[8 4 5 ]
HEARING AND THE OCTOPUS STATOCYST
BY S. J. HUBBARD
Department of Physiology, University College London, Gower Street, W.C. i
{Received 30 May i960)
A detailed study of the anatomy of the octopus statocyst by Young (i960) seemed
to indicate that, although the maintenance of equilibrium was likely to be its most
important function, the possibility of its participating in the function of hearing
could not be ruled out. The work described here was an attempt to test this
possibility.
The general problem of the function of invertebrate statocysts was reviewed by
Lowenstein (1950) who pointed out that structural comparisons with the vertebrate
labyrinth might be misleading and that direct experimental evidence was needed.
Boycott (i960) has described the effects of unilateral and bilateral removal of
statocysts on the maintenance of equilibrium in Octopus, and Wells (i960) has
described the effects of the same procedure, with its consequent displacement of the
retina, on learned discrimination.
METHODS
The work was done at the Stazione Zoologica in Naples and Octopus vulgaris
Lamarck of about 300 g. mass were used. They were placed in pressed asbestos
tanks 100x60x40 cm. deep (Wells & Wells, 1956) through which a continuous
supply of aerated sea water was circulated.
The apparatus used for giving sound stimuli through the water consisted of a
moving-iron telephone earpiece driven from an audio-oscillator. The earpiece was
modified in the following way:
(a) The diaphragm-retaining ring was removed so as to present the maximum
vibrational area to the water, and the diaphragm itself was sealed with 'Araldite'
adhesive to the earpiece all round its outer edge; all screw heads in the surface of
the earpiece were also sealed and filled with the same material.
(b) A hexagonal-headed brass bolt, 12 mm. diameter, 5 cm. long, was drilled
axially so as to allow a screened twin-core, P.V.C.-covered cable to be threaded
through with a tight fit. The outer covering was arranged to lie about 3 mm. back
from the face of the hexagon head and the emergent twin wires and the screening
were sealed in with 'Araldite' adhesive; the exit of the cable at the other end of the
bolt was also sealed. The twin wires were connected to the telephone terminals and
the screening connected to one of them.
(c) A hollow cylindrical mould whose diameter was about 1 cm. greater than
that of the earpiece was made from cold-curing silicone rubber (Midland Silicones
Ltd. K9161, Catalyst N9162). This material will allow the release of at least twenty
846
S. J. HUBBARD
'Araldite' castings. The earpiece and brass bolt were then clamped above the mould
and lowered so that the face of the diaphragm rested on three symmetrically
arranged small spacers of P.V.C. each about I mm. thick. 'Araldite D' to which
had been added 8 % by weight of hardening catalyst was then poured into the mould
to a level just above the hexagon head and its junction with the shank of the bolt.
After about 18 hr. the mould was removed and the unit was ready for fastening to
the tank. Fig. 1 shows the various stages of the above procedure. Five earpieces
were modified in this way and withstood continuous total immersion in sea water
for 3 weeks without breaking down.
Exit cable
Input
terminals.
Araldite seal
Brass bolt
Silicone
mould
Diaphragm
Araldite
casting level
Araldite seal
Diaphragm retaining ring
removed)
Spacers
Fig. 1. Diagram showing how a telephone earpiece was modified to withstand continuous total
immersion in sea water.
The earpieces were mounted singly in the tanks in the middle of the shorter side
through a 12 mm. hole drilled at about half the depth of water (about 25 cm.)
normally in the tank. A rubber washer and a flat brass washer on each side were
sufficient to prevent leakage with only gentle tightening of the fixing nut; the latter
was, however, very firmly tightened to prevent the octopus from turning—and
thus loosening—the whole unit (this occurred several times during the installation
of the units!).
Fig. 2 shows a block diagram of the electrical apparatus. The audio-oscillator
was connected to the earpiece through a shorting key which was opened during
the period required for the sound stimulus; for reasons which will be discussed later
no attempt was made to limit the transients in the diaphragm at the beginning and
end of the stimulus. Preliminary tests with a calibrated hydrophone in metal tanks
rather smaller than those at Naples had shown that many resonances occurred
over the range 50-30,000 cyc./sec.; these were due to the acoustic properties of the
earpiece, mismatching at the diaphragm/water interface and reflexions and absorp-
Hearing and the octopus statocyst
847
tions in the tank itself. To counteract this, in some measure, each unit was calibrated,
before installation, in one of the tanks at Naples, and the oscillator output setting
which gave maximum undistorted output over the range 50-30,000 cyc./sec. was
recorded. The hydrophone used in these tests was suspended in the tank just in
front of the octopus's 'home' (Boycott, 1954) and the output was amplified and
displayed on an oscilloscope. Since the absolute calibration of the hydrophone was
known it was therefore possible to estimate the effective hydrodynamic sound
pressure at any frequency and to see at what level distortion occurred. Boundary
effects at the walls of the tank were, however, large, so the figures for stimulus
sound pressures were regarded only as giving an order of magnitude rather than
an exact absolute figure. Further, this calibration was less useful than might have
been expected since there was insufficient time to carry out training tests at more
than two or three frequencies and, also, the results obtained were such that the
distortion of the wave form from a pure sine curve was not likely to be of much
importance. It was found that between 100 and 5000 cyc./sec. the wave-form of
the hydrodynamic sound stimulus was undistorted in the neighbourhood of the
hydrophone, provided a sound pressure of 17 dynes/cm.2 was not exceeded.
Hydrophone position
for calibration
Water
level
A.F.
oscillator
Oscilloscope
Earpiece
Octopus's home
Fig. 2. Diagram showing the electrical connexions between the units which produced the
sound stimulus in the tanks.
When the earpieces had been installed in the tanks the animals were trained to
attack crabs and when the latencies of attack were about 5 sec, one of two further
methods of training procedure and testing was adopted.
Training '. ittack
A crab was placed in the end of the tank opposite the octopus's home immediately
after a short sound stimulus had been given. After five or six repetitions of this
at about 10 sec. intervals the sound stimulus was given alone and the animal's
response was noted.
S. J. HUBBARD
Training to attack with discrimination
First, a 10 V. a.c. electric shock was given to the octopus in its home after a
iosec. sound stimulus. Then, after 15 min., the two tests which constituted the
training were begun. They consisted of presenting a crab, either alone or preceded
by a 10 sec. sound stimulus. In the first case the octopus was allowed to capture—
and eat—the crab; in the second case the crab was withdrawn and, in some experiments, an electric shock was given. The two presentations of the crab were repeated
at intervals varying from 5 to 30 min. though in any one experiment the interval
between tests was constant. In all cases the measured parameter was the latency
of attack by the octopus after the presentation of the crab.
-D1
30
r
60
Trial number
100
120
130
Fig. 3. Results of trials in training to attack. Abscissa: trial number. Ordinate: latency of attack
in sec. Open circles: latency when crab given. Filled circles: latency when sound given alone.
Solid vertical bars: movement in or slight emergence from home of octopus, usually just after
the sound was switched off.
RESULTS
Training to attack
In these experiments three different frequencies—100, 900 and 2000 cyc./sec.—
were used both with the same and with different animals on separate occasions;
Fig. 3 shows the results from trials with one animal over a period of 9 days.
On days 4 and 5 the frequency used was 100 and 2000 cyc./sec, respectively, but
on all other occasions it was 900 cyc./sec. The trials were 5 min. apart and the
sound was given for either 10 sec.—after which the crab was presented—or for
30 or 60 sec. alone. The 60 sec. period of sound was used on the last 3 days in case
responses of more than 30 sec. latency had been missed in the previous tests.
Open circles in the figure represent attack latencies when the crab was given;
filled circles latencies with the sound alone, 30 sec. being the absolute limit of
time measurement.
Hearing and the octopus statocyst
849
There was no evidence in these experiments to show that sound conditioning had
occurred. It may be seen from the figure that in a total of 130 trials there was only
one positive attack out of a possible fifty-seven occasions on which the sound alone
was used. There were, however, seventeen occasions (shown as thick vertical bars
in the figure) on which the animal showed some movement in its home or emerged
slowly from it, usually after the sound was switched off, but none of these movements
was comparable with the behaviour of the octopus when it was attacking a crab.
60
Trial number
Fig. 4. Results of trials in training to attack with discrimination. Abscissa: trial number. Ordinate:
latency of attack in sec. Open circles: latency when crab given alone. Filled circles: latency
when crab given after 10 sec. sound. Short thick ordinates (S0-S6) represent 10 V. a.c. shocks
given to octopus.
Training to attack zvith discrimination
Most of these experiments were done with a sound frequency of 100 cyc./sec.
and Fig. 4 shows the results of trials on one animal over 5 days. The test intervals
were 5 min. and the sound, when used, was given for 10 sec. In the figure the
short thick ordinates signify when shocks were given to the animal in its home after
a 10 sec. period of sound. No other shocks were given; if an attack was made when
the crab was presented after the 10 sec. sound stimulus it was simply withdrawn
before it could be eaten. The open circles joined by a continuous line represent
attack latencies when the crab was given alone, and the filled circles joined by a
broken line represent the latencies when the crab was given after 10 sec. of sound.
It may be seen that the graph defined by the filled circles tends to lie above that
defined by the open circles; if good discrimination were being performed by the
octopus this separation of the two graphs would be more distinct.
Fig. 5 shows histograms of the attack latencies for the two forms of presentation
and the inset Table gives the results of Mest calculations; these are given for the
54
Exp. Biol. 37, 4
85O
S. J. HUBBARD
whole 5-day period and for the periods between the shocks (See Fig. 4). It may
be seen that the mean latency of attack when the crab was preceded by the sound
was greater than that without the sound except for one instance; but there was only
12
r
Crab alone
30
10
15
20
Latency of attack in seconds
12
Test
interval
/*!,-/*<•
DtoU!
17
So-Si
S1-S2
S2-S3
S3-S4
S4-S6
S6-DSend
1 "4
4-6
3-0
n
in
8
23
18
34
-o-8
i-o
8
10
t
t( P = 0
1-62
3-36
i-35
1-26
2-00
0-40
O-22
2-04
2-31
2-23
0-07
2-3'
2-O7
2-10
= difference of mean latencies,
n = degrees of freedom.
10
8
Crab after sound
3 6
0
5
10
15
20
25
Latency of attack in seconds
Fig. 5. Histograms showing frequencies of latencies of attack when: (a) crab was presented alone,
(6) crab was presented after io sec. of sound. The results are a total for one octopus over a s-day
period. (Inset Table shows f-test calculations for the whole interval and for intervals between
shocks.)
one occasion when the difference of these means was significant (P = 0-05). The
fact that the distribution is truncated (since the upper limit of measurement was
30 sec.) does not significantly affect the calculations.
Hearing and the octopus statocyst
851
DISCUSSION
It is quite clear that, on the evidence presented here, there is little justification for
inferring that the octopus's behaviour was modified by the presence of sound waves,
regardless of whether or not the statocysts were involved. It remains an open
question as to whether the results represent the true sensory situation or are a consequence of the particular experimental methods used.
Before a fully satisfactory investigation of any possible sensory faculty of an
animal can be made it is important to know what background level of stimulus for
that faculty exists in the normal habitat of the animal and, further, what are the
physical limits of the naturally experienced stimuli. In this case, therefore, we
would like to know what background noise exists in the octopus's normal environment and what the amplitude and frequency limits of all possible auditory stimuli
may be. Since this is at present unknown, the selection of test frequencies (for
example) can be no more than arbitrary, although some clues may exist (Dijkgraaf,
1952). It is unlikely that the frequencies used in the present work had much bearing
on the negativity of the results, since distortion (and, thus, many harmonics)
occurred at sound pressures greater than 17 dynes/cm.2 (see Methods) and this
figure was often exceeded; had the results been more positive, pure tones would
have been important in estimating the frequency range of hearing.
In this context the definition of 'hearing' is of some consequence; Pumphrey
(1940) has considered this in relation to insects and his comments are generally
applicable although his definition of hearing excluded water-borne sounds. An
important point which he made was that the separation of sound and tactile senses
was 'based on the intensity factor' and was arbitrary. With this in mind there is no
reason why we should not define hearing in marine animals as occurring when the
animal is 'demonstratively responsive to sound' in this case sound being defined
as low-intensity hydrodynamic disturbances.
The sound pressure used in this work was many times greater than that giving
human threshold of hearing, but Fraser & Purves (i960) have shown, in a discussion
on hearing in Cetacea, that the place of pressure in the response of terrestrial
animals to sound is taken by displacement in marine animals. We can assess the
adequacy of the pressures used here by noting that for any medium, and for a given
sound intensity and frequency, the sound pressure and amplitude are connected by
the equation (Pumphrey, 1940)
* = kplpc,
where x = amplitude of sound, p = pressure of sound, p = density of medium,
c — velocity of sound in the medium, k = constant. So for air xA = kApAjpAcA,
and for water xw = ku.pwlpwcw, where the suffices refer to the appropriate medium.
Dividing one equation by the other gives
(1)
*J*w = (PAIPW) x (PWCJPA CA)>
since kA = kw provided the sound is of the same frequency and intensity in either
medium. Now pwcw/pAcA = 3700 approx. (Vigoureux, i960) and, in these
54-2
852
S. J. HUBBARD
experiments, the minimum hydrodynamic sound pressure was 17 dynes/cm.2.
If we use the air pressure value of 2 x io" 4 dynes/cm.2 for human threshold of
hearing (Sivian & White, 1938) then equation (1) becomes
*J*u> = ( 2 x io-« x 3700V17
= 1/23 approx.
Hence, the hydrodynamic displacements used were about twenty times the corresponding ones in air for human hearing threshold. Pumphrey (1950) has noted the
transparency of marine animals to sound (since the densities on either side of the
water/animal interface are very much nearer the same value than those between
air and terrestrial animals); hence the displacements calculated above will reach the
statocysts with little attenuation. The phase difference with which airborne sounds
reach the mammalian cochlea, however, will be absent at the receptors of marine
animals (except at wavelengths comparable with the inter-statocyst distance) and
therefore the directional properties of hearing will be lost. Fraser & Purves (i960)
have shown how this may be recovered in Cetacea by the acoustic insulation of each
ear; but no comparable structures have been recognized in the octopus.
A special condition which applied to these experiments should be considered
here. In any programme which involves the training of Octopus it is essential to
maintain good sea-water circulation through the tanks in which the animals are
kept (Boycott, 1954). At Naples high oxygenation of the refill liquid entering the
tanks was ensured by means of a jet impinging from above on to the water surface.
The animal was therefore living in a state of high background noise. This was
'heard' qualitatively by the hydrophone but no exact measurement was made of
the sound pressures it produced. In order to counteract this, in some measure, the
jet was diverted a minute or so before each trial was made (whether or not the sound
was used) but this represents a very small fraction of the 24 hr. total circulation.
Hence, the octopus may have achieved considerable adaptation to sound which
did not necessarily disappear when the jet was diverted.
It may be that a more electrophysiological type of experiment—recording from
the statocyst nerve during sound stimulation, for example—would be more rewarding. It would not, of course, answer the question of' hearing' from the behavioural
point of view, but it would at least establish whether a mechanism for such a faculty
did exist.
SUMMARY
1. A description is given of apparatus suitable for transmitting hydrodynamic
sound waves within a rectangular tank.
2. Two types of training experiment for investigating hearing in Octopus are
described, one using simple training to attack, the other training to attack with
discrimination.
3. These results of the experiments, when expressed in latencies of attack on
food, gave little evidence that the octopus was able to hear.
4. Some factors which might have influenced the results are discussed together
with suggestions for any future investigations.
Hearing and the octopus statocyst
853
I would like to record my appreciation of much technical advice from Mr R.
Stubbs and the loan of the hydrophone from the National Institute of Oceanography. Acknowledgement is also due to the Royal Society and the Nuffield
Foundation for part of the expenses involved in this work and also to Dr P. Dohrn
of the Stazione Zoologica at Naples for laboratory facilities there.
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BOYCOTT, B. B. (i960). The functioning of the Statocysts of Octopus vulgaris. Proc. Roy. Soc. B,
152, 78-87.
DIJKCRAAF, S. (1952). Uber die Schallwahrnehmung bei Meeresfischen. Z. vergl. Physiol. 34,
104-22.
FRASER, F. C. & PURVES, P. E. (i960). Anatomy and function of the cetacean ear. Proc. Roy. Soc. B,
152, 62-77.
LOWENSTEIN, O. (1950). Labyrinth and equilibrium. Symp. Soc. Exp. Biol. 4, 60-82.
PUMPHREY, R. J. (1940). Hearing in insects. Biol. Rev. 15, 107-32.
PUMPHREY, R. J. (1950). Hearing. Symp. Soc. Exp. Biol. 4, 3-18.
SIVIAN, L. J. & WHITE, S. D. (1938), Hearing and Brownian movement. J. Acoust. Soc. Amer.
4, 288-98.
VIGOUREUX, P. (i960). Underwater sound. Proc. Roy. Soc. B, 152, 49-51.
WELLS, M. J. & WELLS, J. (1956). Tactile discrimination and the behaviour of blind Octopus.
Pubbl. Staz. Zool. Napoli, 28, 94-126.
WELLS, M. J. (i960). Proprioception and visual discrimination of orientation in Octopus. J. Exp.
Biol. 37, 489-499YOUNG, J. Z. (i960). The Statocyst of Octopus vulgaris. Proc. Roy. Soc. B, 152, 3-29.