226 
PACIFIC SCIENCE, Vol. IV, July, 1950 
Remarks on the Luminescence 
A specimen of Gazza minuta and one of 
Secutor insidiator were put into a tank of 
sea water in a dark room and their lumines- 
cence was studied. No luminescence could 
be seen when the fish were viewed dorsally, 
but if they were viewed laterally a diffuse 
bluish-white luminosity could be seen. If the 
area where the transparent scales are ar- 
ranged was examined directly or obliquely 
from below, a faint bluish light was seen, 
emanating from the chest, thorax, abdomen, 
and tail areas. The light showed uninter- 
ruptedly in these areas. It did not shine bril- 
liantly, but was feeble and diffused. 
If the fish received a strong stimulus or 
was removed from the water, the lower part 
of the body lighted up brilliantly, most 
strongly in the thoracic and abdominal re- 
gions. This sudden increase in brilliancy was 
accompanied by a croaking sound, the pro- 
duction of which is a peculiarity of the 
Leiognathidae. 
If during this brief period of maximum 
brilliancy the fish was enveloped in a cello- 
phane wrapper and was laid on a photo- 
graphic film, it would produce a bright 
image. 
After death the luminous gland was sur- 
rounded by the membrane and the light was 
no longer visible, but if the fish was dissected 
and the membrane covering the gland was 
removed, this membrane was found to be 
luminous and, when kept in an ice box at 
5-10° C, it remained so for 2 days. 
Contents of the Luminous Gland 
When the substance of the luminous gland 
is freshly made up as a 3 per cent salt emul- 
sion, the whole of the emulsion will be lumi- 
nescent, but if it is allowed to stand, only 
the upper layer exposed to the atmosphere 
will glow while the deeper lower layer be- 
comes non-luminous. When shaken up in 
air the whole emulsion becomes luminous 
again. 
Luminescence is greatest at a temperature 
of 26 °— 30° C. in the tropics (in Japan at 
about 20° C.) and becomes feeble at 4° C. 
When the temperature is raised, it increases 
and finally disappears at 45° C., but after 
once attaining this latter temperature it fails 
to recover its luminosity even when cooled 
again. An emulsion in nearly 3 per cent 
NaCl gives the best results; an emulsion in 
distilled water produces no light. If such a 
distilled-water emulsion is centrifuged the 
upper clear solution is non-luminous but the 
sediment is luminous. Microscopically this 
emulsion is made up of disintegrated parti- 
cles of gland cells with innumerable bacteria, 
and it appears from the results of these ex- 
periments that the contents of the luminous 
gland consist of luminous bacteria. In order 
to test this, isolation experiments were car- 
ried out to cultivate them in a 3 per cent 
NaCl agar-agar culture medium. 
Culture of the Luminous Bacteria 
A pure culture of the luminous bacteria 
from the luminous glands was prepared by 
the usual bacteriological techniques. After 
8-10 hours a small, round, transparent and 
luminous colony appeared. From this start 
a pure culture which was free from any con- 
taminating bacteria was obtained. 
It was clearly demonstrated that the same 
kind of fishes always had the same kind of 
luminous bacteria and that the Leiognathidae 
of the tropical sea and those of Japan pos- 
sessed luminous bacteria of the same group, 
with only slight variations probably due to 
the effects of temperature. There is a curious 
feature, viz., the bacteria in culture are not 
as brilliant as are those in the gland, prob- 
ably because of the unsuitability of the me- 
dium for greatest luminosity. The details of 
the bacteriological work will be presented 
elsewhere. 
SUMMARY 
Some luminous fishes show no outward 
peculiarities in structure or appearance and 
