226 
PACIFIC SCIENCE, Vol. XV, April 1961 
maintained in the aquarium 12 days before the 
experiments ) , it was observed that they did not 
feed as frequently as adult manini in the natural 
environment (whereas the juveniles did). The 
feeding by the adults was readily interrupted by 
the approach of an observer. Usually only one 
fish fed at a time, and since the larger one dom- 
inated the smaller and held it in a corner of the 
aquarium for much of the day, the former con- 
sumed the major part of the algae. Yet both 
fish disdained to feed at all when placed in 
separate aquaria. 
Stimulus to Feeding 
The following simple experiments and ob- 
servations were conducted in order to ascertain 
what sensory mechanism or mechanisms are 
utilized by the herbivorous manini in finding 
food. 
The juice from several grams of Enteromor- 
pha was squeezed into an aquarium in which 
two adult manini had been starved for a day. 
This was unfiltered and colored the water green 
when first dropped into the aquarium. No re- 
sponse was observed, even when some of the 
green color was seen to diffuse in the immediate 
vicinity of the nose and mouth of the fish. A 
similar experiment was performed on several 
juvenile manini with Poly sip honia, again with 
no visible response. By contrast, when an ex- 
tract of Polysiphonia was dropped into an aquar- 
ium containing a half -grown Ahudefduf abdom- 
inalis (omnivorous in food habits), this fish 
swam to the surface where the extract clouded 
the water red and snapped its jaws erratically 
in this region. 
Some Enteromorpha and Polysiphonia was 
boiled to the extent that the algae began to dis- 
integrate and lost most of their color. This was 
placed in the aquarium with fresh algae of the 
same kinds. Although juvenile manini fed ini- 
tially on the fresh algae, the boiled algae was 
ultimately completely eaten. 
Some maroon rayon fiber similar in texture 
and color to Polysiphonia was placed in an 
aquarium with juvenile manini. It was taken into 
the mouth temporarily by several of the fish. 
When the rayon was soaked in extract of the 
Polysiphonia , it was not taken in with greater 
frequency. 
When recently voided fecal Enteromorpha 
was placed at the surface of the aquarium in the 
same manner that fresh algae were offered, most 
of the manini approached it, hesitated, and 
swam away. One juvenile drew a small amount 
into its mouth but hastily ejected it. When de- 
fecated filaments were added together with fresh 
filaments of Enteromorpha, only the fresh alga 
was eaten. In the absence of fresh algae, fecal 
algae which have remained in the aquarium for 
several hours may be ingested. The results of 
offering the blue-green algae listed in Table 3 
were similar to that just described for recently 
voided fecal algae. The blue-greens were ap- 
proached, but rarely taken into the mouth. 
The nasal organs of a 26 mm. manini were 
cauterized with a hot wire. Five min. after this, 
the fish was offered some Polysiphonia . It fed 
on this with the same "enthusiasm” as previ- 
ously. 
Another juvenile fish was blinded by wiping 
a crystal of potassium hydroxide over the eye 
( other fish were blinded with silver nitrate crys- 
tals but these invariably died within 36 hr.). 
After contact with the potassium hydroxide, the 
surface of the eye became opaque white; this 
fish survived more than 3 days. It moved slowly 
about the bottom, occasionally swimming by 
chance into a mass of Polysiphonia. It never 
swam directly to the algae. When the algae 
touched the mouth it was often eaten. By the 
third day when the aimless movements carried 
the fish into a mass of algae, it fed voraciously. 
It is concluded from the above that vision 
alone is needed to locate algal food, and the 
olfactory sense functions in feeding only to avoid 
unsavory material. Chemoreceptor organs in the 
mouth and possibly tactile organs as well also 
seem to be associated with the acceptance of 
algae and the rejection of material which is not 
utilized as food. The tendency to feed more on 
one alga than another develops after the algae 
have been sampled. 
DIGESTION 
In the work on nutrition of marine animals 
much effort has been expended on the analysis 
of mode of feeding and stomach contents, 
whereas little has been applied to the important 
