Sound Production in Triggerfishes — Salmon, Winn, and Sorgente 
13 
the surface. Observations were noted on sub- 
mersible writing slates and behavior of fish 
was photographed with a Nikonos underwater 
camera. It was usually possible to sit on the 
bottom 1-2 m from individual fish, observe 
apparently normal behavior, and clearly detect 
by ear sounds correlated with pectoral fin move- 
ments. 
Observations on two groups of 11 B. vetula 
were carried out in Bimini after establishing 
the fish in large, circular, concrete tanks 4 m 
in diameter and 0.6 m deep. Six shelters, con- 
structed from building blocks, were placed 
around the tank periphery. A hydrophone was 
placed in the center of the tank. Color changes, 
sound production, and associated behavior were 
noted during a total of 6 hours of recordings 
on each group of fish, from 0900 to 1400 hours. 
Further observations and recordings were made 
in a fish pen 10 m X ^ m X 2 ~3 m deep 
(depending upon tidal conditions) in which 
40-50 B. vetula and M. piceus were maintained. 
RESULTS 
The Pectoral Fin-Drumming 
Membrane Mechanism 
The anatomy of the pectoral fin and drum- 
ming membrane in R. rectangulus is shown in 
Figure 1. The fin was composed of a single 
stout spine and the rays which supported most 
of the effective surface of the fin. The fin was 
Fig. 1. External anatomy of the sound-producing 
apparatus in Rhinecanthus rectangulus. 1, Pectoral 
fin spine; 2, drumming membrane; 3, pectoral fin rays; 
4, fleshy muscular lobe of pectoral fin. 
supported at the base by a fleshy, muscular lobe. 
The drumming membrane was located just be- 
hind and partly above the fin, and was covered 
with thin scales. The anatomy of the area was 
essentially identical in all other species exam- 
ined, differing slightly in the number and length 
of the rays and the surface area of the tym- 
panum. 
Movements of the pectoral fins during sound 
production by fish held underwater were also 
similar in all species. In R. rectangulus, the 
spine of the fin was pressed against the drum- 
ming membrane and moved back and forth 
across its surface when a sound was produced. 
As the spine was moved posteriorly, the rays 
were folded, then stretched taut as the spine was 
moved anteriorly. In B. vetula and M. piceus, 
the whole fin was moved across the drumming 
membrane, without folding of the rays. The 
number of back-and-forth sweeps of the fin 
across the drumming membrane varied from 
sound to sound within each fish, and ranged 
from one to seven movements in a series. Both 
pectoral fins appeared to complete these move- 
ments synchronously. Oscillographs of these 
sounds are shown in Figure 2. 
No sounds resembling those of pectoral fin 
drumming could be detected from any fish when 
the fins were held immobile against the side of 
the body. Similarly, bilateral removal of the 
entire fin (rays, spine, and fleshy basal lobe) 
in eight fish of three species resulted in no 
further production of sounds. 
The change in peak sound pressure of sounds 
‘‘produced by other operated fish, when compared 
. with their own normal sounds, is shown in 
Figure 3 and analyzed statistically in Table 1. 
Removal of both the spine and rays of one 
pectoral fin (leaving the basal lobe intact) re- 
sulted in only slight reductions in sound pres- 
sure in three fish, and no change in two other 
specimens of R. rectangulus. When the same 
operation was performed bilaterally, sound pres- 
sures averaged about 16 db below those of. 
normal fish. Removal of the - spine from one 
pectoral fin in five specimens of R. rectangulus, 
B. vetula, and M. piceus had little effect on 
sound pressures; but, with the spines removed 
bilaterally, sound pressures averaged 13 db be- 
low those of normal fish. Unilateral or bilateral 
removal of the rays from the pectoral fin did 
