Bimodality in head shape in European eel

Abstract

The existence of two morphotypes, broadheaded and narrowheaded, in European eels Anguilla anguilla is common knowledge among fishermen and eel biologists in Europe. To test whether European eels really are dimorphic in head shape, a total of 277 specimens from two locations in Belgium (Scheldt–Lippenbroek and Lake Weerde), in combination with a larger data set of 725 eels from river systems across Flanders (the northern part of Belgium) were examined. Our biometric data support the hypothesis that a head shape variation in ‘Belgian’ European eel is best described as having a bimodal distribution. Literature data suggest that this may be the result of phenotypic plasticity related to trophic segregation between morphs.