The Biology and Culture of Tilapias

fluvial species invading a recently-inundated area (but see Philippart and

Ruwet, this volume). The key point is the saltatory nature of development

we have described. In a very real sense, these fish can have the best of two

worlds and both life styles. Through heterochronous shifts in development

during ontogeny they can quickly adapt towards a more altricial mode to

take advantage of new habitats (e.g., seasonally flooded lagoons), or revert

to a more precocial mode under other circumstances (e.g., more stable, long

term conditions in large lakes, high competitive diversity of fish tam) while

remaining in the same reproductive guild.

Tilapiine Life Histories and Ontogenetic Inferences

The relevant life history data for guarding and bearing tilapias under
natural conditions are summarized in Table 2. There is clearly a correspon-
dence between the reproductive guild (and, hence the genus; Trewavas 1973a,
1973b, this volume) and other aspects of the life history style. Guarders are
generally more altricial, bearers more precocial. There can be little doubt
that the bearers are more specialized, and almost certainly derived from ances-
tral guarding species (Fishelson 1966a; Peters 1965; Lowe-McConnell1959;
TrewavaB 1973a, this volume). The guarders are more primitive, also, as
proposed in the system of reproductive guilds (Balon 1975b, 1981b). As
already mentioned, bearing may have evolved several times independently
among the tilapias (Trewavas, this volume), and if so it-cannot likely be the
basis for taxonomic distinctions. In any event, the multiple evolution of
bearers is consistent with our hypothesis of altricial and precocial "twin
forms".
Comparisons between tilapias in the two different guilds are informative,
with at least some confirmation of our predictions. There are some apparent
exceptions however. These seem to be the result of adaptive shifts in life
history features within each guild, causing the ranges of certain values to
overlap between guilds. This will become more apparent when we discuss

Table 2. Comparison of tilapiine species in natural populations*, according to reproductive

guilds (modified from Fishelson 1966a; Fryer and Iles 1972; Peters 1973;Lowe-McCon-

nell1975; Balon 1975b, 1977 ; Balarin and Hatton 1979).

Characteristic Guarders Bearers

Fecundity (fertility)

Egg diameter (mm)

Yolk% lipid

Yolk color

Size at onset of feeding (mm)

Age at first reproduction (years)

Total length at first reproduction (cm)

Courtship

Juvenile mortality

Longevity (years)

up to 7,000 eggs

1-1.5

less than 25

clear, pale yellow

5-6

0.5-2

up to 20

prolonged, monogamous

relatively higher

up to 7

up to 1,000 eggs

up to 5

up to 45

opaque orange

9-1 0

1-4

up to 40

brief, polygamous

relatively lower

up to 9

*Extreme values associated with fii stocked in non-native conditions, held in intensive

rearing facilities, or other exceptional circumstances have been excluded.