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. 2018 Nov 6:6:e5773.
doi: 10.7717/peerj.5773. eCollection 2018.

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

Pieterjan Verhelst  1   2   3   4 , Jens De Meyer  5 , Jan Reubens  4 , Johan Coeck  2 , Peter Goethals  3 , Tom Moens  1 , Ans Mouton  2
Affiliations

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

Pieterjan Verhelst et al. PeerJ. .

Abstract

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.

Keywords: Bimodality; Condition; Disruptive selection; European eel; Head dimorphism; Maturation stages; Speed; Telemetry; Unimodality.

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Conflict of interest statement

The authors declare there are no competing interests.

Figures

Figure 1
Figure 1. Study area with the catch location at the tidal weir in Merelbeke (asterisk) and the position of the acoustic listening stations (triangles) in the Zeeschelde.
Figure 2
Figure 2. Head measurements based on the dorsal picture of an eel’s head on graph paper (HL, head length; HW, head width; SL, snout length) (photo credit: Pieterjan Verhelst).
Figure 3
Figure 3. Regression between the ratio head width : head length (HW/HL) and total body length (TL).
Figure 4
Figure 4. Unimodal (A) and bimodal fit (B) of normal distributions (solid lines) on the density distribution of the residuals (dashed lines).
Figure 5
Figure 5. The residual variation according to the six maturation stages (I,FII, FIII, FIV, FV and MII).
The number of eels per stage are indicated above the boxplot.
Figure 6
Figure 6. Unimodal and bimodal fit of normal distributions (solid lines) on the density distribution of the residuals (dashed lines) for each maturation stage (I, FII, FIII, FIV, FV and MII).
Figure 7
Figure 7. The relative condition (Kn) increases with a broader head width (unstandardized residuals).
Figure 8
Figure 8. Migration speeds in relation to the head width (unstandardized residuals).
See this image and copyright information in PMC

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