Temporal stability of niche use exposes sympatric Arctic charr to alternative selection pressures

Abstract

There is now strong evidence that foraging niche specialisation plays a critical role in the very early stages of resource driven speciation. Here we test critical elements of models defining this process using a known polymorphic population of Arctic charr from subarctic Norway. We test the long-term stability of niche specialisation amongst foraging predators and discuss the possibility that contrasting foraging specialists are exposed to differing selection regimes. Inter-individual foraging niche stability was measured by combining two time-integrated ecological tracers of the foraging niche (each individual's d 13 C and d 15 N stable isotope (SI) signatures and their food borne parasite fauna) with a short-term measure of foraging niche use (stomach contents composition). Three dietary subgroups of predators were identified, including zooplankton, gammarid and benthivore specialists foragers. Zooplanktivorous specialists had muscle low in d 13 C, a high abundance of parasites transmitted from pelagic copepods, a smaller head, longer snout and a more slender body-form than gammaridivorous specialist individuals which had muscle more enriched in d 13 C and high abundance of parasites transmitted from benthic Gammarus. Benthivorous individuals were intermediate between the other two foraging groups according to muscle SI-signals (d 13 C) and loadings of parasites transmitted from Electronic supplementary material The online version of this article (both copepods and Gammarus. The close relationship between subgroups identified by stomach contents, time-integrated tracers of niche use (SI and parasites) and functional trophic morphology (niche adaptations) demonstrate a long-term temporally stable niche use of each individual predator. Differential habitat use and contrasting parasite communities and loadings, show differential exposure to different suites of selection pressures for different foraging specialists. Results also show that individual specialisation in trophic behaviour and thus exposure to different suites of selection pressures are stable over time, and thus provide a platform for disruptive selection to operate within this sympatric system.

Summary and perspective on evolutionary ecology of fishes

“…Intriguingly, they followed on previous tests that have shown that stickleback females based their mating preferences on olfactory cues mediated by MHC peptide ligands (Reusch et al 2001;Milinski et al 2005) and now demonstrate that individual MHC diversity is involved in assortative female preference for sympatric males. These results and the similar conclusions by Knudsen et al (2011) lend strong support for the hypothesis that parasite-mediated local adaptation may significantly contribute to diversification between populations. Van der Sluijs et al (2011) review the evolutionary consequences of anthropogenic changes to the sensory environments of fishes.…”

“…Fish feeding(Kahilainen et al 2011;Knudsen et al 2011), communication (van der Sluijs et al 2011) and mating(Eizaguirre et al 2011) are all involved into the formation of ecologically divergent populations. Sexual signal evolution may even outpace the ecological divergence along morphological or diet-related traits, as demonstrated for the adaptive radiation of electric fish species (Paramormyrops sp.…”

Creatures of habit: foraging habitat fidelity of adult female Australian sea lions

“…The accessibility of multiple suitable habitats may require an sea lion to develop specialist foraging techniques to successfully exploit one, which may preclude exploitation of others (McLaughlin et al 1999, Bolnick et al 2003, Knudsen et al 2011. Whilst no ecotype-specific bias to either detected seasonality existed, tracking and isotopic data from the present study suggest individual adult female Australian sea lions are repeatedly targeting the same foraging locations and similar trophic levels of prey.…”

Divergent growth patterns between juveniles of two sympatric Arctic charr morphs with contrasting depth gradient niche preferences

“…habitat choice), which are stable over seasons including the long (*6 months) winter period, and across years (Klemetsen et al 2003b;Knudsen et al 2006Knudsen et al , 2010Amundsen et al 2008;Amundsen and Knudsen 2009). The differences in growth related to temperature can be explained as the LO-morph is a trophic generalist, utilising all habitats and dietary resources through their ontogeny (Knudsen et al , 2011Amundsen et al 2008) and experiencing large temperature fluctuations through the year. Arctic charr are generally able to take advantage of the seasonally fluctuating resource accessibility and temperature regimes (Klemetsen et al 2003a).…”

“…The ability of the PBmorph to locate half-buried benthic prey in the soft sediments and to feed effectively during the polar night in deep water under the cover of thick ice and snow with very dim light conditions Amundsen et al 2008) and may also in part be explained by their heritable substantially larger eye size compared with the LO-morph (Klemetsen et al 2002). In contrast, the eye size is only marginally different between upper-water Arctic charr with divergent trophic niches (Knudsen et al 2007(Knudsen et al , 2011. Similar adaptations in eye size of deep-water morphs compared with upper-water layer morphs are also found among European whitefish morph pairs (Harrod et al 2010;Siwertsson et al 2013b), indicating that vision is also an important trait for deep-water dwelling morphs.…”