Biology Reference
In-Depth Information
Smith, 1990),
D. marginatus
(Shpigel and Fishelson, 1986) and
D. carneus
(Asoh and Yoshikawa, 2003). The sequence of gonadal differentiation in
D. fl avicaudus
proceeds from ovarian stage I to stage II and to the formation
of ovarian lumen, a stage corresponding to non-vitellogenic peri-nuclear
stage in zebrafi sh (Wang et al., 2007). More or less the same sequence of
gonadal differentiation has also been reported for
D. albisella, D. trimaculatus,
D. aruanus
and
D. carneus.
From a detailed study on the distribution of
mixed stage (consisting of both ovary and testis at different stages) and
other gonadal categories with increasing size of
A. fl avicaudus
from the
northern and southern Vaipahu outer-reef populations in Moorea of French
Polynesia, Asoh (2004) has come to the following conclusions: 1. Beyond the
ovarian stage II,
D. fl avicaudus
may follow further gonadal differentiation
in one of the two pathways: (a) Toward mature testis through degeneration
of oocytes and development of spermatogenic tissue or (b) Toward mature
ovary through growth of oocytes—very similar to the testicular and ovarian
differentiation in the presumptive males and females of zebrafi sh. 2. (a) With
the scarcity or absence of mature males in northern Vaipahu population,
most fi sh with mixed stage of gonads postpone functioning as pure male,
i.e., a sort of Okinawan hermaphrodism is maintained in which male-active
hermaphrodites function as males and (b) With female's maturity at the
smaller sizes of 3.2 cm in northern Vaipahu (as against > 5.2 cm in the
southern Vaipahu and outer-reef populations),
D. fl avicaudus
may prove to
be a functional protogynic hermaphrodite. Asoh (2004) has also not ruled out
the occurrence of sex change from vitellogenic oocytes to testis. However, it
must also be noted that the evolution of sequential hermaphroditism from
Dascyllus-
like gonochorism is not a one-way traffi c; there are also instances
of hermaphroditic sparid
Leptoscarus vaigiensis
returning to gonochorism
(see Pandian, 2010, 2011).
3.4 Structural and temporal costs
Simultaneous hermaphroditism is a rarity among vertebrates. It occurs
only among 14 species (i.e., 0.05% species of fi shes). It has to bear the costs
of investments in morphological apparatus and functional mechanism
of both sexes; competitive superiority of pure males could easily render
hermaphroditism evolutionarily unstable (see Pandian, 2010).
Sex change among hermaphroditic fish is also a relatively rare
phenomenon, and occurs only in 4% teleost families, i.e., 19 out of about
450 teleost families and includes about 109 (0.4%) species (Table 25). In
these teleosts, sex change involves gonad, gonadal ducts and in some cases,
secondary sexual characters (e.g., second ventral fi n ray of
Cirrhilabrus
temmincki;
anal fi n color,
Pseudolabrus sieboldi,
see Pandian, 2010). Sex
change is too costly that an α female changes to male only in the absence
