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tions (as indicated by gonadosomatic index, or GSI) are in keeping with sperm-competition predictions: initial-phase males produce lots of sperm while terminal-phase
males only produce a little. GSI refl ects the level of sperm competition experienced
by the two male types: initial phase males spawn in multi-male groups where sperm
competition is intense; terminal phase males spawn in pairs where sperm competition is relaxed (Robertson and Warner 1978 ; Warner and Robertson 1978 ) (Fig. 3.5 ).
[See also Chap. 8 .]
0
4
8
J
F
M
A
M
J
J
A
S
O
N
D
GSI
Month
a
b
0
1
2
J
F
M
A
M
J
J
A
S
O
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D
GSI
Month
Fig. 3.4 Gonadosomatic index ( GSI , gonad weight/body weight * 100) of species ( a ) with, brown
surgeonfi sh ( Acanthurus nigrofuscus ), and ( b ) without, gag grouper ( Mycteroperca microlepis ).
Higher GSI indicates larger gonad size relative to body weight and refl ects greater exposure to
sperm competition. Main spawning seasons are February and March for gag grouper and May to
August for brown surgeonfi sh. Females open circles ; males closed squares (Adapted from
Fishelson et al. ( 1987 ) , Hood and Schleider ( 1992 ) )