378 
No fish were caught within a 3-hour period 
after 2030, indicating that they scatter to feed 
some distance away from their daytime haunts, 
perhaps as far as adjacent sandy areas as de- 
scribed by Hiatt and Strasburg (I960). Few if 
any sounds were recorded from the area after 
the school had left and until it returned shortly 
before dawn, although the fish did produce 
sounds at night when confined in aquaria. 
Nocturnal activity of laboratory populations 
was similar to that of fish in the field, i.e., they 
began to swim more actively out of the cave 
and showed color changes typical of specimens 
caught by hook and line at night. The period 
of nocturnal feeding corresponded to the time 
of greatest locomotory activity by isolated fish 
in the activity chamber, as was the case with 
H. rufus (Winn et ah, 1964). 
Differences in behavior between H. rufus 
and M. berndti were observed in (1) the types 
of sounds produced and in their diel distri- 
bution, (2) responses of laboratory popula- 
tions to sound playbacks, and (3) movements 
in the field. It is possible to explain these dif- 
ferences by comparing their nonreproductive 
social organization. 
Individuals of H. rufus are territorial, but 
fish may maintain territories a few meters apart 
and certainly within acoustic range. These fish 
produce at least three different types of sounds: 
hand-held sounds, which presumably communi- 
cate the presence of a predator by a captured 
fish; staccatos, emitted by individuals when star- 
tled or when a predator approaches; and grunts, 
produced during territorial defense, especially 
involving intraspecific aggression but also the 
chasing of a nonpredatory fish of another spe- 
cies from the territory. Display behavior, in- 
volving fin erection, nipping, shuddering, and 
lateral displays are additional components of 
territorial defense. “Mobbing” may occur, at 
least under laboratory conditions, when a 
predator swims through closely spaced terri- 
tories of a number of fish. Winn et al. (1964) 
have pointed out the similarity between ele- 
ments of the acoustical system of H. rufus and 
certain behavior patterns of birds which roost 
together though maintaining territories, and 
which will mob a predator, show crepuscular 
peaks of sound production, and have analogous 
behavioral responses to alarm calls. The acous- 
PACIFIC SCIENCE, Vol. XXI, July 1967 
tical system of H. rufus aids in maintaining 
territories by individual fish and also promotes 
the survival of all fish in adjacent areas with 
a warning call. The peaks in production of 
staccato sounds at dawn and dusk are believed 
to be the response of territorial squirrelfish 
to movements of other species through their 
territories. The initial response of laboratory 
populations to playbacks of staccato sounds 
consisted of retreat by each individual into the 
open can within his territory, followed imme- 
diately after the playback by orientation to and 
investigation of the sound source by a few fish. 
The evidence presented here indicates that 
M. berndti is nonterritorial. Fish in the labora- 
tory were never observed to defend particular 
areas of the cave from others. The presence of 
large groups of fishes in the field, schooling 
under broad ledges or inside open caves, sup- 
ports the contention that menpachi live in non- 
territorial aggregations during the day. Further 
evidence was the absence of any aggressive 
behavior or associated sound production to- 
ward individuals of other species of nonpreda- 
tory fishes introduced to populations in the 
laboratory, or to diurnally active groups of reef 
fishes frequently observed to enter and leave 
habitats occupied by menpachi in the field. The 
presence of appeasement postures, shown by 
several fish in three populations, could be ex- 
pected in this type of a social system. Lastly, 
nocturnal scattering, probably some distance 
from their daytime haunts, would make terri- 
toriality a highly transitory phenomenon. 
The most common type of sound produced 
by menpachi was a series of knocks. It is as- 
sumed that these sounds are associated with 
the chasing of a small fish by a larger one in 
field populations, because only under these cir- 
cumstances were the sounds produced in the 
laboratory. The hypothesis presented here is 
that, while territoriality promotes spacing of 
individuals in H. rufus , chasing and knock 
sounds function to maintain distance between 
individuals in M. berndti . This does not mean 
that some fish would be driven into open water, 
but that they would tend to space themselves 
throughout a given cave or ledge area, reduc- 
ing the danger that more than one individual 
could be caught by a predator and increasing 
the likelihood that a predator approaching from 
