Seabirds and their parasites are the model used in a research study aimed at improving our understanding of host-parasite trophic relationships, published in the journal PloS One. The study, led by the experts Jacob González-Solís and Elena Gómez-Díaz from the Department of Animal Biology and the Institute of Research of the Biodiversity (IRBio) at the University of Barcelona, takes a novel approach to deciphering the trophic relationships between hosts and parasites, based on the application of isotopic markers used for the first time to analyse parasitism in seabirds.
"Host-parasite trophic relationships are difficult to analyse. In many cases, it is not easy to determine the parasite's source of food, making it harder to confirm whether the symbiotic relationship is really parasitic or if it can be better defined as mutualism or commensalism," explains González-Solís. The research group behind the study examined colonies of Cory's shearwater. The two taxa of this seabird, which belong to the genus Calonectris, are found in large areas of the Mediterranean and the North-East Atlantic, respectively, and are considered endangered by human activity. The study focuses on the trophic relationships between the shearwater and selected ectoparasites: three species of feather lice (Halipeurus abnormis, Austromenopon echinatum and Saemundssonia peusi) and one species of flea (Xenopsylla gratiosa).
Trophic relationships in host-parasite systems are usually studied through analysis of parasite feeding behaviour or host use. As an alternative technique, this study is based on the detection of stable isotopes of carbon and nitrogen. "We are what we eat: the pattern of host isotopes found in the tissue of parasites can be predicted, so by analysing the isotopic signature of these elements we can obtain greater insight into the feeding preferences of the parasitic fauna and therefore better understand host-parasite relationships and segregation in the coexistence between different parasite species," explains González-Solís.
The study has identified links between the isotopic composition of food sources with isotopic levels in the parasites. So, for example, in the case of lice, the feather is the principal food source but the isotopic signature varies between the species considered, which suggests differences in feeding habits such as variation in the consumption of secretions, the lipids that coat the feathers or the particles and spores that cling to the plumage. In the case of the flea species, which is a blood-feeding parasite, the isotopic signature corresponds to that found in the blood of the seabird.
The group has found that the results coincide for both the Mediterranean and Atlantic colonies. González-Solís explains that, "Baseline isotopic levels in the host vary according to the geographical area being analysed, in this case the Mediterranean or the Atlantic, where the water compositions are different. However, the levels of enrichment, which form the basis for predicting the isotopic signature of the parasite, are equivalent in all of the areas studied."
The group also plans to begin new research into the genetic, morphological and taxonomic characteristics of feather mite populations -- unique to each species of shearwater -- and to analyse their spatial and trophic segregation profiles in these symbiotic systems to shed greater light on the ecological structure of the parasite community. "The more specialized the parasite, the greater its dependence on the host; the coevolutionary relationship is closer, and the genetic structure of the host coincides with that of the parasites. In more general parasites, which are less dependent on the evolutionary history of the host, the genetic structure is more closely related to the geographical surroundings," says González-Solís.
Shearwaters are one of seabird species most seriously threatened by human activity, in particular longline fishing. Numbers of the Balearic shearwater are receding, largely due to the impact of longline fishing vessels on the adult population, and if measures are not taken, the species could disappear altogether within the next 40 years. From a more general perspective, the work led by Jacob González-Solís and Elena Gómez-Díaz provides more detailed insight into the coevolution of hosts and parasites, their phylogenetic relationships and the difficulties in the conservation of seabirds in the North East Atlantic and the Mediterranean. Seabirds are under increasing threat from fishing activity, competition for food due to overfishing, loss of breeding habitats, the introduction of predators into islands used for nesting, and environmental contamination -- the latter caused primarily by hydrocarbon spills such as the recent oil slick produced by explosions at the BP oil rig in the Gulf of Mexico.
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