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ARTICLE
Glass eels (Anguilla anguilla) imprint the magnetic
direction of tidal currents from their juvenile
estuaries
Alessandro Cresci 1,2*, Caroline M. Durif2, Claire B. Paris1, Steven D. Shema3, Anne Berit Skiftesvik2&
Howard I. Browman2
The European eel (Anguilla anguilla) hatches in the Sargasso Sea and migrates to European
and North African freshwater. As glass eels, they reach estuaries where they become pig-
mented. Glass eels use a tidal phase-dependent magnetic compass for orientation, but
whether their magnetic direction is innate or imprinted during migration is unknown. We
tested the hypothesis that glass eels imprint their tidal-dependent magnetic compass
direction at the estuaries where they recruit. We collected 222 glass eels from estuaries
flowing in different cardinal directions in Austevoll, Norway. We observed the orientation of
the glass eels in a magnetic laboratory where the magnetic North was rotated. Glass eels
oriented towards the magnetic direction of the prevailing tidal current occurring at their
recruitment estuary. Glass eels use their magnetic compass to memorize the magnetic
direction of tidal flows. This mechanism could help them to maintain their position in an
estuary and to migrate upstream.
https://doi.org/10.1038/s42003-019-0619-8 OPEN
1Department of Ocean Sciences, Rosenstiel School of Marine & Atmospheric Science, 4600 Rickenbacker, Causeway, FL 33149-1098, USA. 2Institute of
Marine Research, Austevoll Research Station, Sauganeset 16, N-5392 Storebø, Norway. 3Grótti ehf., Grundarstíg 4, 101 Reykjavík, Iceland.
*email: alessandro.cresci@rsmas.miami.edu
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The European eel (Anguilla anguilla) is a migratory species
that crosses the Atlantic Ocean twice during its life (Fig. 1).
After hatching in the Sargasso Sea1, eel leptocephali larvae
move with the gulf stream more than 5000 km until they reach
the continental slope of Europe2,3. There, they metamorphose
into the post-larval transparent glass eel4. At this stage, glass eels
migrate across the continental shelf to the coast2,5. After reaching
the coast, glass eels enter estuaries, where some of them continue
their migration upstream into freshwater6. The eels that enter
freshwater spend most of their lifetime (5–25 years) there,
growing first into the adult yellow eel stage, and then into silver
eels2. Silver eels then navigate back to the Sargasso Sea where they
spawn and die1,7,8.
The European eel is a commercially important species that is
critically endangered [International Union for Conservation of
Nature (IUCN)]: eel populations have declined precipitously
since the 1980s9–11. Much research on the conservation and
management of eels has been undertaken12,13, driven to some
extent by the requirement for member states of the European
Union to develop management plans for the recovery of eel
populations. The arrival of the early-life stages to estuaries
(recruitment), is an important phase of their migration, and the
number of recruiting glass eels have been consistently declining
over the past decades10,14. Thus, a better understanding of the
dynamics of this step of the migration, including deeper knowl-
edge of the orientation mechanisms involved, is needed.
Glass eels use multiple spatial and sensory cues for orienta-
tion15–17, which could hypothetically be imprinted and used years
later during their return journey to the spawning areas. It is
possible that eels imprint spatial information from the
environment at several steps during their migration, such as at
the time of hatching or during the journey to the European coast.
Imprinting is associated with a broad variety of processes in
animals, including food and habitat preference, host selection in
parasitic animals, and homing18–20. Imprinting is a fast-learning
process that typically occurs during a specific life history
phase18,19,21. Imprinting of spatial cues, such as olfactory cues in
salmon22, occurs in several aquatic animals. Animals can also
imprint information from the magnetic field of the Earth. Aquatic
species such as pacific salmon (Oncorhynchus nerka)23 and sea
turtles (Caretta caretta)20 imprint specific features of the Earth’s
magnetic field and use this to orient during migration. The
European eel is a long distance migrator that also uses magnetic
fields to orient during migration24,25. However, whether
imprinting of magnetic cues occurs, either at the larval or glass eel
stage (or both), remains unknown.
Glass eels recruit at tidal estuaries along the European coast,
where they are exposed to the alternation of ebbing and flooding
tidal currents. In aquatic environments, fish display an uncon-
ditioned response to water currents (rheotaxis), which can be
positive (the fish swims into the current) or negative (the fish
swims in the same direction as the current)26,27. Rheotaxis is a
major component of fish orientation in tidal estuaries in which
currents can be fast and visibility low27. In such situations, fish
use rheotaxis for both upstream migration and upstream-oriented
station holding behavior to minimize energy expenditure in
flowing water28. Glass eels display both of these rheotactic
behaviors when they recruit to estuaries, showing rhythmic pat-
terns of positive or negative rheotaxis synchronized to the tidal
phase29. This tidal-dependent orientation is important during
Seawater
Freshwater
Brackish water
Brackish water
Leptocephalus
(larva)
Glass eel
(post-larva)
(testing stage)
Elver
(juvenile)
Yellow eel
(non-mature adult)
Silver eel
(migratory adult)
H.P.I.
S
p
a
w
n
i
n
g
m
i
g
r
a
t
i
o
n
(
S
a
r
g
a
s
s
o
S
e
a
)
Fig. 1 Life history of the European eel (Anguilla anguilla). Eels hatch as leptocephalus larvae in the Sargasso Sea. As larvae, they drift across the Atlantic
Ocean to the continental slope of Europe, where they metamorphose into post-larval, transparent glass eels. The glass eels migrate across the continental
shelf and eventually reach the brackish water of estuaries. After metamorphosing into pigmented juveniles, called elvers, they start the ascent into
freshwater, where they will grow into adult yellow eels. After some years, yellow eels undergo another metamorphosis into silver eels, which migrate for
thousands of kilometers to the Sargasso Sea where they spawn and die. Eels used in this study were at the stage of glass eel (red font), and the
Hypothetical Period of Imprinting (H.P.I.) is highlighted by a dashed blue polygon. Artwork credit A. Cresci
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migration, as glass eels exploit tidal flows to enter brackish and
freshwater using selective tidal stream transport (STST)30–33. This
constitutes a deeply rooted behavior in many species of fish and
invertebrates inhabiting tidal estuaries34.
In order to orient with respect to water flow, fish rely on
multiple sensory cues, including visual, vestibular, and tactile35,36.
Rheotaxis in fish is also influenced by changes in the magnetic
field at intensities as low as the Earth’s37,38, suggesting that
magnetic fields could be one of the reference cues that fish use to
orient in the presence of flowing water. Glass eels exhibit rheo-
taxis in the presence of tidal currents and sense magnetic cues:
our previous work showed that glass eels use the magnetic field
for orientation using a magnetic compass mechanism, switching
magnetic direction according to an endogenous tidal rhythm in
the absence of a current16. Specifically, eels oriented to the
magnetic south with the ebb tide and to the north with the flood
tide16. While these results show that glass eels use a magnetic
field-based compass mechanism, the stimuli that cause glass eels
to swim toward specific magnetic directions at this stage of the
migration are unknown.
We tested the hypothesis that the magnetic orientation direc-
tion of glass eels (i.e., the orientation direction relative to the
direction of the magnetic field) is imprinted from the tidal cur-
rents at the estuaries where they recruit. To test this hypothesis,
222 glass eels were collected at stream estuaries flowing toward
different cardinal directions (north, southeast, south, and north-
west). The magnetic orientation of glass eels was observed in a
magnetic laboratory facility, using an electric coil system that
modified the direction of the magnetic field, while depriving the
glass eels of any other external cues that they could use for
orientation. We show that glass eels orient toward the magnetic
direction of the prevailing tidal current occurring at their estuary.
This is interpreted as evidence that glass eels form and retain a
magnetic memory of tidal flows.
Results
Tests in the magnetic laboratory. The magnetic orientation
behavior of glass eels was observed following the methods
described in our previous work (see the Methods section)16.In
brief, a circular transparent chamber (Drifting In Situ Chamber,
DISC; Supplementary Fig. 1b)39 was submerged in a circular
black tank in a magnetic laboratory (MagLab; https://fishlarvae.
org/facilities/magnetoreception-test-facility/) located in Austevoll,
Norway (60.1175 N, 5.2118 E; Supplementary Fig. 1). The
MagLab is equipped with a triaxial electric coil system that
allowed us to manipulate the magnetic field to which glass eels
were exposed in the experimental tank.
Glass eels were collected at four stream estuaries located
around the Austevoll archipelago (Fig. 2), before they migrated
into freshwater. The tidal estuaries were selected according to
their geographical features, as we searched for estuaries flowing
toward different cardinal directions. Glass eels were collected at
Vasseide (60.1122 N and 5.2298 E, flowing to the north),
Torvesund (60.0294 N, 5.3016 E, flowing to the southeast),
Vinnesvåg (60.0088 N, 5.2583 E, flowing to the south), and
Stolmen (60.0082 N and 5.0788 E, flowing to the northwest)
(Fig. 2). Upstream of these tidal areas (for example, in Torvesund,
where the stream changes direction), there was no action of the
tides and only pigmented elvers were present. The data set used in
this study is composed of the orientation data of 222 glass eels.
These include the re-analysis of the orientation data from our
previous work of 49 glass eels tested in the MagLab in 2015 (eels
coming from Vasseide)16, and 173 glass eels newly collected from
Vinnesvåg, Torvesund, and Stolmen (S, SE, and NW oriented
estuaries; Table 1).
Glass eels were individually exposed to different configurations
of the magnetic field: the magnetic north in the laboratory was
rotated toward one of the four cardinal points of the Earth’s
magnetic field, and each eel was exposed to only one of the four
magnetic conditions used in this study (Supplementary Fig. 2).
The orientation of each glass eel was then assessed with respect to
the rotated magnetic north in the laboratory (Supplementary
Fig. 2).
Each eel was tested in the magnetic lab between the peaks of
high and low tide, during one of the rising/lowering tides (ebb/
flood) occurring along the Austevoll archipelago. The magnetic
orientation direction of each eel was then analyzed with respect to
the magnetic direction of the tidal current occurring contempor-
aneously at the eel’s recruitment estuary. Thus, in our reference
system, we considered the magnetic direction of the tidal flow
with respect to the Earth’s magnetic field) as 0°. Finally, we
computed the angular difference between the magnetic orienta-
tion direction of each eel and the magnetic direction of the
tidal flow.
Magnetic orientation. The overall proportion of glass eels
showing a preferred magnetic orientation direction was 70% (155
out of 222). This proportion, however, changed depending on the
estuary, but it was always >50% (Table 2). Glass eels significantly
oriented to the magnetic direction of the prevailing tidal flow that
was occurring at their recruitment estuaries during the tests.
Their average magnetic orientation direction was 359° (N =155,
Rayleigh’sp=0.0018, r=0.2; Fig. 3), and it matched the mag-
netic direction of the incoming tidal flow (0°, Fig. 3).
Discussion
Glass eels use their magnetic compass to memorize the magnetic
direction of the currents at the estuaries where they recruit. This
is evidence that these fish are capable of forming and retaining a
magnetic memory of the direction of water currents, and to use it
to orient in moving water during migration.
Satellite tracks of loggerhead sea turtles (Caretta caretta) and
fur seals (Callorhinus ursinus) indicate that they detect the
downstream direction of currents in open waters, implying the
use of magnetic cues40,41. It was also hypothesized that elasmo-
branchs, that navigate by detecting electromagnetic fields42, could
sense the direction of oceanic currents through the electricity
produced by the friction of water moving over the sea bottom43.
When visual reference points were present, the interaction
between the magnetic sense and orientation to water flows was
observed in shoaling zebrafish (Danio rerio), which change their
orientation in flowing water according to the direction of the
magnetic field37.
The results described in this study add evidence that eels detect
and form a memory of the magnetic direction of currents, sup-
porting the possibility that these fish—and possibly others—have
the sensory capacity to integrate magnetic and rheotactic infor-
mation and use them for orientation. Analogous results involving
learning of magnetic cues were reported at a later stage in the life
cycle of the eel: silver eels displaced between different locations
learned the compass direction of their displacement24. In that
study, orientation was dependent on temperature, demonstrating
their ability to modulate their response to the magnetic field
according to other environmental signals.
The magnetic memory of the flows at the recruitment estuaries
described in this study might represent a specific case of
imprinting. In its original definition21, imprinting describes a
learning process that occurs in a restricted, sensitive time of the
life of an animal, and that this memory is stable and retained over
time. Considering the glass eels, it is possible that they imprint the
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10 km
0 2E4E6E8E
58N
60N
62N
5E 5.5E
59.9N
60N
60.1N
60.2N
60.0075
60.0080
60.0085
60.0090
5.080
Longitude
Latitude
Stolmen
60.0080
60.0085
60.0090
60.0095
5.078 5.079 5.257 5.258 5.259 5.260
Longitude
Latitude
Vinnesvåg
60.0290
60.0295
60.0300
5.300 5.301 5.302 5.303
Longitude
Latitude
Torvesund
Latitude
Latitude
Longitude
Longitude
NW
N
SE
S
N
N
North Sea
Norway
Austevoll
North Sea
Geographical orientation of the estuary
60.1115
60.1120
60.1125
60.1130
5.229 5.230 5.231
Longitude
Latitude
Vasseide
Flood tide Ebb tide
F
E
F
E
F
E
F
E
02040 m
02040 m
02040 m02040 m
50 km
Fig. 2 Estuaries where glass eels (Anguilla anguilla) were collected and the direction of tidal currents. Maps show Norway (upper left) and the Austevoll
archipelago. Red points show the location of the estuaries. Blue arrows and blue cardinal points show the magnetic direction toward which the estuaries
flow; arrows start from the freshwater side, and point toward the seawater side. The satellite images show the aerial view of each one of the four estuaries.
Sky-blue and yellow arrows show the magnetic direction of the tidal currents at each of the estuaries (E =ebb current, F =flood current)
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rhythm of the tide and the magnetic direction of flows during a
specific, possibly sensitive period, when they first enter the
estuary, which constitutes a life history transition from seawater
to freshwater physiology. Previous studies on salmon smolts show
that imprinting can occur in such transition periods associated
with surges in plasma thyroxine44. Furthermore, otolith analyses
revealed that the time that glass eels spend at the estuaries could
be very long as some never enter freshwater6. Thus, this memory
could be retained and used for a significant portion of the life of a
glass eel, before the elver stage. However, it is also possible that
these results may represent a phenomenon of continuous ongoing
learning, and that this magnetic memory of flows could remain
flexible over time. For example, eels might adjust their magnetic
heading to varying hydrodynamic conditions at the estuary, or
further upstream, where the magnetic direction of the current
could change. These are interesting scenarios that we intend to
address in future work. Furthermore, the estuaries selected in this
study are quite rectilinear systems, but it is possible that highly
curvilinear estuaries are harder for glass eels to navigate and
future work should investigate whether this affects recruitment to
such sites.
In movement ecology, the concept of imprinting has been
mostly associated with the return of animals to natal areas
(homing). Among aquatic species, the most studied case is that of
salmon. As smolts, salmon record the specific combination of
odors from their natal stream, which they will use as adults to
guide their spawning migration upstream22,45. In addition to
olfactory cues, there is evidence that salmon imprint magnetic
information when they make first contact with seawater, which
they use later in life to find the coastal area where their natal
estuary is located23. Similarly, sea turtles imprint the geomagnetic
field associated with their natal beach, which they use years later
to return and nest20. In the case of the eel, the magnetic
imprinting observed in glass eels has a different importance for
their migration. Glass eels learned the magnetic direction of
currents at their recruitment estuary. This estuary will be the
same as that which they encounter years later during the descent
from the stream to start their oceanic spawning migration as
adult silver eels. Whether eels keep a magnetic memory of their
estuary through adulthood, and whether this constitutes a refer-
ence to start the marine phase of the spawning migration in the
right direction, is unknown. For example, it is possible that adult
eels recognize the direction of the alternating tidal flows experi-
enced as glass eels when they descend the estuary, and that this
constitutes a trigger to undertake the last physiological and
behavioral changes at the start of their long marine migration.
However, this hypothesis needs to be tested in future work.
The use of the magnetic sense to imprint the direction of
currents at estuaries could also help glass eels at this earlier phase
of their migration. During their estuarine residency, glass eels
undergo physiological and morphological changes, developing
pigmentation, developing jaws and teeth, and adapting to fresh-
water2. Thus, glass eels need to maximize the energetic efficiency
of their swimming behavior at the estuary, both to keep their
position and to migrate upstream46. In environments such as
streams, rivers, or estuaries, fish orient with respect to the current
by using visual and tactile cues (such as the bottom)35,47. Our
results show that glass eels also use their magnetic sense as a
reference to orient against the direction of the flow. This could
help them in several ways, such as keeping the right compass
course when visual reference points are lost or obscured (turbid
water in rivers or muddy streams), or when there is no physical
contact with the substrate. This would be of great importance
especially for glass eels recruiting to large, long tidal estuaries
such as the Gironde in France, where the effect of the tidal flows
extends for kilometers. Moreover, in such large estuaries the
water is turbid, and temperature and salinity are subjected to high
variability. Thus, the use of the magnetic compass system
described in this study would have obvious advantages in such
environments, providing a fixed reference for orientation.
Table 1 The estuaries where the glass eels (A. anguilla)were
collected are listed
Estuary Orientation of the
estuary
nDates of the tests
Vasseide N4916–22 April 2015
Vinnesvåg S369–16 May 2016
Torvesund SE 24 13–16 May 2016
Stolmen NW 113 28 April–18
June 2017
Orientation of the estuary is the cardinal direction toward which each of the estuaries flows. n
number of glass eels tested in the magnetic laboratory listed by stream of provenience. The
dates of the tests are also indicated
Table 2 Proportion of glass eels (A. anguilla) showing a
significant magnetic orientation direction
Estuary nOrienting
glass eels
Proportion of
orienting eels
Vasseide 49 35 71%
Vinnesvåg 36 20 56%
Torvesund 24 15 62%
Stolmen 113 85 75%
Total 222 155 70%
n: the number of glass eels tested in the magnetic laboratory listed by stream of provenance.
The Table shows the number of glass eels displaying significant orientation and the proportion
of eels that oriented
Tidal
flow
90
Downstream
270
Fig. 3 Magnetic orientation of glass eels (Anguilla anguilla) with respect to
the magnetic direction of the tidal flows. In the circular plot, the outer gray
circle represents the x-axis. The angle between the magnetic orientation of
each glass eel that significantly oriented (Rayleigh’sp< 0.05 on individual
tracks) and the direction of the tidal flow (top of the plot) is shown as a
navy-blue data point (N =155). The bottom of the plot represents the
downstream direction of the flow. Significant (Rayleigh’sp< 0.05) collective
orientation toward the upstream magnetic direction of the tidal flow is
shown as a black arrow pointing toward the top of the plot. The direction of
the arrow point toward the mean orientation direction of the glass eels.
Dashed gray lines show the 95% confidence interval around the mean
direction of the eels. For clarity, the data are displayed binned by 5°
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Although glass eels oriented along the magnetic direction of
tidal currents at their recruitment estuaries, there was unac-
counted for interindividual variability (Rayleigh’sr=0.2). Var-
iation in orientation response is dependent on the internal state
(or behavioral traits) of the animals, which significantly con-
tribute to their decision to move48. The internal state (often
classified as proactive and reactive, or migratory and non-
migratory phenotypes) plays an important role in the magnetic
sense of fish, up to the point that it can make the difference
between responding or not responding to magnetic stimuli38.
European eels display significant differences among individuals
concerning their tendency to migrate. Experiments using flume
tanks showed that glass eels (A. anguilla), elvers, and yellow eels
display different tendencies to migrate and can be separated into
migrants that actively move upstream or downstream and non-
migrant that do not show a particular tendency to migrate46,49.It
is possible that those glass eels not showing any preferred mag-
netic orientation direction have a nonmigrant phenotype. Future
work should repeat these experiments on magnetic imprinting
after dividing the glass eels according to their motivation to
migrate and dividing migrating glass eels into upstream vs.
downstream migrants. This could be accomplished using an
experimental setup similar to that used by Bureau Du Colombier
et al., with a flume tank connected to upstream and downstream
traps46.
Glass eels learn the features of the environments that they
encounter during migration. Future work should investigate
whether migrating silver eels that are translocated at the glass eel
stage have more difficulty in finding their way toward the sea
compared with nontranslocated ones. If they do, then the results
of this study would be relevant to management plans that include
the restocking of glass eels in freshwater where the population is
most depleted46.
Methods
Animals and maintenance. The glass eels were collected in Norway, before their
upstream freshwater migration between March and June of 2015, 2016, and 2017.
They were caught using hand nets searching under rocks and sediment at low tide.
None of the animals used in this study were pigmented (developmental stage: V–VI
2), and they did not have food in the gut. Glass eels were kept in 20 -L maintenance
tanks, where they were re-acclimated to near full salinity seawater (32 ppt) after
capture. They were kept in aerated aquaria in a temperature-controlled room set to
ambient conditions similar to those of the local Langenuen fjord (ranging between
6 and 10 °C). An imals were not fed (they were at the pre-feeding stage) and were
kept in 14 h light and 10 h dark cycle (following the daylength at the study location
during the observation period). Two-thirds of the volume of each aquarium were
replaced with filtered seawater every 48 h to maintain water quality. The seawater
was provided by the filtration system at the Institute of Marine Research’s Aus-
tevoll Research Station, which collects seawater from the Langenuen fjord at a
depth of 160 m. Before the tests, glass eels were taken from the large aquaria and
placed in individual 500 -mL white plastic containers filled with seawater at the
same temperature as the aquaria and transported to the magnetic laboratory
(MagLab) in a cooler to keep the temperature stable. No permits were required by
the Norwegian authorities because no glass eels (Anguilla anguilla) were harmed
while performing the experiments, and after use, they were either returned to the
wild or killed using an approved method. Eels were collected under research catch
permit #11/11448 issued by the Norwegian Fiskeridirektorat.
Experiments and data analysis. The experiments in the laboratory followed the
same protocol as described by Cresci et al.16. The MagLab is designed to study the
magnetic orientation of aquatic animals. It is equipped with a triaxial electric coil
system (Supplementary Fig. 1c), with a design described by Merritt et al.50. The
coils are connected to a power supply (max. 3 A). A black circular fiberglass tank
(diameter, 1.40 m; height, 0.90 m; see Supplementary Fig. 1c) filled with seawater is
located at the center of the coils. The seawater is pumped from the sea (300 m
away). The building (see Supplementary Fig. 1a) is made of nonmagnetic material,
and is distant from any magnetic interference (163 m from the nearest electrical
disturbance and 365 m from the closest building; Supplementary Fig. 1a).
For this study, we used the DISC (“Drifting In Situ Chamber”, Supplementary
Fig. 1b)39 as a behavioral chamber, submerged in the circular dark tank (see
Supplementary Fig. 1c). The chamber of the DISC in which individual glass eels
swam one at a time was 40 -cm wide (diameter) and 15 cm deep. The chamber was
semi open, as the bottom was rigid and made of acrylic, while the walls were made
of transparent fine mesh, which allows water and dissolved gas exchange. The top
of the chamber was covered with opaque white plastic, which diffused light
uniformly in the chamber. Light intensity in the tank was low (0 lum/ft2from
HOBO light sensor on the bottom plate of the DISC frame).
The behavior of glass eels in the DISC was observed using a GOPRO HERO 4
camera placed on the bottom plate of the DISC acrylic frame, underneath the
chamber (Supplementary Fig. 1b, d). The DISC was equipped with an analog
compass attached to the acrylic poles of the DISC frame and placed below the
circular arena. This positioning eliminates the possibility that the compass would
be a visual reference for the eel.
The MagLab has two nested electric coil systems. We used one of them to cancel
out the horizontal component of the ambient field. We used the second system to
create a magnetic field of 48.8 to 50 µT, which is the same total intensity as the
ambient field, and to rotate the magnetic north. We did not change the intensity
and inclination inside the coil compared with the ambient field (48.8 to 50 µT and
73°, with a deviation of <1°).
We recorded each eel for 15 min, with the first 5 min as an acclimation
period16,51. We simulated four magnetic field conditions, with the magnetic north
rotated towards the east, south, west, or north of the geomagnetic field (see
Supplementary Fig. 2 and Supplementary Data 1). We exposed each glass eel to
only one of the four conditions. This technique eliminated any nonmagnetic bias
that could have influenced the orientation response of the animals. The MagLab is
a bespoke facility that was designed to study magnetic orientation in marine
organisms and to eliminate other environmental cues (water flows, odor plumes,
sunlight, or any celestial cues). The walls are constructed of aluminum and wood,
and they are insulated to protect from sound and temperature fluctuation. Every
metal fitting, screw and bolt, is made of nonmagnetic material (aluminum, brass, or
high-quality stainless-steel). The experimental room is completely isolated, and the
top of the facility has a roof (i.e., no celestial cues). The facility is located at the top
of a fjord, hundreds of meters from electromagnetic disturbances. To water flow in
the tank, no pumps, aerator, or ventilation systems are present. The observation
tank is supported on an autonomous concrete block, separating it from the rest of
the building and isolating it from vibrations.
All tests were conducted during daytime. The data about the tide were obtained
from the Norwegian Mapping Authority (www.kartverket.no). The direction of
flow in the estuaries was assessed in situ using an analog compass.
The orientation of glass eels was determined through the analysis of the
GOPRO images, tracking the position of the head of the eel in the circular arena
every second for 10 min. Magnetic north had a different orientation in the
laboratory during each test, and the position of the eels with respect to the
magnetic north in the laboratory was monitored using the analog compasses. The
video frames were processed using the DISCR tracking procedure, utilizing R and a
graphical user interface provided by imageJ software52,53. Using this tracking
procedure, we collected the positional data (in units of magnetic degrees) of the
glass eel with respect to the center of the chamber, which were considered as
bearings. The code utilized is available at the web page Drifting In Situ Chamber
User Software in R (https://github.com/jiho/discr written by Jean-Olivier Irisson
(Université Pierre et Marie Curie UPMC), released under the GNU General Public
License v3.0.
We assessed the mean orientation of each individual from the bearings collected
using the video tracking analysis (Supplementary Fig. 3). The mean of 600 data
points, which represent the bearings of the eel in the chamber at each second (one
position/s over 10 min period), was considered to be the orientation of each
individual51,52. The orientation of each eel was then corrected with respect to the
rotated magnetic North induced with the coils in the laboratory (Supplementary
Fig. 3). As last step, the magnetic orientation was corrected with respect to the
magnetic direction of the prevailing current occurring at the estuary where the eel
was collected from (Supplementary Figs. 3, 4).
Statistics and reproducibility. If the eel displayed a magnetic bearing in the
chamber of the DISC, we considered it as indicative of directionality39,52. Whether
the directionality was significant was assessed with the Rayleigh’s test of uniformity
for circular data52–54. An outcome was considered statistically significant when p<
0.05 (alpha =0.05)52. This analysis was performed on 222 glass eels tested
individually.
After having assessed the orientation of each individual, the following step of
the analysis evaluated whether the eels tested in the DISC show a significant pattern
in their orientation, or whether they go toward a common direction. To assess
collective patterns in orientation direction, we used the same statistical test, the
Rayleigh test of uniformity on all the mean individual bearings of the eels that
displayed orientation (N =155), testing whether the frequency distribution of the
directions displayed by the individuals was significantly different from random
(95% confidence interval, alpha =0.05)52. Possible effects on the results caused by
the larger sample size of the eels collected in Stolmen was assessed statistically
(analysis and results are displayed in Supplementary Fig. 5).
Reporting summary. Further information on research design is available in
the Nature Research Reporting Summary linked to this article.
ARTICLE COMMUNICATIONS BIOLOGY | https://doi.org/10.1038/s42003-019-0619-8
6COMMUNICATIONS BIOLOGY | (20 19) 2:366 | https://doi.org/10.1038/s42003-019-0619-8 | www.nature.com/commsbio
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Data availability
All of the data displayed in Fig. 3 are available in the Supplementary Data 2.
Received: 5 April 2019; Accepted: 17 September 2019;
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Acknowledgements
We thank Josefina M. Olascoaga and Joseph E. Serafy for their help with the data analysis
and for providing insightful critical comments on the manuscript. This research was
supported by the Paris Lab at the Rosenstiel School of Marine and Atmospheric Science
of the University of Miami, and by funds awarded to H.I.B. by the Norwegian Institute of
Marine Research project “Fine-scale interactions in the plankton”(project # 81529) and
the Research Council of Norway (project # 234338). A.C. was supported by the U.S.
National Science Foundation NSF-OCE 1459156 to C.B.P. and by the Norwegian
Institute of Marine Research project # 81529 to H.I.B.
COMMUNICATIONS BIOLOGY | https://doi.org/10.1038/s42003-019-0619-8 ARTICLE
COMMUNICATIONS BIOLOGY | (2019) 2 :366 | https://doi.org/10.1038/s42003-019-0619-8 | www.nature.com/comm sbio 7
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Author contributions
A.C. designed the study; collected, analyzed, and interpreted the data; and wrote the
paper. C.M.F.D. designed the study; collected, analyzed, and interpreted the data; and
wrote the paper. C.B.P. designed the study; analyzed and interpreted the data; wrote the
paper; and funded the research. S.D.S. collected and analyzed the data and helped
drafting the manuscript. A.B.S. designed the study, collected and interpreted the data;
wrote the paper; and funded the research. H.I.B. designed the study; collected and
interpreted the data; wrote the paper; and funded the research.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s42003-
019-0619-8.
Correspondence and requests for materials should be addressed to A.C.
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