Overview of Tilapia Life HistoriesThe question of reproductive guilds is an obvious one to apply to tilapias.
Several authors have proposed categorizations of these fishes on this basis
(e.g., Baerends and Baerends-van Roon 1950; Wickler 1966a; Barlow 1974;
Keenleyside 1979). The usual distinction has categorized the cichlids,
including especially tilapias, as either substratespawning or mouthbrooding
species (Lowe-McConnell 1975). The distinctions are obvious and clear, with
few if any ambiguities assigning species to either category. The only problem-
atic species are those few which behave initially as substrate-spawners(i.e., they
do not take the fertilized eggs into the buccal cavity but direct custodial care
towards embryos outside the body), but subsequently (up to as long as a few
days later) take the embryos into the mouth and complete the cycle in a
fashion essentially similar to that of a typical mouthbrooding species (Timms
and Keenleyside 1975; Keenleyside 1979).
Further elaborations of these schemes include considerations as to whether
young are taken back into the parent's mouth after they are first released (in
mouthbrooding species), and the precise location of spawning. These generally
assume (usually implicitly) that release from the mouth or hatching from the
egg envelope(s) is a significant event for interspecific comparisons in onto-
geny. However, as we have shown, this is misleading, as these events are not
involved in defining any of the developmental intervals.
The distinction between mouthbrooding and substrate-spawning habits is
also significant in taxonomic considerations of tilapiine fishes (Trewavas
1973a, 1978, this volume), consistent with the generic divisions of Saro-
therodon and Tilapia, respectively. However, if mouthbrooding has evolved
independently a number of times from nest-spawning guarders, it may not
represent an evolutionary trend.
In our scheme of reproductive guilds, the substrate-spawning species are
categorized as nest-spawning guarders (guarders, for convenience). The
mouthbrooders are mouthbrooding external bearers (bearers, for con-
venience). There is little significant difference between the (usually) accepted
classification of reproductive styles of these fish (Barlow 1974) and our
guilds. The (apparently) intermediate species have been omitted at present,
for lack of detailed information to decide on their guild. The utility of the
guild concept extends beyond tilapias, however, so our adherence to it is
more than just an attachment to our particular terminoloy.
The life histories of the tilapias can be profitably reviewed from the
combined model of ontogeny and reproductive guilds. The close association
of these, and their apparent relationship to other ecologic'al aspects of life
histories have been described for a hbeotropheus species (Balon 1977) and
cham (Salvelinus species; Balon 1980). There is good reason, both empirical
and theoretical, for believing that heterochronous shifts in early development
have been of major importance in the evolution of not only life history
styles, but also consequently of independent species (Balon 1980, 1981a).
This could be a plausible mechanism for sympatric speciation, perhaps to
account for some of the endemism in African cichlids (Fryer and Iles 1969,
1972). We will return to this suggestion later.
We have surveyed published life history data for tilapias, especially as