Assessing acoustic communication active space in the Lusitanian toadfish

Abstract

The active space of a signal is an important concept in acoustic communication as it has implications on the function and evolution of acoustic signals. However, it remains mostly unknown for fish since it has been measured in only a restricted number of species. We combined physiological and sound propagation approaches to estimate the communication range of the Lusitanian toadfish's (Halobatrachus didactylus) advertisement sound, the boatwhistle (BW). We recorded BWs at different distances from vocalizing fish in a natural nesting site at circa 2-3 m depth. We measured the representation of these increasingly attenuated BWs in the auditory pathway through the auditory evoked potentials technique (AEP). These measurements point to a communication range ranging between 6 to 13 m, depending on the spectral characteristics of the BW. A similar communication range (circa 8 m) was derived from comparing sound attenuation at selected frequencies with auditory sensitivity. This is one of the few studies that combines auditory measurements with sound propagation to estimate the active space of acoustic signals in fish. We emphasize the need for studies to consider that active space estimates should take informational masking into account.

Full text

RESEARCH ARTICLE
Assessing acoustic communication active space in the Lusitanian
toadfish
Daniel Alves
1,
*, M. Clara P. Amorim
2
and Paulo J. Fonseca
1
ABSTRACT
The active space of a signal is an important concept in acoustic
communication as it has implications for the function and evolution of
acoustic signals. However, it remains mostly unknown for fish as it
has been measured in only a restricted number of species. We
combined physiological and sound propagation approaches to
estimate the communication range of the Lusitanian toadfish’s
(Halobatrachus didactylus) advertisement sound, the boatwhistle
(BW). We recorded BWs at different distances from vocalizing fish
in a natural nesting site at ca. 2–3 m depth. We measured the
representation of these increasingly attenuated BWs in the auditory
pathway through the auditory evoked potential (AEP) technique.
These measurements point to a communication range of between 6
and 13 m, depending on the spectral characteristics of the BW. A
similar communication range (ca. 8 m) was derived from comparing
sound attenuation at selected frequencies with auditory sensitivity.
This is one of the few studies to combine auditory measurements with
sound propagation to estimate the active space of acoustic signals in
fish. We emphasize the need in future studies for estimates of active
space to take informational masking into account.
KEY WORDS: Information masking, Fish, Communication range,
AEP technique, Auditory evoked potential, Boatwhistle
INTRODUCTION
Acoustic communication is a widespread phenomenon across
vertebrates (Bradbury and Vehrencamp, 1998), as well as other
taxa (e.g. insects; Hedwig, 2014) and it is used in a great variety of
contexts such as advertisement, courtship, spawning, agonistic
interactions, competitive feeding or disturbance (Bradbury and
Vehrencamp, 1998; Hedwig, 2014). To be effective, acoustic
signals produced by the sender must be correctly perceived by the
receiver (Bradbury and Vehrencamp, 1998).
In some behavioural contexts, such as mate attraction, it is
advantageous for the emitter to maximize its communication range,
i.e. the area/volume around an individual where communication
with conspecifics can occur (Clark et al., 2009). However, in close-
range interactions, acoustic signals with decreased active space may
also evolve (e.g. Reichard and Anderson, 2015). Independently
of sound source characteristics (e.g. amplitude level), the
communication range of an acoustic signal will be limited by the
environmental sound propagation properties and ambient noise
conditions that will act as an ‘acoustic filter’(Fine and Lenhardt,
1983). Therefore, the effective communication distance has
important implications for the evolution and function of
acoustically mediated behaviour (Bradbury and Vehrencamp,
1998). This parameter has been studied in a variety of terrestrial
animals (e.g. insects –Kostarakos and Römer, 2010; anurans –
Kuczynski et al., 2010; birds –Brenowitz, 1982; reptiles –Todd,
2007; mammals –de La Torre and Snowdon, 2002) but poorly
addressed in aquatic animals such as fish (e.g. Radford et al., 2015).
In general, underwater acoustic communication has received less
attention than terrestrial acoustic communication, probably due to
technical difficulties. Most studies on the communication range of
aquatic animals have been carried out with cetaceans (e.g. Janik,
2000; Sirovićet al., 2007; Tervo et al., 2012). Although teleost fish
are considered the largest group of vocal vertebrates (Ladich, 2004),
communication range estimation in this group is so far restricted to a
small number of species in shallow water conditions (e.g. Fine and
Lenhardt, 1983; Myrberg et al., 1986; Mann and Lobel, 1997; Lugli
and Fine, 2003; Locascio and Mann, 2011; Ghahramani et al., 2014;
Holt and Johnston, 2015; Radford et al., 2015). Sound propagation
is reduced in shallow waters, where low frequency sounds, such as
most fish vocalizations (Amorim, 2006), are strongly attenuated
with distance (Bass and Clark, 2003; Mann, 2006). Depending on
the species, estimated ranges vary from a few centimetres to tens of
metres (Amorim et al., 2015).
The Lusitanian toadfish Halobatrachus didactylus (Bloch and
Schneider 1801) is a member of the family Batrachoididae that
inhabits coastal waters and estuaries (Roux, 1986). It is a benthic
species with an unusually rich vocal repertoire (Amorim et al., 2008)
that produces sounds in both reproductive and agonistic contexts
(dos Santos et al., 2000; Vasconcelos et al., 2010). During the
breeding season, males aggregate in nesting areas close to the
substrate and produce advertisement calls –the boatwhistle (BW) –
to attract mates (Jordão et al., 2012; Vasconcelos et al., 2012). The
BW is the most commonly produced acoustic signal in this species
throughout the year (Amorim et al., 2006, 2008, 2010).
Halobatrachus didactylus has been used in both behavioural (e.g.
Vasconcelos et al., 2010; Ramos et al., 2012; Conti et al., 2015) and
physiological (e.g. Vasconcelos and Ladich, 2008; Vasconcelos
et al., 2011a,b) studies, making it an excellent model species for the
assessment of active space of acoustic signals in fish. Here, we aimed
to estimate the communication range in the Lusitanian toadfish using
complementary physiological and sound propagation approaches.
MATERIALS AND METHODS
Auditory evoked potential (AEP) technique
Test subjects
Lusitanian toadfish were collected in the Tagus estuary (Portugal)
from trawling by local fishermen during the months of December
2013 to February 2014. After collection, fish were transported to the
laboratory at the University of Lisbon (Portugal), where they were
kept in aerated 80 l stock tanks equipped with protein skimmers,
Received 18 November 2015; Accepted 4 February 2016
1
Departamento de Biologia Animal and cE3c - Centre for Ecology, Evolution
and Environmental Changes, Faculdade de Ciências, Universidade de Lisboa,
Lisbon 1749-016, Portugal.
2
MARE –Marine and Environmental Sciences Centre,
ISPA-Instituto Universitário, Lisbon 1149-041, Portugal.
*Author for correspondence (dbalves@fc.ul.pt)
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Journal of Experimental Biology

under a 12 h:12 h light:d ark cycle, and fed with shrimp once a week.
Water temperature ranged between 15 and 17°C, falling within
natural values. We used a total of 13 adult fish (8 males and 5
females), with an average standard length of 26.8 cm (range 16.3–
37.9 cm), and an average body mass of 590 g (range 110–1450 g).
All experiments were performed in accordance with local
regulations.
Measuring AEPs in response to conspecific sounds
Fish were anaesthetized in a 0.01% ethyl p-aminobenzoate (Alfa
Aesar, Karlsruhe, Germany) saltwater bath and then immobilized
by an intramuscular injection of gallamine triethiodide (10–
15 µg kg
−1
; Sigma-Aldrich, Saint-Louis, MO, USA). Test
subjects were positioned just below the water surface, except for
the uppermost part of the head where the recording electrode was
placed, in the middle of a round plastic experimental tank (diameter
36 cm, water depth ca. 18 cm), with the otolithic endorgans kept at
about 7 cm above the vibrating disc of the sound-generating device.
The tank was placed on a vibration isolation table inside a Faraday
cage. All recording and sound-generating equipment were kept
outside the Faraday cage. Fish gills were perfused with saltwater
through a T-split tube positioned in the mouth using a simple
temperature-controlled (21±1°C) gravity-fed water system.
Acoustic stimuli (BWs), generated by a PC, were fed via the D/A
output of an Edirol UA-25EX (Roland Corporation, Tokyo, Japan;
48 kHz, 16 bit), amplified (custom-built amplifier) and delivered,
with alternated phase, through a custom-made underwater sound-
generating device. This device, centred within the tank, was
composed of an immersed Plexiglas disc (8 cm diameter, 8 mm
thick) attached to a rod, which was driven by a damped mechanical
wave driver (SF9324, PASCO, Roseville, CA, USA) kept below the
experimental tank. Because the mechanical wave driver is
uncoupled from the experimental tank, it does not transmit
extraneous mechanical noise to it. The rod crossed the tank
bottom through a water-restraining flexible device, which not only
prevented water drainage but also kept the rod vertically aligned.
Before each experiment, auditory stimuli were calibrated using a
precision hydrophone (8104 Bruël & Kjaer, Naerum, Denmark;
sensitivity −205 dB re. 1 V µPa
−1
) with its acoustic centre located
7 cm above the disc, the same position occupied by the fish’s
hearing organs during the recordings. The hydrophone was
connected to a sound level meter (Mediator, 2238, Bruël & Kjaer)
and the acoustic signal was digitized (Edirol UA-25EX, Roland
Corporation; 48 kHz, 16 bit) and then monitored by a laptop
running Adobe Audition 3.0 (Adobe Systems Inc., CA, USA). This
allowed us to verify, and if needed to adjust, the amplitudes of the
auditory stimuli.
The recording electrode was pressed against the skin of the fish’s
head directly above the hindbrain, while the ground electrode was
placed between the eyes. The AEPs were amplified (Grass CP511,
Grass Instruments, USA; gain 20,000×, high pass 10 Hz, low pass
1000 Hz), digitized with the A/D input of the same Edirol device
(48 kHz sampling frequency) and recorded with the same PC
running Adobe Audition 3.0. The auditory stimuli and the AEP
recordings were continuously monitored with an oscilloscope.
Sound stimuli
We made synchronous recordings of BWs produced by breeding
territorial males in the natural nesting habitat (Air Force Base 6,
Montijo, Portugal; 38°42′N, 8°58′W). BWs were registered at
different distances from the sound-producing fish (0.1, 2.5, 5, 7.5,
10, 12.5 and 15 m; hydrophones kept at 0.1 m above the substrate).
These recordings were made with equalized hydrophones (High
Tech 94 SSQ, High Tech Inc., Gulfport, MS, USA; frequency
response: 30 Hz to 6 kHz ±1 dB; voltage sensitivity: −165 dB re.
1VμPa
−1
) connected to a multichannel recording device (M-Audio
Fast Track Ultra 8R, M-Audio, Irwindale, CA, USA; 8 kHz, 16 bit)
controlled by a laptop computer running Adobe Audition 3.0. All
BWs were recorded with water levels ranging from 2.2 to 2.6 m. The
recording obtained at 0.1 m was adjusted to a playback amplitude of
140 dB sound pressure level (SPL; re, 1 µPa), corresponding to the
amplitude of BWs produced by a vocalizing nesting male
(Vasconcelos and Ladich, 2008). The sounds recorded at the
other distances were modified by the same factor in order to
preserve the amplitude gradient measured in the nesting habitat with
distance. These BWs were then played back to the subject fish and
the AEP responses recorded. Each stimulus was presented 500 times
at opposite polarities, with intervals between presentations equal to
50% of the stimulus duration.
To represent the variability of the Lusitanian toadfish BW
(Amorim and Vasconcelos, 2008), four different BWs were used as
stimuli (Fig. 1), differing in duration (418–1006 ms) and dominant
frequency [ca. 50–150 Hz; fast Fourier transform (FFT) 8192
points, Hamming window]. Each experimental subject was tested
with two different BWs.
Amplitude modulation representation
We used the protocol described in Vasconcelos et al. (2011a) to
estimate the maximum distance at which each fish correctly
represented the features of a BW. In brief, we compared the
envelope (amplitude modulation) of the BW recorded at 0.1 m with
the envelope of the AEP responses to the same BW recorded at
various distances (both positive and negative envelopes). Stimuli
and response envelopes were extracted with a moving average
(7 ms) of maximum amplitude values of a waveform. The variations
of envelopes of stimulus and AEP response were compared using
Pearson’s correlation. Response thresholds were estimated with
Pearson’s correlations between the envelopes of the stimulus BW
and the AEP response to silence in all the experimental trials
(hereafter called the threshold criteria). The threshold was defined as
the average plus twice the standard deviation of the values of these
correlation coefficients. The longest distance at which the Pearson’s
correlation coefficient between the envelopes of BW and respective
averaged AEP was above threshold was considered the maximum
distance at which the BW was correctly perceived.
Signal averaging was performed with custom-written software
(P.J.F. and M. Vieira). This software is available from the authors.
The Pearson’s correlation analysis was performed with Statistica
12.0 (StatSoft, Inc., USA).
Sound propagation
To measure sound attenuation in the natural breeding habitat, we
compared recordings of BWs of nesting fish registered at 0.1 m with
simultaneous recordings of the same BWs obtained at different
distances (see above).
We further compared played back sounds recorded with a
hydrophone (High Tech 94 SSQ) positioned at doubling distances
(0.5, 1, 2, 4, 8, 16 m) from an underwater speaker (Clark AQ339,
Lubell Labs, Whitehall, OH, USA), using as a reference the recording
of a similar hydrophone kept at 0.1 m from the speaker. These
measurements were conducted at Air Force Base 6 (Montijo,
Portugal) with a 2- and 5-m-high water column. The sea had very
small waves on an almost windless day. The sounds, fed to the
loudspeaker through an amplifier (Blaupunkt GTA 260, Hildesheim,
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Germany), were produced with a multifunction I/O USB device (NI
USB –6521, National Instruments, Austin, TX, USA) controlled by
software custom-written (P.J.F.) in LabView (v8.2, National
Instruments), and consisted of frequency sweeps (0.01 to 1000 Hz)
with a 500 ms duration, repeated 25 times with 100 ms pauses. The
output of both the reference and testing hydrophones was
simultaneously recorded with the same multifunction board.
Comparisons of the reference sound with attenuated sounds were
made by computing with the same LabView software the averaged
transfer function spectra and the coherence function of the distant
hydrophone relative to the reference hydrophone. The sound
sampling was delayed relative to the recording of the hydrophone
close to the speaker to take into account the sound propagation delay
from the speaker to the distant hydrophone. The hydrophone
recordings were calibrated by comparison with a Bruël & Kjaer type
8104 hydrophone conditioned by a Sound Level Meter (Mediator
2238) with a linear frequency weighting and set on fast measuring
mode. The coherence function is a normalized measure of the
degree of causality (or degree of relationship) between two signals
(here, the voltage signals measured by both hydrophones) and
varies between 0 and 1. High values indicate that the recorded sound
is highly related to the stimulus sound. We only considered data
with coherence values above 0.9. Hence, we selected 120, 240 and
540 Hz as these are multiples of 60 Hz, a common fundamental
frequency in this species (Vasconcelos et al., 2010). Notice that we
excluded 120 Hz at 2 m depth and 16 m distance, and 540 Hz at 5 m
depth and 16 m distance because they did not meet the coherence
value criterion. Moreover, we could not measure the attenuation
around the fundamental frequency because the loudspeaker was not
able to transmit such frequencies with enough power.
RESULTS
BW propagation loss
The BWs (BW1–4) recorded in shallow waters of the natural
toadfish breeding habitat suffered strong attenuation with distance
(Figs 2 and 3A). When compared with the measurements of the
reference hydrophone (0.1 m), the attenuation averaged 17 dB in the
first 2.5 m. An additional 7 dB attenuation was found 5 m away
from the vocal fish, and the progressive loss of sound energy was to
about −35 dB at a distance of 15 m. From 5 m onward, the
attenuation was almost constant, with an average of 2.1–2.7 dB per
2.5 m. As expected, the attenuation varied throughout the frequency
range of the BW, with lower frequencies exhibiting a stronger
attenuation with increasing distance (Fig. 3B,C). As different fish
produce BWs with different spectral energy distributions (BW1–4;
Fig. 1), they suffered different attenuations. BW1 and BW2 were
more severely attenuated than BW3 and BW4 (cf. Fig. 2), as the
100 ms
0 100 200 300 400
Frequency (Hz)
–10
–30
–50
–10
–30
–50
–10
–30
–50
–10
–30
–50
Relative amplitude (dB)
BW1
BW2
BW3
BW4
Fig. 1. Oscillograms and power spectra for the four boatwhistles (BWs) that were used as stimuli. The boatwhistles were recorded in the field 0.1 m
away from the vocal male. Power spectra [fast Fourier transform (FFT), 8192 points, Hamming window] are shown as relative amplitude. The dashed line
highlights the typical fundamental frequency of the BWs.
Relative amplitude (dB)
0
–5
–10
–15
–20
–25
–30
–35
–40
–45
0246810121416
Distance (m)
BW1
BW2
BW3
BW4
Fig. 2. Relative amplitude of each of the four BWs used in this study as a
function of the distance from the sound-producing fish in the natural
habitat. Amplitudes (total root mean square power) are shown relative to the
intensity registered at 0.1 m from the vocal male. The line indicates the mean.
See Materials and methods for recording conditions.
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former BWs presented stronger lower frequency components than
the latter (note differences in the fundamental frequency around
50 Hz; Fig. 1), which attenuate faster in shallow waters.
Single frequency propagation
We measured the attenuation close to the substrate of selected
frequencies corresponding to the first and third harmonics of the
BWs in the same toadfish breeding habitat and at different water-
column heights. The use of single frequencies allows the reduction of
confounding effects caused by non-linearities of the loudspeakers,
and also the detection of possible anomalies in the sound
propagation caused, for instance, by acoustic interactions or filter
properties of the environment (e.g. effect of substrate and surface).
Fig. 4 depicts the propagation loss for sound at three different
frequencies (120, 240 and 540 Hz). At 2 m depth, these
measurements exhibit stronger attenuation than BWs produced by
a vocalizing fish (cf. Fig. 2). For example, while BWs attenuated
15–19 dB and 30–38 dB at 2 and 15 m from the source,
respectively, these frequencies decreased 15–29 dB and 37–
44 dB, at 2 and 16 m, respectively. Propagation at 120 Hz was
strongly affected by depth; attenuation was greater at 2 m than at 5 m
depth for 120 Hz at all tested distances (e.g. 13 dB versus 29 dB at
1 m distance; 26 dB versus 38 dB at 4 m distance; Fig. 4A). While
the 540 Hz tone seemed to follow this trend (although the pattern
was less obvious than with 120 Hz), this seemed to be reversed for
the 240 Hz tone up to 10–12 m from the source (Fig. 4B). At the
distance corresponding to the average communication range (8 m),
the attenuation varied between 30 and 45 dB. In addition to these
selected frequencies, we investigated the propagation loss of the
fundamental and the third harmonic of the BWs used in our study
(Fig. 3C). Attenuation for 60 and 180 Hz followed a similar pattern
to that for 120 and 240 Hz and at ca. 8 m (7.5 m) the average
attenuation was comparable, varying between 27 and 37 dB.
In general, propagation loss fell between the ranges predicted by
spherical and cylindrical propagation models. The only exception
was the 120 Hz tone with the lower 2 m water column, which
attenuated stronger than the predictions of the spherical propagation
model (Fig. 4A).
AEP response to natural sounds
To estimate the distance at which the receiver’s auditory pathway
can represent a BW, we compared the envelopes of the AEP
responses with the envelope of the stimulus BW recorded at 0.1 m
from the vocal fish, using both the upper and lower AEP envelopes
for the Pearson’s correlation. Fig. 5 shows an example of AEP
responses to a BW (BW2) recorded at different distances from the
source and the corresponding averaged AEP. Additionally, Fig. 5A
depicts the positive amplitude envelopes for the more intense sound
and its corresponding AEP.
Amplitude fluctuations of BWs recorded at 0.1 m from the fish
were well represented in the AEP response (Fig. 5A). The responses
to the 5 m-attenuated stimuli (Fig. 5B) showed lower amplitude as
expected (compare the amplitude scales in Fig. 5). Nevertheless,
amplitude fluctuations were still partly preserved. The AEP
response to the same BW recorded at 10 m (Fig. 5C) no longer
represented the envelope of the original sound.
AB
C
0.1 m
5 m
10 m
100 ms
0.1 m
5 m
10 m
60 Hz
180 Hz
0 2 4 6 8 10121416
Distance (m)
0 100 200 300 400 500
Frequency (Hz)
–15
–35
–55
–75
10
–10
–30
–50
Relative amplitude (dB)
Fig. 3. Representation of attenuation of BWs in the natural environment. (A) Oscillograms of BW2 recorded at different distances from the vocal fish.
(B) Power spectra (FFT, 8192 points, Hamming window) obtained for BW2 at the same distances as in A. (C) Average propagation loss at 60 and 180 Hz of the
BWs used in our study for all recorded distances. Values are relative to the intensity registered at 0.1 m from the vocal male (error bars represent s.d.).
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Table 1 shows an estimation based on threshold criteria of the
maximum distance at which the amplitude fluctuations of each BW
were still represented in the AEPs and thus probably perceived by
each fish. The mean maximum distance varied between 6.4 m
(BW2) and 13.2 m (BW4). Table 1 also shows differences in
putative detection distances for the four BWs.
DISCUSSION
In this study, we estimated the communication range of the
Lusitanian toadfish advertisement BW, the most commonly
produced sound in this species (Amorim et al., 2006, 2008). We
used two different methods to provide an estimate for the
communication range of the BW: a sound-propagation experiment
in the natural environment and a physiological evaluation of hearing
ability for BWs recorded in the field.
BW propagation in the natural environment
BWs suffered strong attenuation with distance in the natural toadfish
breeding habitat as expected for shallow water. Low-frequency
sounds with wavelengths longer than the height of the water column
are strongly attenuated in natural environments (Bass and Clark,
2003; Mann, 2006). Because BWs have fundamental frequencies of
ca. 50–60 Hz (sound wavelength λ≈25–30 m), water depth at
our breeding study site (<5 m) will impact sound propagation.
Indeed, frequency components corresponding to a wavelength
approximately four times longer than the water depth are below the
cut-off frequency dictated by the high-pass filter properties of the
medium and their propagation will be strongly hampered (Mann,
2006).
In addition, the radiation pattern of the sound source should also
influence sound attenuation. For example, the sound-producing
swimbladder in the oyster toadfish, Opsanus tau, was described as a
complex mixed-sound radiator with monopole, dipole and
quadrupole components (Fine et al., 2001). The acoustic near-
field of such a source (usually up to λ/2πmetres) can be quite
complex with acceleration, velocity, net fluid displacement and
sound pressure decreasing faster than expected for a geometric
spreading model (6 dB per doubling distance, which is the
attenuation in the far-field; Bass and Clark, 2003). Although the
swimbladder of the Lusitanian toadfish presents a different
morphology, a complex radiation pattern should also be expected.
This may explain why the attenuation of the BWs in the natural
environment was very high close to the sound-producing fish, with a
steep slope in the first few metres. At greater distances, the
attenuation was lower and became more uniform (Fig. 2).
Amorim and Vasconcelos (2008) reported an attenuation of
21 dB from 0.5 to 4 m for BWs emitted from a speaker. Although
the depth was not mentioned, these values are in accordance
with our findings. Other Batrachoidids present similar attenuation
sounds in the natural environment. For example, in the plainfin
midshipman, Porichthys notatus, the advertisement hum attenuated
0
–10
–20
–30
–40
–50
0
–10
–20
–30
–40
–50 024681012141618
024681012141618
Distance (m)
120 Hz, 5 m
120 Hz, 2 m
Spherical
Cylindrical
240 Hz, 5 m
240 Hz, 2 m
540 Hz, 5 m
540 Hz, 2 m
A
B
Relative amplitude (dB)
Fig. 4. Propagation loss of selected frequencies at a breeding site of
Lusitanian toadfish (Tagus estuary) at different water-column heights
(2 and 5 m). Frequencies: 120 Hz (A); 240 and 540 Hz (B). Amplitudes are
shown relative to the intensity registered 0.1 m away from the loudspeaker.
A
B
C
100 ms
10 µv
0.5 µv
0.5 µv
Fig. 5. Example oscillograms of stimuli and respective auditory evoked
potential responses, as well as the amplitude modulations (envelopes)
obtained in each case for a fish responding to BW2. Stimuli (upper traces)
recorded at 0.1 m (A), 5 m (B) and 10 m (C) together with the respective
auditory evoked potentials (AEPs; lower traces). To estimate the distance at
which the receiver’s auditory pathway can represent a BW, we compared the
envelopes of the AEP responses with the envelope of the stimulus BW
recorded at 0.1 m from the vocal fish, using both the upper and lowerenvelopes
for the Pearson’s correlation. Although only the upper trace is depicted, both
the upper and lower envelopes were used. Note that the scale bars to the right
of the oscillograms represent the same amplitude because the amplitude of the
stimulus in A is much larger than that in the other panels.
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close to 20 dB from 0.5 to 3 m (5 m depth) from a vocal male (Bass
and Clark, 2003). The advertisement BW in O. tau was reduced by
17 dB between 1 and 5 m distance (0.75–1.2 m depth) from the
male (Fine and Lenhardt, 1983), while in the Gulf toadfish,
Opsanus beta, it was attenuated by approximately 22 dB between
0.1 and 2.5 m distance (∼1 m depth) (Remage-Healey and Bass,
2005). For fish of other families, the attenuation described has been
lower (e.g. 7–9 dB attenuation from 1 to 3 m from the vocal male, 1–
4 m depth, in the Hawaiian sergeant damselfish, Abudefduf
abdominalis; Maruska et al., 2007), and the model for sound
degradation in the black drum, Pogonias cromis (7 m depth),
predicts much lower attenuations (Locascio and Mann, 2011).
Single frequency propagation
Water-column height had an important effect in the propagation of
the selected frequencies as there were significant differences in
sound attenuation at distances above 0.5 m between 2 and 5 m water
depth (Fig. 2).
At 2 m depth, the 120 Hz tone (a frequency strongly represented
in many BWs; see Fig. 1) had an amplitude loss around 45 dB 8 m
away from the speaker (see Fig. 4). Considering a level of 140 dB re.
1 µPa at 0.1 m (Vasconcelos and Ladich, 2008), a loss of 45 dB
would mean that the 120 Hz frequency is close to the hearing
threshold for the BW of ca. 98 dB measured with the AEP technique
(Vasconcelos et al., 2011a). Note that the attenuation for the
60 Hz component of the BW had a similar value (ca. 40 dB at a
distance of 7.5 m; see Fig. 3C). Therefore, a receiver would have
very little energy available to listen to a conspecific at around 8 m
distance.
The remaining studied frequencies, while still within the
hearing range of this species (Vasconcelos et al., 2011b), have a
smaller contribution for the overall BW energy (Fig. 1). These
frequencies were also attenuated at 8 m from the speaker to
amplitude values that were close to the 98 dB hearing threshold
(Vasconcelos et al., 2011a). Thus, 8 m seems to be a reasonable
approximation for the maximum communication range estimated
with this method.
The propagation of almost all the selected frequencies fell
between a spherical and a cylindrical propagation model (Fig. 4; for
a review of these models, see Bass and Clark, 2003; Mann, 2006).
However, the attenuation of the 120 Hz tone with a 2 m water
column was even greater than for the spherical propagation model.
Using eqn 8 in Mann (2006) with the velocity for sound propagation
in a sandy substrate (1600 m s
−1
), we obtain a cut-off frequency of
539 Hz for a 2 m water column and a cut-off frequency of 216 Hz
for a 5 m column. As 120 Hz is below these cut-off frequencies, as
well as most BW energy, this probably contributes to further limit its
propagation. This intense attenuation of low-frequency sounds in
the breeding environment should restrain the communication range
in this habitat. Moreover, propagation of these frequencies in such
shallow waters is a very complex phenomenon where reflection and
refraction play an important role (Bass and Clark, 2003; Mann,
2006). However, these factors are hard to model because they
generate very complex sound fields and the properties of the
substrate have also to be taken into account. All these factors may
explain why the models do not accurately account for our
propagation data. These factors could also be a possible
explanation for the fact that the 240 Hz tone suffered less
attenuation with a 2 m than with a 5 m water column (Fig. 4).
In summary, the water-column height seems to play a very
important role in shaping the communication range in this species,
as expected. Although we did not register or test the propagation of
BWs at 5 m or deeper waters, we expect that at these depths the
attenuation of low frequencies should be less intense (Fig. 4) and,
because the BWs have more energy at low frequencies (Fig. 1), the
structure of the BWs should be maintained at larger distances.
Congruently, tide level has been shown to influence the number of
BWs produced by nesting males and the characteristics of these
BWs. Males calling from nests in the upper infralitoral zone
exhibited a marked circatidal rhythm, with a higher BW production
in the high tide (Amorim et al., 2011). These changes in vocal
behaviour probably reflect a strategy to minimize vocal activity
costs, i.e. the relationship between the vocal effort and
communication range.
AEP response to natural sounds
Our results indicate that the BWs can be detected up to a distance of
6–13 m (Table 1). This difference seems to depend mostly on the
spectral content of the sound and not on redundancy conferred by
longer duration. BW2 and BW3 have similar durations but differ by
about 4 m in maximum detection distance, while BW1 and BW2
present similar maximum detection distances but have a 300 ms
duration difference. However, spectral characteristics affect sound
attenuation (see above) and hence its detection by fish. BWs with
more energy in the lower frequency components (BW1 and BW2)
propagated less and presented shorter detection distances as
estimated by AEPs (7 and 6 m) than BWs (BW3 and BW4) with
more energy at higher frequencies (10 and 13 m). The expected
communication ranges for the different BWs estimated with AEPs
are in agreement with the estimations using sound propagation
methods. These distances are slightly longer than the ones reported
for other Batrachoidids such as P. notatus (Ibara et al., 1983) and
O. tau (Fine and Lenhardt, 1983). However, both of these studies
were conducted with lower water-column heights. Communication
range estimations in fish of other families have varied from tens of
centimetres in the Padanian goby, Padogobius bonelli (Lugli and
Fine, 2003), to tens of metres in Pogonias cromis (Locascio and
Mann, 2011). Direct comparisons should be made with caution as
Table 1. Estimated maximum distance at which the fish is able to correctly encode the amplitude modulations of the stimulus boatwhistle
BW1 BW2 BW3 BW4
Fish Detection distance (m) Fish Detection distance (m) Fish Detection distance (m) Fish Detection distance (m)
2 7.5 2 10 1 12.5 1 15
6 10 3 7.5 4 7.5 3 7.5
7 5 5 5 6 10 4 12.5
85 85 910 515
10 10 9 7.5 10 12.5 7 15
11 5 11 5 12 10 12 15
13 5 13 12.5
Mean 7.1 Mean 6.4 Mean 10.4 Mean 13.2
Fish are grouped by the different boatwhistles (BWs) used as a stimulus (see Materials and methods).
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Journal of Experimental Biology

differences in the environmental conditions of the experiments,
such as water-column height or distance from the emitter fish to the
substrate, will influence these estimations.
The present study points to the Lusitanian toadfish
communication range being more limited by audition than
affected by the background noise in the studied habitat. Most
experimental fish stopped perceiving the BWs correctly at distances
where the amplitudes registered were well above the hearing
threshold described for this species. Using the attenuation values in
Fig. 2, at 10 m from the sound-producing fish, the BW would have
an amplitude close to 110 dB, considering a source level of 140 dB
re. 1 µPa at 10 cm (Vasconcelos and Ladich, 2008). This value is
over 10 dB above the described threshold for this species (ca.
98 dB) (Vasconcelos et al., 2011a). This contrasts with what
Locascio and Mann (2011) have reported for P. cromis. In their
work, they compared the propagation of calls in the natural
environment with the hearing capabilities of one fish. They
concluded that the limiting factor for communication is
environmental noise because environmental noise is higher than
the hearing threshold. Most other studies that have estimated the
acoustic communication range of fish have either determined
the maximum distance for signals to become indistinguishable from
the background noise (Fine and Lenhardt, 1983; Mann and Lobel,
1997; Lugli and Fine, 2003) or assumed that the limiting factor for
the communication range is environmental noise (Radford et al.,
2015). Notably, thresholds in our study are not based on the
presence of a biological response in the auditory pathway but on a
correct representation of the acoustic signal envelope. Therefore, we
are testing informational masking of acoustic signals and not
energetic masking (Clark et al., 2009). Signal degradation should
make signals lose their informational content before its energy is
undetectable from background noise. Therefore, animals may have
trouble retrieving information from acoustic signals in the
environment at the communication ranges estimated through
energetic masking that characterize many studies.
Demonstration of the communication distance at which fish are
capable of extracting relevant information from conspecific sounds is
probably only possible with behavioural experiments. To the best of
our knowledge, there are no studies that measure the communication
range in the field based on information criteria. For instance,
Myrberg et al. (1986) report that females are attracted to playbacks of
male sounds up to 10 m away from the speaker and that they show
preference for the sound of a larger male. However, because no other
sounds were tested, it remains to be clarified whether the phonotaxis
range described was due to information extraction from the sound or
simply triggered by cues such as frequency or energy.
In summary, we show that the communication range of the
Lusitanian toadfish advertisement BW varies on average between 6
and 13 m, an estimation that is corroborated by comparing the
attenuation of selected frequencies measured in the natural habitat
with the hearing sensitivity of the fish. We highlight the importance
of estimating communication active space by taking into account the
perception of the information content in acoustic signals and not just
energy detection.
Acknowledgements
We thank Air Force Base No. 6of Montijo (Portugal) for allowing us to conduct sound
recordings of toadfish adults in their military establishment. We would also like to
thank Andreia Ramos for her help with the fieldwork and Manuel Vieira for help with
the averaging software.
Competing interests
The authors declare no competing or financial interests.
Author contributions
D.A. conceived the experimental design, carried out the AEP experiments and
analysed their results, and drafted the article. M.C.P.A. helped design the
experiments, supervised the experimental work and critically reviewed the article.
P.J.F. conceived the experimental design, supervised the experimental work,
carried out the sound propagation experiments and analysed their results, and
critically reviewed the manuscript.
Funding
This study was funded by the Science and Technology Foundation, Portugal [ grant
SFRH/BD/48015/2008 to D.A. and pluriannual programme UI&D 331/94 and UI&D
329, and the strategic projects UID/MAR/04292/2013 and UID/BIA/00329/2013
granted to MARE (M.C.P.A.) and to cE3c (P.J.F.)].
Data availability
Custom-written software (P.J.F. and M. Vieira) for signal averaging is available on
request from the authors.
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