168
C. F. HICKLWG
when very young fry, feed wholly on plankton when they first enter
the limans in April. In May, they begin to take zoobenthos, and in
June, when the fry are growing into fingerlings, zooplankton drops to
27% while zoobenthos rises to 51% of their food. I n July, zoobenthos
falls to 3y0, while 97% consists of microbenthos. Yashouv and BenShachar (1967) found that the young fry of Mugil cephalus feed freely
on copepods, chironomid larvae and cladocerans. Microcystis and
various diatoms were found whole and undigested in the rectum.
Pillay (1954) found that Mugil tade (Forsk.) is an " iliophage ",
because decayed organic matter, filamentous algae and diatoms form
its main food, apart from inorganic soil. But very small M . tade feed
mainly on blue-green algae, and fry of 2.1 to 4.6 cm begin to feed on
detritus and also on important numbers of small crustacea such as
copepods, cladocerans and mysids. At this stage, sand or mud is not
eaten, so it would seem that the relatively short gut, like that of a
carnivore, which the fish has when very small, has an enzyme system
able to cope with this kind of food.
Odum (1968), working on M . cephalus in Georgia, found that the
fish takes its food by sucking up the surface layer of the mud or by
grazing on submerged surfaces such as Zostera leaves or stones. He
found that the major constituents of the stomach contents were (i)
microalgae, including epiphytic and benthic diatoms, dinoflagellates
and green and blue-green algae, (ii) decaying plant detritus, and (iii)
inorganic sediment particles. He regards the microalgae as the main
source of nutrition, though plant debris, no doubt much enriched by
bacteria, also appears to be important.
Ghazzawi (1933) found that, even when he examined fish from water
where planktonic diatoms were abundant, they were not feeding on
these but on littoral diatoms and algae with epiphytic diatoms. Sand
grains were always present in the gut.
Hickling (1970) found that the gut contents of Crenimugil labrosus
in English waters included only 5-20y0 of organic matter, the rest
being soil. Of this small proportion of organic matter, detritus was
always an important part, but equally important were benthic diatoms
such as the naviculoids, blue-green algae and small animals such as
harpacticoid copepods and nematodes. The very long gut present in
this species increases in length relative to the total length of the fish,
from 3 times in fish of 20 cm to 5 times in fish of 50 cm. This, and
the fast rate of passage of food through the gut, are probably
adaptations for the digestion of food of very poor quality, and not an
adaptation to a herbivorous habit.
Tilapia mossambica also feeds on the algal pasture. There are many
C. F. HICKLWG
when very young fry, feed wholly on plankton when they first enter
the limans in April. In May, they begin to take zoobenthos, and in
June, when the fry are growing into fingerlings, zooplankton drops to
27% while zoobenthos rises to 51% of their food. I n July, zoobenthos
falls to 3y0, while 97% consists of microbenthos. Yashouv and BenShachar (1967) found that the young fry of Mugil cephalus feed freely
on copepods, chironomid larvae and cladocerans. Microcystis and
various diatoms were found whole and undigested in the rectum.
Pillay (1954) found that Mugil tade (Forsk.) is an " iliophage ",
because decayed organic matter, filamentous algae and diatoms form
its main food, apart from inorganic soil. But very small M . tade feed
mainly on blue-green algae, and fry of 2.1 to 4.6 cm begin to feed on
detritus and also on important numbers of small crustacea such as
copepods, cladocerans and mysids. At this stage, sand or mud is not
eaten, so it would seem that the relatively short gut, like that of a
carnivore, which the fish has when very small, has an enzyme system
able to cope with this kind of food.
Odum (1968), working on M . cephalus in Georgia, found that the
fish takes its food by sucking up the surface layer of the mud or by
grazing on submerged surfaces such as Zostera leaves or stones. He
found that the major constituents of the stomach contents were (i)
microalgae, including epiphytic and benthic diatoms, dinoflagellates
and green and blue-green algae, (ii) decaying plant detritus, and (iii)
inorganic sediment particles. He regards the microalgae as the main
source of nutrition, though plant debris, no doubt much enriched by
bacteria, also appears to be important.
Ghazzawi (1933) found that, even when he examined fish from water
where planktonic diatoms were abundant, they were not feeding on
these but on littoral diatoms and algae with epiphytic diatoms. Sand
grains were always present in the gut.
Hickling (1970) found that the gut contents of Crenimugil labrosus
in English waters included only 5-20y0 of organic matter, the rest
being soil. Of this small proportion of organic matter, detritus was
always an important part, but equally important were benthic diatoms
such as the naviculoids, blue-green algae and small animals such as
harpacticoid copepods and nematodes. The very long gut present in
this species increases in length relative to the total length of the fish,
from 3 times in fish of 20 cm to 5 times in fish of 50 cm. This, and
the fast rate of passage of food through the gut, are probably
adaptations for the digestion of food of very poor quality, and not an
adaptation to a herbivorous habit.
Tilapia mossambica also feeds on the algal pasture. There are many