Acoustic communication in the Lusitaniantoadfish, Halobatrachus didactylus:evidence for an unusual large vocalrepertoire

Abstract

The Lusitanian toadfish Halobatrachus didactylus (Bloch & Schneider) (Batrachoididae) is a well-known sound producer that has an unusual large acoustic repertoire for fish. This repertoire consists so far of five distinct sound categories: boatwhistles, grunt trains, croaks, double croaks and a mixed grunt–croak call. Sixteen males that spontaneously occupied artificial concrete nests placed in the intertidal zone of the Tagus estuary (Portugal) were recorded over 8 days in June/July 2006. During the analysis of the recordings new sound emissions were found. Long grunt trains that sounded to the human ear like a running engine were heard. These sounds differ from the normal grunt trains by having a lower amplitude, a much longer duration (tens of seconds versus <1 second) and more grunts per call. Other new sound emissions (e.g. triple croaks) were also registered but were heard less frequently. The incidence of the various sound types is given.

Full text

Acoustic communication in the Lusitanian
toadfish, Halobatrachus didactylus:
evidence for an unusual large vocal
repertoire
m.c.p. amorim
1
, j.m. sim ~
oes
1
and p.j. fonseca
2
1
Unidade de Investigac¸a
˜o em Eco-Etologia, I.S.P.A., Rua Jardim do Tabaco 34, 1149-041 Lisboa, Portugal,
2
Departamento de
Biologia Animal e Centro de Biologia Ambiental, Faculdade de Cie
ˆncias da Universidade de Lisboa, Bloco C2 Campo Grande,
1749-016 Lisboa, Portugal
The Lusitanian toadfish Halobatrachus didactylus (Bloch & Schneider) (Batrachoididae) is a well-known sound producer
that has an unusual large acoustic repertoire for fish. This repertoire consists so far of five distinct sound categories: boatwhis-
tles, grunt trains, croaks, double croaks and a mixed grunt–croak call. Sixteen males that spontaneously occupied artificial
concrete nests placed in the intertidal zone of the Tagus estuary (Portugal) were recorded over 8 days in June/July 2006.
During the analysis of the recordings new sound emissions were found. Long grunt trains that sounded to the human ear
like a running engine were heard. These sounds differ from the normal grunt trains by having a lower amplitude, a much
longer duration (tens of seconds versus ,1 second) and more grunts per call. Other new sound emissions (e.g. triple
croaks) were also registered but were heard less frequently. The incidence of the various sound types is given.
Keywords: acoustic communication, Lusitanian toadfish, vocal repertoire
Submitted 12 October 2007; accepted 21 February 2008; first published online 24 June 2008
INTRODUCTION
Vocal fish, like other vertebrates, use acoustic signals during
social behaviour to communicate. Sound characteristics
depend largely on the mechanism of sound production.
Fish evolved the largest diversity of sonic organs among ver-
tebrates. Well-known and common mechanisms of sound
generation used by fish are the vibration of the swimbladder
via the contraction of specialized rapid intrinsic or extrinsic
sonic muscles and stridulationthatresultsfromrubbing
bony elements against each other (Ladich & Fine, 2006).
Other mechanisms include plucking enhanced tendons of
pectoral fins and vibration of pectoral girdles (Ladich &
Fine, 2006). While stridulatory mechanisms generate
broad-band pulsed sounds with frequencies up to a few
kHz, the vibration of the swimbladder results in the emis-
sion of low-frequency (,1 kHz) sounds made up of pulses
that can be repeated at different rates, depending on the
sonic muscles’ contraction rate (Ladich, 2004; Ladich &
Fine, 2006).
Despite the diversity of sound-producing mechanisms, fish
acoustic signals present a much lower variability than found in
the sounds of other taxa and the acoustic repertoire of a fish
species is typically restricted to one or two different types
(Amorim, 2006). One exception is the mormyrid fish
Pollimyrus adspersus (Gu¨ nther, 1866) that is known to
produce five different sounds (Crawford, 1997). Fish can com-
municate by either producing different sound types or chan-
ging the characteristics of one sound type according to
context (Amorim, 2006). Behavioural studies have demon-
strated that temporal features within a sound, including
pulse rate and number, can be important to its communicative
value (Ladich, 2004). For example, differences in the number
and repetition rate of pulses can potentially encode species
(Myrberg et al., 1978; Amorim et al., in press) or individual
(Thorson & Fine, 2002; Amorim & Vasconcelos, 2006) iden-
tity, and the motivational state of the sound-emitter (e.g.
Mann & Lobel, 1998).
Although fish probably represent the largest group of sound-
producing vertebrates, with several thousands of vocal species
(Ladich, 2004), the knowledge of their vocal ability to communi-
cate (e.g. extent of the vocal repertoire; the role of different sound
types) remains largely unexplored in comparison with amphi-
bians, birds and mammals (Ladich, 2004). This study aimed at
describing the acoustic repertoire of the Lusitanian toadfish
Halobatrachus didactylus (Bloch & Schneider) (Batrachoididae).
This species is known to have an unusual large acoustic repertoire
for fish composed of five different sound categories (four different
soundtypesandamixedcall)that are produced by the contrac-
tions of paired intrinsic sonic muscles attached to the swimblad-
der wall (dos Santos et al., 2000; also see Amorim et al., 2006). In
this study, new sound types, several mixed (blends) of sound
types, and the incidence of various sound types are described
demonstrating that this species has the widest acoustic repertoire
knowninfish.
Corresponding author:
M.C.P. Amorim
Email: amorim@ispa.pt
1069
Journal of the Marine Biological Association of the United Kingdom, 2008, 88(5), 1069–1073. #2008 Marine Biological Association of the United Kingdom
doi:10.1017/S0025315408001677 Printed in the United Kingdom

MATERIALS AND METHODS
Study species
Lusitanian toadfish males defend nests in estuarine shallow
waters during the breeding season (May to July). Females
are attracted to the nests by the males’ conspicuous advertise-
ment calls, the boatwhistles, and leave their fertilized eggs
attached to the nest’s ceiling after spawning. After mating
with several females, males continue to defend the nest
alone until the young are free-swimming (dos Santos et al.,
2000; personal observation). Besides the boatwhistle, four
other sound categories were recognized in the Lusitanian
toadfish: grunt trains, croaks, double croaks and a mixed
grunt–croak call (for sound descriptions see dos Santos
et al., 2000; Amorim & Vasconcelos, 2006; Amorim et al.,
2006; Figure 1).
Sound recording and data analysis
Twenty artificial concrete nests 50 cm long, 30 cm wide and
20 cm high (internal dimensions) with a hemicylinder shape
and closed at one end were placed 1.5 m apart in an intertidal
area of the Tagus estuary (Portugal, Montijo, Air-Force Base 6;
388420N, 88580W). These nests were only exposed to air during
spring tides (at very low tides), which only happened twice
during the study period. During the course of this study
water level in the nest area varied between 0 m and 2.8 m.
Two groups of eight males (total length (TL)—mean
(range): 41.6 cm (35.3–47.7 cm); eviscerated weight (We):
1195 g (857 –1612 g)), that spontaneously occupied these arti-
ficial concrete nests, were recorded over a period of eight days
in June/July 2006, during the peak of the reproductive season.
Each male was recorded for an average of 36 hours (11 hours –
56 hours). Mean water temperature was 23.58C (218–288C).
All recorded fish experienced similar water temperature
ranges and variability during recordings of the various
sound types. Nests’ entrances were closed with a plastic net
to prevent males from escaping and to ensure male identity
throughout the study. Plastic nets did not affect acoustic
signals and allowed possible visual interactions. All unoccu-
pied nests were also wrapped in plastic nets to prevent
further occupations during the study. Simultaneous multi-
channel recordings were made to a laptop connected to USB
audio capture devices (Edirol UA25, Roland; 16 bit 6 kHz
acquisition rate per channel) controlled by Adobe Audition
2.0 (Adobe Systems Inc., 2005). One hydrophone (High
Tech 94 SSQ hydrophone, sensitivity 2165 dB re 1 V/mPa,
frequency response within +1 dB from 30 Hz to 6 KHz) was
placed at about 10 cm from the entrance of each occupied
nest. Sounds could be attributed to particular males because
of the high attenuation of the sounds produced by the males
restrained in nearby nests (.27 dB), observed in the simul-
taneous multi-channel recordings of the different males.
Sound analysis was carried out with Adobe Audition 2.0,
and Raven 1.2.1 for Windows (Bioacoustics Research
Program, Cornell Laboratory of Ornithology, Ithaca, NY,
USA). The occurrences of each sound type emitted by each
male were counted. The proportion (%) of each sound type
on total sound production was calculated based on male
calling rate, i.e. number of sounds produced per hour.
During sound analysis new undescribed sound types were
recognized. From these only long grunt trains (LGT) were
characterized and analysed in detail because of their
common occurrence. Long grunt trains were analysed for
the number of grunts per train, call duration (from the start
of the first grunt to the end of the last grunt, s), grunt duration
(measured in three grunts randomly chosen, ms), average
grunt period (measured for all the grunts in a train, ms),
number of pulses per grunt (measured in three grunts
chosen at random), and dominant frequency (measured
from the average power spectrum of at least three consecutive
grunts). When LGT occurred combined with other sounds only
the longer LGT present in the sequence was analysed. A total of
78 LGT were analysed from 12 males. Grunt trains (GT) were
also measured for the same parameters. Comparisons between
Fig. 1. Oscillograms (top) and sonograms (bottom) of boatwhistles (A), grunt trains (B), croaks (C), double croaks (D), and a mixed grunt–croak call made up of
croak element followed by four grunts (E). The sonograms used a 30 Hz filter bandwidth in all sound types.
1070 m.c.p. amorim, j.m. simoes and p.j. fonseca

these two sound types were carried out with Mann – Whitney
non-parametric tests, using mean values for each male. A total
of 36 GT were analysed from 7 males. To compare the ampli-
tude of LGT and GT, the relative peak amplitude of 25 calls of
each type (from 10 males) was measured from power spectra.
Average power spectra were computed with a 2048 points FFT
conditioned by a Hamming window, with a time overlap of
50.0% and a 10 Hz filter bandwidth. Amplitude measurements
were only taken when LGT and GT were produced within 60
seconds. In such cases it was assumed that the recording con-
ditions and the distance from the male to the hydrophone
were similar during the production of both sounds. Amplitude
differences were tested with Wilcoxon non-parametric tests.
The potential relation between LGT production rate and GT
and total sound production rate, and male total length (TL,
cm) and condition factor [CF, (We/TL
3
)1000] was explored
with Spearman rank-correlation tests.
RESULTS
All males in the nests emitted sounds. On average, 345 sounds
(mean +SD (range) ¼344.5 +689.7 (7 –2296)) were regis-
tered per male. As expected boatwhistles were the most
frequent sounds (Figure 2), composing 78% (+22.7 (28.6 –
100.0) %) of the total sound production activity observed
Fig. 2. Mean percentage of the different sound types emitted per hour by 16
nesting males during one week in the peak of the breeding season.
Fig. 3. During the present study, nesting Lusitanian toadfish males emitted sounds that have not previously been described such as triple croaks (A), long grunt
trains (B) and combinations of long grunt trains with other sound types (C). In (C) a long grunt train (thin line) combines with a grunt train (double line) that ends
in a croak (thick line), which blends into a boatwhistle (dashed line), which is then followed by another long grunt train (thin line). Note that in (C), the LGT is
hardly visible in the oscillogram due its much lower amplitude than the other sounds. Sonograms used a 30 Hz filter bandwidth.
acoustic communication in lusitanian toadfish 1071

per hour. Interestingly, the second most frequent sound type
were long grunt trains (8.0 +9.5 (0.0 – 33.3) %), a new
sound type for the Lusitanian toadfish. Less frequent sounds
(Figure 2) were double croaks (3.7 +5.7 (0.0 – 21.4) %), fol-
lowed by grunt trains (2.0 +4.3 (0.0 –14.3) %) and croaks
(0.6 +0.8 (0.0–2.3) %). Approximately 1% of the registered
sounds (0.9 +1.7 (0.0 –5.6) %) did not fit in either of the pre-
vious categories. These were composed mostly by grunt –
croak calls but also by other mixed calls and triple croaks, a
sound type also described for the first time (Figure 3A).
Triple croaks resemble double croaks with a further frequency
modulated element. It is likely that males produced this
variety of sounds influenced by the vocal activity of
neighbours.
Long grunt trains sounded to the human ear like a running
engine (Figure 3B) and were made up of a long sequence of
grunts (70.7 +47.3 (14 –302) grunts, N ¼78), lasting on
average 10.3 seconds (+6.7 (1.2–39.4) seconds). 74% of regis-
tered LGT were combined either with other single sound type
or with combinations of joined sounds such as boatwhistles,
croaks and double croaks (see example in Figure 3C). LGT dif-
fered from the typical GT because of their lower amplitude
(peak amplitude difference: 24.6 +5.2 (16.4 –36.1) dB, N ¼
25; Wilcoxon test: T ¼0.00, P,0.001)), an increased
number of grunts per call (Mann – Whitney test: Z ¼3.55, P
,0.001), and a longer duration (Mann– Whitney test: Z ¼
3.55, P,0.001) (Figure 4). Grunt period did not differ
between sound types (Mann – Whitney test: Z ¼20.76, P.
0.05). The grunts in a LGT were also made up of fewer
pulses than in GT calls (Mann–Whitney test: Z ¼22.79,
P,0.01) but had similar durations and dominant frequencies
(Mann–Whitney test: Z ¼21.52– 20.33, P.0.05)
(Figure 4). LGT production rate was neither correlated with
sound production rate (GT or total sound production)
(Spearman rank-correlation: N ¼16, R ¼0.18 – 0.34, P.0.05)
nor with the male’s physical features (TL and CF) (Spearman
rank-correlation: N ¼16, R ¼20.10–0.03, P.0.05).
DISCUSSION
Toadfish and midshipmen (Batrachoididae) have long been
known for their conspicuous ability to produce sounds and
a great deal of what is known in terms of acoustic communi-
cation and its underlying neurophysiological mechanisms in
fish has been based on studies focusing on batrachoidids
(Bass & McKibben, 2003). The Lusitanian toadfish is
however the species with the most extensive acoustic reper-
toire, contrasting with Opsanus spp. and Porichthys notatus
Girard, 1854 that produce mating tonal sounds (boatwhistles
or hums) and agonistic grunts and growls (Bass & McKibben,
2003).
The present study has shown that during the breeding
season the Lusitanian toadfish commonly produces a new
undescribed sound type (long grunt trains, LGT) besides the
five known sound categories described by dos Santos et al.
(2000). LGT differed from GT calls by their significantly
lower amplitude, higher number of grunts per call and
longer durations. Perhaps the only other fish species that is
comparable in terms of acoustic signal diversity is the
weakly electric fish Pollimyrus adspersus (Mormyridae).
Crawford (1997) has named this species as the strongly acous-
tic fish because of its wide acoustic repertoire and the con-
spicuousness of its calls. Pollimyrus adspersus produces
hoots and pops during agonistic interactions and long con-
spicuous acoustic courtship displays that can last for tens of
seconds and consist of long sequences of grunts and moans
followed by growls (Crawford, 1997).
The long grunt trains produced by nesting toadfish males
were low amplitude long sound sequences that commonly
preceded or followed other sound types. The production of
LGT was not correlated with either the male’s calling rate
(GT and total sound) or its physical features (TL and CF).
However, this ‘rumbling’ activity suggests a general physio-
logical ‘excitement’ from the nesting males, that may
perhaps be associated with the production of more intense
sounds, such as the boatwhistles, that are used to attract
females from long distances. LGT have probably not been
detected before because previous studies on this species were
based on recordings where the hydrophones were placed at
unknown distance from the calling males (dos Santos et al.,
2000; Amorim & Vasconcelos, 2006; Amorim et al., 2006).
In the present study, hydrophones were positioned 10 cm
from the males (just in front of the nests) so even low ampli-
tude sound emissions could be detected and attributed
undoubtedly to a particular male.
The present study also demonstrated the ability of this
species to mix and blend different sound types, or modify
existing sound types (e.g. triple croaks). Other species are
known to exhibit a continuous variation in their vocalizations.
For example, the courting male haddock Melanogrammus
aeglefinus (Linnaeus, 1758) (Gadidae) produces a graded
series of repeated ‘knocks’ that become longer and faster as
the male’s level of arousal increases (Hawkins & Amorim,
2000). The wealth in acoustic signals and in signalling
Fig. 4. Long grunt trains (LGT) were significantly longer (A), were made up of
more grunts (B), but had similar grunt periods (C) than grunt trains (GT).
Grunts in LGT had similar duration (D) and dominant frequency (F), but
had a higher number of pulses (E) than grunts in GT. Medians and quartiles
are depicted. Mann–Whitney tests, ,P,0.001; ,P,0.01.
1072 m.c.p. amorim, j.m. simoes and p.j. fonseca

plasticity show that the Lusitanian toadfish has a richer ability
to communicate acoustically than previously thought that
seems exceptional among fish. Future studies will focus on
testing the function of the different sound types.
ACKNOWLEDGEMENTS
The authors would like to thank the Air Force Base No. 6 of
Montijo (Portugal) for allowing this study in their military
establishment. This study was supported by the project
PDCT/MAR/58071/2004 and by a grant POSI SFRH/BPD/
14570/2003 (MCPA) from FCT, Portugal.
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Correspondence should be addressed to:
M.C.P. Amorim
Unidade de Investigac¸a
˜o em Eco-Etologia
I.S.P.A., Rua Jardim do Tabaco 34
1149-041 Lisboa, Portugal
email: amorim@ispa.pt
acoustic communication in lusitanian toadfish 1073