Heavy metals are persistent pollutants, the concentrations of which have increased in many natural environments as a result of anthropogenic activities. Accumulation of heavy metals in coastal benthic habitats is common. Nematoda are the most abundant and species-rich phylum in these benthic habitats, and shifts in their assemblage composition are considered good indicators of environmental change. At the same time, single-species assays can be highly informative of the organismal and population-level effects of pollution. Litoditis marina is a marine nematode inhabiting decaying macroalgae in the littoral zone of coasts and estuaries; it represents a cryptic species complex, and local co-occurrence of several cryptic species is common. Impact studies often assume that phylogenetically related nematodes exhibit similar sensitivities to disturbance, and commonly used biomonitoring indices often utilize scores defined at the level of guilds or families. Nevertheless, studies on other phyla have shown that even closely related taxa such as the cryptic species analysed in this study, may differ in their tolerance to pollution. Here we test the effects of Cu and Pb on the growth, fecundity and behaviour of two cryptic species of L. marina, PmII and PmIV, in single-species assays. Pb is a neurotoxicant and we expected that it would strongly affect behaviour (measured here as taxis to food). Increased Cu concentrations cause oxidative stress and can interfere with osmoregulation and endocrine pathways; Cu was therefore expected to impact more on growth and fecundity. Both cryptic species of L. marina were sensitive to heavy-metal pollution, but PmIV was substantially more sensitive than PmII, especially to copper. In contrast to our expectation, copper yielded the strongest effects on all endpoints, including behaviour. Lead yielded more discrepant results, inhibiting reproduction but stimulating growth and taxis to food at low (PmII) to intermediate (PmIV) concentrations. This study demonstrates that also in nematodes, closely related species, such as the cryptic species PmII and PmIV analysed in this study, can substantially differ in their response to contaminants, necessitating species-level rather than family or guild-based analyses of pollution effects. This is further supported by a comparison with literature data on the sensitivity of other nematode species to the same heavy metals. This comparison highlights that sensitivity to pollution can vary equally strongly between as within nematode families and guilds, and thereby challenges many guild- or taxonomic-relatedness-based approaches in impact studies using nematodes. |
