Searching the onset of head shape bimodality in European eels (Anguilla anguilla)

The life cycle of the European eel (Anguilla anguilla) remained a mystery until the 20th century, when Schmidt discovered that the Sargasso Sea was its spawning area. However, many aspects of the eelU+2019s life cycle remain poorly understood. Among these is the bimodal distribution in head shape, with broad- and narrowheaded phenotypes reported in the yellow eel stage. Although this has been linked to dietary preferences of the yellow eels, very little is known about why, how and when this dimorphism arises during their ontogeny. To find out whether this dimorphism indeed appears in relation to trophic niche segregation, we examined head shape variation at an earlier ontogenetic stage, the glass eel stage, as at this stage, eels are considered to be non-feeding. Head shape was studied in glass eels captured from the Yser river mouth (N=275), the Leopold Canal (N=112; Belgium) and from the rivers Severn, Trent and Parret (N=153; U.K.) by both taking measurements (head width/head length) and using an outline analysis. Our results show that thereU+2019s already considerable variation in broadness and bluntness of the head at the glass eel stage, but no unambiguous support for head shape dimorphism was found. However, as variation in head width/head length ratios in non-feeding glass eels shows a similar range as in feeding yellow eels, head shape in European eel might be at least partially determined through other mechanisms than trophic segregation.

Publication year:2014