Mechanisms underlying the production of carapace vibrations and associated waterborne sounds in the American lobster, Homarus americanus
University of New Hampshire, Zoology Department, Durham, NH 03824, USA
* Author for correspondence (e-mail: win{at}unh.edu)
Accepted 28 June 2005
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Summary |
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Key words: sound, acoustics, American lobster, Homarus americanus, crustacean, antenna, vibration
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Introduction |
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Sound production in the American lobster (Homarus americanus Milne
Edwards 1837) may be unique amongst crustaceans because it seems to be
produced by contraction of internal musculature. Fish
(1966) first hypothesized this
mechanism when he observed differences in the spectral qualities of American
lobster sounds versus sounds from stridulatory sound producers. While
sound production by stridulation (such as in spiny lobsters and many insects)
typically consists of wide band sounds with no harmonic component, Fish
observed low-frequency (100130 Hz in six animals), tonal,
short-duration sound pulses in American lobsters. These sound pulses are, in
fact, most similar to sounds produced by some insects, such as cicadas, and
some teleost fish species, including toadfish, searobins, sculpins, black
angelfish and plainfin midshipmen (Barber
and Mowbray, 1956
; Moulton,
1958
; Tavolga,
1964
; Winn, 1964
;
Fish and Mowbray, 1970
;
Rome et al., 1996
;
Vance et al., 2002
;
Ladich and Bass, 2003
;
Connaughton, 2004
). These fish
typically produce sound by contracting muscles that vibrate the swim bladder
or pectoral girdle.
Fish (1966) further
suggested that the remotor muscle, located at the base of the second antenna,
is the muscle responsible for the low-frequency, waterborne `buzzing' sounds
he recorded from the American lobster. The remotor muscle is a large muscle,
divided into two distinct bundles, often termed the large and small bundles
(Mendelson, 1969
;
Bevengut et al., 1993
), which
originate at the coxopodite of the second antenna and insert into the carapace
of the cephalothorax (Mendelson,
1969
; Rosenbluth,
1969
; Bevengut et al.,
1993
). Besides being a putative sonic muscle, the remotor muscle
serves to articulate the basipodite and coxopodite at the base of the antenna,
causing antennal movement.
The two fiber bundles found within the remotor muscle have distinct
compositions and are likely to serve distinct functions. The small bundle is
composed of myofibrils with long sarcomeres (>6 µm), while the large
bundle has myofibrils with short sarcomeres (24 µm)
(Bevengut et al., 1993). The
length of the sarcomere is directly related to the speed of fiber
contractions, with long sarcomere muscles contracting slower than those with
short sarcomeres. Therefore, the large bundle of the remotor muscle can
contract faster than the small bundle and has been termed the fast division
(Mendelson, 1969
). The fast
division of the remotor muscle also has a prominent array of T-tubules and
voluminous sarcoplasmic reticulum (up to 60% by volume versus 15% in
the small division; Bevengut et al.,
1993
), which are common anatomical features of sonic muscles and
an adaptation for producing fast contractions
(Mendelson, 1969
;
Bevengut et al., 1993
).
Additionally, mitochondria are limited in the fast division, probably because
these fibers do little mechanical work and are only intermittently active
(Rosenbluth, 1969
). Mendelson
(1969
) suggested that the fast
division of the remotor muscle is capable of contracting at frequencies up to
100 Hz, which is comparable to the 100130 Hz sounds recorded by Fish
(1966
). Fish
(1966
) further speculated that
these high-frequency contractions were made possible by a pair of motor
neurons that alternately activated the muscle. The anatomical composition of
the remotor muscle suggests that each division might serve a unique function,
such that the small bundle (slow division) is responsible for antennal
movement and posturing, while the fast division is used to produce sound.
While the anatomical composition of the remotor muscle suggests that it is
a sonic muscle, data supporting this hypothesis are limited. There are only
two published reports concerning sound production in the American lobster:
Fish (1966) recorded
waterborne sounds from six animals and felt body vibrations in 25 (of 100)
lobsters, and Mendelson (1969
)
successfully recorded remotor muscle activity in lobsters while they produced
body vibrations; however, he did not rigorously determine whether the muscles
caused the vibrations. The overall goal of the present study was to build on
this limited empirical data and develop a more complete understanding of the
mechanisms underlying carapace vibrations and sound production in the American
lobster. First, we recorded the body vibrations produced by lobsters and
characterized them in terms of frequency and duration. Second, we tested
whether the body vibrations produced by lobsters give rise to waterborne
acoustic signals by making simultaneous vibration and sound recordings using
an accelerometer attached to the carapace and a hydrophone. Finally, to
determine if the muscles of the second antenna were, in fact, the underlying
cause of body vibrations and waterborne sounds, we recorded electromyograms
(EMGs) from antennal muscles, while simultaneously monitoring body vibrations,
in both normal lobsters and lobsters with lesioned muscles. Our results
demonstrate that lobsters use both the remotor and promotor muscles of the
second antenna to produce vibrations of the carapace, which in turn give rise
to waterborne sounds.
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Materials and methods |
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Vibration and sound recordings and spectral analysis
In the laboratory, acoustic signals and carapace vibrations were recorded
from known vibration-producing lobsters and characterized according to both
their frequency and duration. To determine whether body vibrations and
waterborne signals were produced simultaneously, acoustic signals and carapace
vibrations were recorded from lobsters in an aquarium by two means: (1)
measuring vibrations of the carapace using an accelerometer glued to the
carapace and (2) recording the associated waterborne acoustic component with a
hydrophone placed near the lobster. Henceforth, signals recorded with the
accelerometer are reported as vibrations whereas those recorded with the
hydrophone are referred to as sounds.
A waterproof, general-purpose accelerometer (1.25 cmx2.70 cm; Sales Technology Inc., League City, TX, USA) was used to record carapace vibrations. The threaded end of the accelerometer was screwed into a nut embedded in a curved epoxy tab, which was secured to the lobster's dorsal carapace with cyanoacrylate glue. The output of the accelerometer was displayed and recorded on a Macintosh laptop using a Powerlab 8SP analog-to-digital interface (ADInstruments, Colorado Springs, CO, USA) and Chart Software version 4.2 (ADInstruments). A sampling rate of 4000 samples s1 was used to ensure proper representation of the accelerometer signal. The accelerometer was tested with known frequencies of sound generated by a low-frequency oscillator (model 202c; Hewlett Packard, Palo Alto, CA, USA) in the range of 102000 Hz, to confirm that it could collect accurate data at this rate of sampling.
Lobsters' vibrations were recorded by the accelerometer in a small chamber
(26.7 cmx29.2 cmx15.2 cm, width x length x depth)
filled with chilled seawater (1015°C). Successful recordings were
obtained from a total of 17 lobsters out of 50 that were tested for use
in the electrophysiological studies. Twelve of the lobsters, from three
different size classes [<65 mm CL (56F, 63M, 54M, 61M, 65M), 7482 mm
CL (75F, 78F, 82M, 74M) and >90 mm CL (92M, 90M, 90M)] were intermolt,
while five lobsters were soft, postmolt animals (69M, 81F, 84F, 90M, 95M).
Soft lobsters (69M and 84F) not falling into the aforementioned size
categories were excluded for size class comparisons. Lobsters were induced to
produce sound either by lightly grasping or tapping on the dorsal carapace or
tail or by passing a shadow above them.
In order to test whether lobsters were producing waterborne acoustic signals concurrent to body vibrations, a hydrophone (model AQ-9; Aquarian Audio Products, Anacortes, WA, USA) was placed in the test chamber, anterior to, and within 10 cm of, four lobsters. Both types of signals (hydrophone and accelerometer) were simultaneously displayed and recorded using the aforementioned MacLab hardware and software, and the spectral characteristics of both recordings were measured and compared. Hydrophone recordings of waterborne acoustic signals were recorded from four additional lobsters using the spectral software Canary version 1.2.4 (Cornell Ornithology Laboratory, Ithaca, NY, USA).
A one-way ANOVA (Systat version 10) was used to compare differences in body vibration frequencies and durations between size classes, and unpaired t-tests (Instat version 2.01; GraphPad Software Inc., San Diego, CA, USA) were used to compare differences between sexes and molt stages (postmolt versus intermolt). Typically, intensity was not determined from accelerometer recordings because the location of the accelerometer on the carapace in relation to the sound source and the amount of contact the epoxy tab made with the carapace varied among individual animals. For waterborne acoustic signals recorded using Canary software, spectrograms were produced and peak frequency (in Hz) and mean intensity (in dB) were determined.
Vibration production mechanism: sonic muscle electrophysiology
To determine if the remotor and promotor muscles were active during the
production of body vibrations, electrical activity was recorded from the left
and right promotor and remotor muscles of 10 lobsters while they were induced
to vibrate [see fig. 1 in Bevengut et al.
(1993) for an excellent
illustration of these muscles]. Two small holes were created in the carapace
over each muscle using a 26-gauge hypodermic needle (Becton-Dickinson and Co.,
Franklin Lake, NJ, USA), and pairs of electrodes, made of 30-gauge insulated
wire with 2 mm of insulation stripped from the tips, were inserted into the
holes and secured to the carapace with cyanoacrylate glue and squares of duct
tape. The electromyograms (EMGs) were amplified and filtered using A.C.
pre-amplifiers (P5 series; Grass Medical Instruments, Quincy, MA, USA) and
recorded with a Powerlab 8SP interface (ADInstruments) and Chart version 4.2
software (ADInstruments). t-tests (Instat version 2.01) were used to
examine differences in the frequency and duration of vibrations produced with
one set of muscles (i.e. left or right remotor and promotor) versus
those produced with both sets of muscles simultaneously.
The remotor and/or promotor muscles of 12 lobsters were lesioned to test whether the activity of the muscles was responsible for producing carapace vibrations and whether both sets of muscles (right and left antennae) and/or both types of muscles (remotor and promotor) were necessary for production of carapace vibrations and waterborne sounds. Lesions were performed on one set of muscles at a time (right or left antennae). In six cases, both muscles were lesioned; in three cases, just the remotor was lesioned; and in three further cases just the promotor was lesioned. Lesions were performed by cutting a small window in the carapace just posterior to the insertion of the target muscle(s) onto the carapace and separating the muscle tissue from the insertion on the carapace; the rest of the muscle tissue, including the origin at the antenna, was left intact. The window in the carapace was then covered with duct tape, and lobsters were allowed to recover in a holding tank for a minimum of 1 h. After this recovery period, the electrical activity of both the intact and lesioned muscles, as well as carapace vibrations, was recorded as previously described. t-tests (Instat version 2.01) were used to evaluate differences in the vibration frequency between lobsters with intact and lesioned muscles.
To further test the hypothesis that the muscles of the second antenna are responsible for production of carapace vibrations in the American lobster, the remotor and promotor muscles were electrically stimulated while vibrations were recorded using an accelerometer, as described above, in five lobsters. To stimulate the muscles, wire electrodes inserted into the muscles of interest were connected to S9 stimulators (Grass Medical Instruments), and muscles were stimulated with trains of pulses of varying frequencies, intensities and durations.
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Results |
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Body vibrations gave rise to simultaneous waterborne sounds of similar frequencies (Fig. 4). Seven waterborne sound events, with a mean frequency at peak intensity of 182.9±21.7 Hz and a mean intensity of 18.5±0.5 dB (dB references: standard seawater reference, 0.65 aW m2), were recorded in Canary Software from four lobsters with a hydrophone located approximately 10 cm in front of the lobster. Each of the seven sound events was closely associated with an exoskeleton vibration of comparable duration and frequency.
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When the posterior insertions of both the remotor and promotor muscles were lesioned, carapace vibrations ceased (Fig. 7), even though muscle contractions still occurred in the lesioned muscles. When the muscles on only one side of the body were lesioned, lobsters could still produce vibrations by using the intact muscles of the other side (Fig. 7). Lobsters continued to alternate between the muscles of each side of the body as they did before the lesion, rather than avoiding the use of the lesioned muscles. Lesions of the promotor muscle alone (N=3) did not inhibit carapace vibrations nor did they significantly change the frequency of carapace vibrations in comparison with signals produced by the same animals with intact promotor muscles (non-lesioned vibrations, N=10, mean frequency 161.7±14.6 Hz; lesioned vibrations, N=19, mean frequency 158.3±9.2 Hz; P=0.84). However, when the remotor muscle alone was lesioned (N=3), carapace vibrations ceased.
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In four lobsters, we directly stimulated the remotor and promotor muscles
to induce carapace vibrations (Fig.
8). The stimulation parameters that yielded vibrations that most
closely resembled natural vibrations were: frequency 150 Hz, duration 20
ms, intensity 34 V. Vibrations were produced when both muscles were
stimulated or when just the remotor was stimulated, but not when the promotor
muscle was stimulated alone. In every case, the stimulation duration matched
the body vibration duration, indicating that the stimuli caused the vibrations
rather then causing the lobsters to be disturbed so that they vibrated on
their own (Fig. 8).
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Discussion |
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A year-long survey of 1723 lobsters demonstrated that: (1) all size classes of lobsters can produce body vibrations; (2) the tendency is similar for males and females; (3) the prevalence of body vibrations between months, while statistically different in some months, varied within a small range of 2.9614.10% and (4) carapace vibrations were often associated with meral spread posturing. These findings suggest that these carapace vibrations and/or the associated waterborne signals are used by all lobsters, possibly for defensive purposes rather than playing a role in a behavior, such as mating, which is unique to a particular subset of lobsters.
Manually induced carapace vibrations and their associated waterborne
signals had frequencies of 183 Hz, with durations between 68 and 1720 ms.
All lobsters produced signals of varying frequencies and durations, and it's
likely that changes in duration, and possibly frequency, were influenced by
the level of excitation and/or fatigue. Some differences in the frequency and
duration of vibrations were seen between the three size classes sampled, but
there were no clear trends across the size classes. The phenomenon of body
size-dependent sound frequency production, as seen in mormyrids, drums and
tiger fishes (Ladich and Bass,
2003
), is related to the increasing size of the internal sound
production and resonating structures as the animal grows. However, in the case
of the American lobster, the differences seen in the data probably represent
different levels of excitation and fatigue, not anatomical differences in the
muscle-driven mechanisms, as all lobsters produced signals with a wide range
of frequencies and durations. Differences in the frequency of body vibrations
between soft, postmolt and hard, intermolt lobsters are probably not related
to differences in the underlying sound-producing mechanism but are rather due
to differences in the transfer and subsequent distortion of body vibrations
when transmitted through a softer versus a harder carapace. It is
also possible that differences are due to changes in the insertions of these
muscles onto the carapace during the molt cycle. Soft lobsters were also more
likely to vibrate as compared with their harder counterparts, possibly because
they were more vulnerable to attack than lobsters with hard shells.
Both the remotor and promotor muscles are active during the production of
carapace vibrations and there is a one-to-one relationship between
contractions of these muscles and the carapace movements recorded with an
accelerometer attached to the shell. This type of relationship is also seen in
the sound-producing mechanisms of the toadfish (Opsanus tau) and the
western diamondback rattlesnake, Crotalus atrox
(Rome et al., 1996). Lobsters
only need to use one pair of muscles to produce vibrations, and they often
alternated the use of muscles from one side to the other when creating a
series of signals. Northern searobins also use a mechanism of alternating
muscle contractions to create sounds
(Connaughton, 2004
). However,
while searobins alternately contract two sonic muscles during a single event,
in order to increase the frequency of airbladder vibrations, it is not clear
why lobsters use two antagonistic muscles to vibrate the carapace, given that
the remotor muscle appears to be sufficient.
The remotor muscle has been previously implicated as an American lobster
sonic muscle (Fish, 1966;
Mendelson, 1969
;
Rosenbluth, 1969
), yet this is
the first study to demonstrate involvement of the promotor muscle.
Interestingly, the promotor muscle is the primary muscle responsible for the
movement of spiny lobster antennae during stridulatory sound production
(Patek, 2003). Both our lesioning and stimulation experiments indicate that
the remotor muscle, the larger of the two muscles, is the primary muscle
involved with sound and vibration production, while the promotor muscle may
serve to modulate the signal. Results from lesioning manipulations and
promotor muscle stimulation experiments indicate that the promotor muscle is
not opposing the remotor as a way to hold the antennae still, nor is it
essential to sound production. However, results from remotor-only and combined
remotor and promotor stimulation experiments suggest that the promotor is
important for maintaining the appropriate waveform and intensity of the
vibrations. It is possible that the promotor muscle acts to tune the sound
waves and facilitates transmission of sounds by modulating the tension of the
carapace in a manner similar to the way dove superfast muscles control the
tension of the syrinx as a means of trill pitch control
(Elemans et al., 2004
).
While various behavioral experiments designed to record the natural
production of carapace vibrations and/or acoustic signals proved unsuccessful,
the lobsters used in the experiments presented here predominantly produced
signals when disturbed or threatened. Reports abound in the crustacean
(Meyer-Rochow and Penrose,
1976; Mulligan and Fischer,
1977
), fish (Winn,
1964
; Connaughton, 2003) and insect
(Dunning and Roeder, 1965
;
Sandow and Bailey, 1978
;
Masters, 1979
,
1980
;
Evans and Schmidt, 1990
)
acoustic literature of vibration and sound production in response to
disturbance or predator presence. These signals are often interpreted as
defensive in nature; however, the true function of these signals is not well
established, except perhaps in studies with insects
(Dunning and Roeder, 1965
;
Masters, 1979
;
Evans and Schmidt, 1990
). Moths
of the family Arctiidae produce ultrasounds at night
(Dunning and Roeder, 1965
).
Playback recordings of these sounds decrease the predatory effectiveness of
echolocating bats, presumably because the sounds startle the bats or act as a
warning of the moths' noxiousness (Dunning
and Roeder, 1965
; Evans and
Schmidt, 1990
). In addition, Masters
(1979
) found that both wasp
and beetle species use sounds produced by stridulation to deter predators,
including spiders and mice. In this example, soniferous individuals were less
aggressively attacked and more likely to survive than their aphonic
counterparts, suggesting that these sounds are used as startle or aposematic
signals. Similarly, for American lobsters, it is not hard to imagine that the
carapace vibration of a grasped lobster could startle a fish or skate predator
enough to cause them to momentarily release the lobster. Additionally,
waterborne sound signals of lobsters, while not intense in nature, could serve
as close contact warnings to predators, similar to the weak airborne sound
signals produced by some insects in the presence of predators
(Masters, 1980
), and the more
intense sounds of some fish and spiny lobsters when threatened
(Mulligan and Fischer, 1977
;
Connaughton, 2003). Many of the fish known to attack lobsters can detect
sounds in the range produced by disturbed lobsters
(Popper, 2003
). In our related
work on sound detection, we found that lobsters can detect waterborne sounds
produced by conspecifics at distances of one meter or less (H. P. Henninger
and W. H. Watson, manuscript submitted), which supports the possibility that
these signals could also serve a role in intraspecific communication. Studies
are currently underway to determine when lobsters produce carapace vibrations
and sounds in their natural habitat.
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Acknowledgments |
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