The Biology and Culture of Tilapias

are little known beyond their native rivers, although T. tholloni has been

much used in laboratory studies.

Table 1. Frequency distribution of vertebrae and dorsal spines in Tilapia zilliiand T.

rendalli.

T. zillii T. rendalli

Vertebrae 27 5

28 33

29 2

30

Dorsal spines XI11 2

XIV 60

XV 467

XVI 63

XW

In the eastern rivers north of the Zambezi Tilapia was never reported
before the widespread introductions of the last three decades. But the fish
fauna of Mozambique and Tanzania was little known and we cannot now be
sure that the presence of T. rendalli in the rivers of Tanzania is not natural.
We do know, however, that Kenyan waters were devoid of this genus until
the introduction of T. zillii.
The tilapias most cultivated are species of Sarotherodon, the mouth-
brooders.
It is now well known that the details of the mouthbrooding habit are not
the same in all mouthbroodihg tilapias (Heinrich 1967; Peters and Berns
1978). The type-species of Sarotherodon, S. melanotheron, is, as it happens,
the most atypical. In nature it lives in brackish water and it is a paternal
mouthbrooder. Many studies of its behavior show that in comparison with
S. mossambicus for example, it has retained vestiges of some of the repro-
ductive habits and wen structures of the substratespawners. So has its
relative in fresh waters, S. galilaeus. When we subdivide the mouthbrooders,
it is these two species (S. melanotheron and S. galilaeus) that typify Sarothe-
rodon in the strict sense, that is, subgenus Sarotherodon. S. mossambicus, S.
niloticus, S. aureus and other east and central African species have a repro-
ductive schema that excludes the males from the care of the brood. They are
in breeding time sexually dimorphic and the female takes the eggs as soon as
they are fertilized to special nursery areas where she holds them in her mouth
until the yolk is sufficiently reduced for them to swim freely. Even then, for
several days she takes them back into her mouth at night or when danger
threatens. The east African maternal brooders have received a second sub-
generic name, the earlier available being Oreochromis (now raised to genus;
see Addendum).
The meaning of Oreochromis is 'mountain cichlid'. (Chromis was an
early name given to both cichlids and a marine genus and is now restricted
to the latter). It was first given to Oreochromis hunteri Giinther (1889), an
inhabitant of a small rocky crater-lake on the slopes of Mount Kilimanjq.
Little is known about the habits of this species, but from its structure