228 
length increased from 3.5: 1 to 7.2: 1. He demon- 
strated a correlation between the increase in 
length of intestine and the substantial rise in the 
amount of larger algal types in the stomach con- 
tents and the reduced number of smaller food 
items like diatoms and blue-green algae. No sud- 
den change-over in the size of algal food which 
is eaten by the manini occurs at any size range 
once transformation has taken place, and the 
curve of alimentary tract length plotted against 
standard length is without sharp inflection. The 
great increase of the gut length which occurs 
when manini transform from the acronurus to 
the juvenile state is discussed in the section on 
transformation. 
The inner surface of the intestine is covered 
with small villi. The wall of the intestine is 
thin, its average thickness, not including any 
villi, is about 0.05 mm. The mucosa is about 
three to four times as broad as the muscle tunic. 
Time for Algae to Pass through Gut 
In order to find out the time which is required 
for algae to pass through the entire alimentary 
canal of the manini, two adults, 123 and 128 
mm. in standard length, were fed only the red 
alga, Polysiphonia , for a period of 2 weeks. The 
tank was cleaned and the diet switched to the 
green alga, Enteromorpha. Two hr. and 25 min. 
later the fish were observed defecating the green 
alga. They had already deposited some on the 
bottom of the aquarium, so the following day 
the experiment was repeated by switching back 
to the red alga. In 2 hr. and 4 min. the first of 
the red alga was voided. In view of the great 
length of the digestive tract, this short interval 
of time seems extraordinary. However, when 
considered in the light of the nearly constant 
diurnal feeding and the volume of algae con- 
sumed, it becomes more understandable. 
The same procedure was repeated for small 
juvenile manini, ranging from 26 to 30 mm. 
in standard length. The Polysiphonia passed 
through their alimentary tracts in 1 hr. and 45 
min. 
At night, when feeding ceases and the fish 
enter a state of torpor, the gut is not completely 
emptied in 2 hours or so. One 39 mm. manini, 
for example, which was caught at 1:15 A.M. 
PACIFIC SCIENCE, Vol. XV, April 1961 
still had a small amount of algae in the stom- 
ach and in the last 26 mm. of the intestine. The 
total length of the alimentary tract of this speci- 
men was 177 mm. A 123 mm. manini was ob- 
served to defecate Enteromorpha 14 hours after 
it last fed on this alga. 
Digestion of Algae 
Lefevre (1940) has shown that different 
kinds of algae resist digestion by gastrointestinal 
juices of fishes in various degrees. Although sup- 
porting growth of manini as well or better than 
the Polysiphonia , the Enteromorpha appeared 
less digested. Filaments of this alga were found 
to be only slightly altered after their rapid pas- 
sage through the gut. The cellulose cell walls 
seemed unaffected, and the cell contents still 
in place. The only discernible change in most 
cells was the breaking up of the parietal grassy 
green chloroplast to brownish-green granular 
clumps. A visual comparison was made of the 
quantity of starch granules in the Enteromor- 
pha cells following staining of fresh and fecal 
algae, and no obvious differences were apparent. 
Clearly, more work is needed to elucidate the 
problem of algal nutrition of the manini. The 
possibility that the pectic sheath material of 
algae is digested should be investigated. 
Digestive Enzymes 
No reference was found in the literature to 
any assay of the digestive enzymes of a strictly 
herbivorous fish. From work on herbivorous an- 
imals other than fishes the generalization can be 
made that proteinase is less active in herbivorous 
forms than in carnivores and amylase more ac- 
tive. There are indications that this is true for 
fishes, at least for amylase. Kenyon ( 1925 ) com- 
pared starch digestion in the carp ( Cyprinus 
carpio) which, though omnivorous, eats large 
amounts of algae, and the carnivorous pike 
( Esox lucius ) . He wrote, . . the carp, which is 
largely a vegetarian, possesses amylase in tre- 
mendous amounts in the hepatopancreas and to 
a less extent throughout the intestinal mucosa. 
The pickerel, on the contrary, . . . possesses only 
a negligible quantity of amylase, having little 
in the pancreas, esophagus, intestine, and prac- 
tically none in the stomach.” Vonk (1927) 
