Four coexisting morphs or arctic charr, Salvelinus alpinus (L.), occur in the lake Thingvallavatn, Iceland. They can be identified by their head morphology, which appears to he related to feeding habits, benthivorous or planktivorous-piscivorous. Laboratory-rearing experiments indicate that these morphological differences have a genetic basis with a significant maternal effect and support the suggestion that there are three populations of arctic charr in the lake. The ontogenetic mechanism of these differences most readily explained as development heterochrony, that is the heterochrony that shape reflects embryonic phenotypic characteristics while the planktivorous-piscivorous head shape is more differentiated from the embroyonic phenotype.

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