not higher in diapausing than non-diapausing beetles. This indicates that this species does not increase diapause related cold tolerance via homeoviscous adaptation, and might have developed other means to cope with suboptimal temperatures, such as behavioral adaptations.

Combining stable isotope and intestinal parasite information to evaluate dietary differences between individual ringed seals (Phoca hispida botnica)

The diet and foraging behaviour of nine individual Baltic ringed seals ( Phoca hispida botnica Gmelin, 1785) in the Bothnian Bay were studied by combining results from stable isotope analyses (δ13C and δ15N) with data on intestinal parasites whose occurrence varied among the fish hosts. The patterns of infection with three acanthocephalan parasites, Corynosoma semerme (Forssell, 1904), Corynosoma magdaleni Montreuil, 1958, and Corynosoma strumosum (Rudolphi, 1802), and with a cestode larva, Schistocephalus solidus (Müller, 1776), were examined. The ringed seals become infected with these intestinal parasites by feeding on the fish hosts and hence have different parasite species and different parasite burdens according to their dietary history. δ13C and δ15N values were determined from diaphragm muscle of the seals and from tissues of potential prey items. A dual isotope plot of δ13C and δ15N values for individual seals and mean values for key potential prey species, together with the parasitological data from the seals, allowed inferences to be drawn about the feeding of individual seals. It appeared that two seals foraged particularly on fourhorn sculpin ( Myoxocephalus quadricornis (L., 1758)) and one seal consumed a high proportion of the isopod Saduria entomon (L., 1758). Three seals apparently preferred coastal benthic prey in their diets, while two other seals fed more on pelagic herring ( Clupea harengus membras L., 1761). One older female seal evidently also fed on salmon ( Salmo salar L., 1758).

The impact of sexual selection on Corynosoma magdaleni (Acanthocephala) infrapopulations in Saimaa ringed seals (Phoca hispida saimensis)

In free-living animals sexual selection is a central force shaping the spatial distribution of individuals in a population as well as sexual size dimorphism. We studied the influence of sexual selection on spatial distribution, female-to-male body size ratio, and female mating success of acanthocephalans in a natural host population of Saimaa ringed seal (Phoca hispida saimensis) harbouring a single intestinal helminth species, Corynosoma magdaleni. The acanthocephalans were always found along the full length of the small intestine; however, the site selection varied among the individual seals according to the age of the infection. The distribution of male acanthocephalans was not random with respect to females, with larger males tending to aggregate around non-mated females. A higher proportion of C. magdaleni females had copulated in seals with relatively more male worms. Male-male competition for access to females can be intense in C. magdaleni infrapopulation and may select for large males. We found that the larger the infrapopulation size, the smaller the males compared to females. In addition, the greater the female bias in the infrapopulation, the smaller the testes of males. Our study shows that sexual selection may be an important determinant of spatial distribution, male body size and female mating success of C. magdaleni in Saimaa ringed seal.

Intestinal helminths of a landlocked ringed seal (Phoca hispida saimensis) population in eastern Finland

A small, landlocked, endangered ringed seal (Phoca hispida saimensis) population lives as a postglacial relict in Lake Saimaa in eastern Finland. In this study, the intestinal metazoans were examined from a total of 61 Saimaa seals found dead from 1981 to 2001. The helminth fauna was very depauperate. Only one acanthocephalan species, Corynosoma magdaleni, has been able to survive during isolation in the freshwater environment. In addition, only two cestode species were found: Diphyllobothrium ditretum and Schistocephalus sp. However, neither of these larvae developed in the ringed seals. As the newborn pups of Saimaa seals are nursed for about 2 months only one of them was infected by C. magdaleni. At 2-5 months the pups harboured worms; however, the mean intensity did not rise distinctively until the seals were adults. C. magdaleni infection increased with increasing age and the abundances were significantly different between the age groups. There was no correlation between the number of parasites and the adult seal body weight; however, seals weighing 36 kg or more carried the majority of the worms. The distribution of the parasites was aggregated; six seals carried 65% (n=1,182) of all worms (total n=1,809) and 26 seals were uninfected. Variation in the number of worms in individual seals may indicate differences in feeding behaviour and in the diet of seals, and/or variation in the immune status of the seals.