Luminous Organs of Fish — H AN EDA 
223 
Gazza minuta. Both specimens were the same 
as those found in tropical seas. 
In 1943-1944 I collected in Singapore the 
species as listed in the accompanying table. 
Around Singapore and the Malay Penin- 
sula these kinds of fishes are easily obtained 
in all seasons from the fishing trap known as 
the Kelong. Fowler (1938) reported 12 spe- 
cies of this family in Malaya. 
The Luminous Organ 
Gazza minuta : Externally this fish does 
not present any unusual features, and it was 
only by a careful observation of the living 
fish by night that the bluish-white lumines- 
cence of the lower half of the body was 
revealed. 
The source of the luminescence is a swol- 
len ring of gland-like substance which en- 
circles the oesophagus. It is covered with a 
white membrane except at two points, one 
located dorsally and the other ventrally. These 
appear as two small bean-shaped areas of a 
yellowish-white color. The ring contains a 
gland which has two openings (Fig. 4, op) 
which lead into the oesophagus. 
The swim bladder and the thoracic and 
ventral muscles are so modified as to increase 
the efficiency of the light-producing mech- 
anism. The swim bladder is lined with a 
remarkably bright silvery reflecting surface 
and the muscles of the thorax and the ven- 
tral parts of the body are translucent but 
somewhat cloudy. The swim bladder has no 
reflecting surface at its posterior' end, which 
is translucent. Part of the dorsal silver lining 
is continued under the spinal column and 
acts as a reflector for the light which is trans- 
mitted backward through the translucent end 
of the bladder. Light is emitted from the 
two bean-shaped areas only, the dorsal one 
illuminating the swim bladder, the ventral 
one the thoracic and ventral muscles. 
If the diagram of the longitudinal section 
of the fish through the swim bladder is stud- 
ied, it will be seen that the anterior end of 
the swim bladder forms a parabola, and that 
a bean-shaped light spot (one part of the 
luminous ring gland) is at its focus. We 
Fig. 4. a, Diagrammatic figure of the luminous 
organ of Gazza minuta (Bloch), b, Longitudinal 
section of the luminous gland of Gazza minuta 
(Bloch): phot, luminous gland; refl, reflector; 
lens, lens; vise, internal organ; vesi, swim bladder; 
chr, chromatophore; memb, membrane; op, open- 
ing of luminous gland; oeso, oesophagus; lary, 
larynx; vent, stomach. 
