Research ArticleANIMAL MIGRATION

Glass eels (Anguilla anguilla) have a magnetic compass linked to the tidal cycle

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Science Advances  09 Jun 2017:
Vol. 3, no. 6, e1602007
DOI: 10.1126/sciadv.1602007
  • Fig. 1 Drifting in situ chamber.

    (A) The main underwater unit is composed of a chamber (of a size selected to accommodate the swimming abilities of the glass eels being tested), the imaging system, and a set of sensors. The DISC configuration for the glass eels was a 41-cm-diameter chamber placed 35 cm above the camera. General protocol: During deployment, the DISC was submerged alongside a small boat, and one animal was inserted through the top the chamber. The DISC was then slowly released at a selected depth and location for a duration of 20 min (5-min acclimation and 10-min observation). (B) Example of an image from the upward-looking camera showing the position of the glass eel inside the chamber, the three analog compasses, the water surface, and the downwelling sunlight as seen through Snell’s window.

  • Fig. 2 Study location.

    (A) Northern Europe. (B) Austevoll location where experiments were conducted. The Austevoll archipelago is located between the North Sea (west) and the Bjørnafjorden fjord channel (east). The yellow circle (•) indicates the site of the in situ experiments, and the yellow asterisk () represents the location of the magnetic laboratory facility. (C) Zoomed satellite image of the stream estuary. The yellow triangle (▲) indicates the location of the stream estuary where A. anguilla glass eels were collected. The brown zone between the triangle and the water is a rocky gravel intertidal area. The map was created using RStudio version 0.98.1103 (package ggmap, 2009–2014, RStudio Inc.) (www.rstudio.com). Source of the images: Google Maps.

  • Fig. 3 Orientation of A. anguilla glass eels in situ and in the magnetic laboratory.

    The mean values of the individual bearing angles are presented as red dots for the eels tested in situ and blue dots for the eels tested in the magnetic laboratory. N, E, S, and W represent the four main magnetic cardinal points. The length of the black arrows corresponds to the Rayleigh r value in each graph and their orientation to the mean group direction. Red dashed lines indicate 95% confidence intervals. When the arrow and dashed lines are present, there is a significant directionality in orientation at the population level, according to the Rayleigh test of uniformity (5%). W.p are P values of the Watson’s two-sample test of homogeneity. (A) Orientation of glass eels observed in situ during the flood tide (n = 26; Rayleigh’s P = 0.43, r = 0.18). (B) Orientation of glass eels observed in situ during ebb tide (n = 27; Rayleigh’s P = 0.02, r = 0.38). (C) Orientation of glass eels observed in the magnetic laboratory during flood tide (n = 20; Rayleigh’s P = 0.02, r = 0.43). (D) Orientation of glass eels observed in the magnetic laboratory during ebb tide (n = 15; Rayleigh’s P = 0.04, r = 0.46). The orientation behavior in (C) and (D) was displayed by the glass eels tested under rotated magnetic fields under laboratory conditions.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/6/e1602007/DC1

    The magnetic laboratory

    Analysis and data collection in R

    Activity level of the animals

    Protocol of the tests with the DISC in situ in the fjord

    Collection and maintenance of the glass eels

    Statistics

    Details of the tests

    Analysis of the orientation at the individual level

    Magnetic laboratory protocol details

    fig. S1. Magnetic laboratory facility and the DISC inside the experimental tank.

    fig. S2. Swimming speed of glass eels tested.

    fig. S3. Schematic diagrams of the magnetic protocol.

    table S1. Day, hour, and tidal phase of the tests conducted in situ.

    table S2. Day, hour, and tidal phase of the tests conducted in the magnetic laboratory.

    table S3. Mean orientation of the glass eels tested in situ.

    table S4. Mean orientation of the glass eels tested in the magnetic laboratory.

  • Supplementary Materials

    This PDF file includes:

    • The magnetic laboratory
    • Analysis and data collection in R
    • Activity level of the animals
    • Protocol of the tests with the DISC in situ in the fjord
    • Collection and maintenance of the glass eels
    • Statistics
    • Details of the tests
    • Analysis of the orientation at the individual level
    • Magnetic laboratory protocol details
    • fig. S1. Magnetic laboratory facility and the DISC inside the experimental tank.
    • fig. S2. Swimming speed of glass eels tested.
    • fig. S3. Schematic diagrams of the magnetic protocol.
    • table S1. Day, hour, and tidal phase of the tests conducted in situ.
    • table S2. Day, hour, and tidal phase of the tests conducted in the magnetic laboratory.
    • table S3. Mean orientation of the glass eels tested in situ.
    • table S4. Mean orientation of the glass eels tested in the magnetic laboratory.

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