Unimodal head-width distribution of the European eel ( Anguilla anguilla L.) from the Zeeschelde does not support disruptive selection

• Published in: PeerJ
• Main Authors: Verhelst, P., De Meyer, J., Reubens, J., Coeck, J., Goethals, P., Moens, T., Mouton, A.M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2018
• Subjects: Speed; Telemetry; Anguilla anguilla
• Online Access: https://www.vliz.be/imisdocs/publications/320687.pdf

Since the early 20th century, European eels ( Anguilla anguilla L.) have been dichotomously classified into `narrow' and `broad' heads. These morphs are mainly considered the result of a differential food choice, with narrow heads feeding primarily on small/soft prey and broad heads on large/hard prey. Yet, such a classification implies that head-width variation follows a bimodal distribution, leading to the assumption of disruptive selection. We investigated the head morphology of 272 eels, caught over three consecutive years (2015-2017) at a single location in the Zeeschelde (Belgium). Based on our results, BIC favored a unimodal distribution, while AIC provided equal support for a unimodal and a bimodal distribution. Notably, visualization of the distributions revealed a strong overlap between the two normal distributions under the bimodal model, likely explaining the ambiguity under AIC. Consequently, it is more likely that head-width variation followed a unimodal distribution, indicating there are no disruptive selection pressures for bimodality in the Zeeschelde. As such, eels could not be divided in two distinct head-width groups. Instead, their head widths showed a continuum of narrow to broad with a normal distribution. This pattern was consistent across all maturation stages studied here.