Individual specialisation can lead to the exploitation of different trophic and habitat resources and the production of morphological variability within a pop- ulation. Although the ecological causes of this phenome- non are relatively well known, its consequences on individual fitness are less recognised. We have investigated the extent of individual specialisation in resource use and trophic morphology and its fitness consequences through a combination of tagging–recapture, stable isotope analyses and telemetry. The European eel (Anguilla anguilla) was the model species as it displays significant variability in head shape. Independent to their body length, individuals with broader heads displayed a significantly higher trophic position (d15N) than individuals with narrower heads. This corresponded with a significantly higher proportion of prey fish in their diet compared with invertebrates and was associated with the use of a habitat niche located further from the river bank. The European eel therefore provides a rare empirical example of individual specialisation in resource use and trophic morphology in a natural popula- tion occurring at a very small spatial scale. Individuals with intermediate head morphology displayed lower body con- dition (a proxy of fitness) than individuals with extreme head morphology (i.e. narrower and broader headed indi- viduals), demonstrating the existence of disruptive selec- tion associated with individual specialisation.
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