Biology Reference
In-Depth Information
is intermediate, the fi sh may have a fi xed age, at which they mature. There
are also other predictive models, which account for 60% variations in the age
of fi rst reproduction for 31 populations representing 23 species (Roff, 1992).
A large body of literature is available on variations in age and size at which,
50% of the collected sample fi shes are identifi ed to have attained sexual
maturity. Table 18 lists such variations reported for a few selected examples
of fi shes, which are known for their different modes of spawning.
Causes for such wide variations in age/size at maturity are related
to social and/or environmental factors. Age at puberty can be delayed or
advanced, depending on the environmental conditions; for instance, both
age and size at puberty have been reduced in several heavily exploited
populations (e.g. Engelhard and Heino, 2004, Olsen et al., 2005; see also
Pandian, 2010), indicating the phenotypic plasticity or genetic changes due
to size selective fi shing and thereby reducing both age and size at puberty
(Dieckmann and Heino, 2007). A reason for sexually dimorphic growth and
differences between size and age at puberty (Table 18) has been traced to
different relationships between body size and reproductive success in males
and females (Taranger et al., 2010). Selection often favors early maturing
individuals, because their higher survival to maturity and relatively
greater contribution to population growth. But it may also favor delayed
maturation, when fi tness increases with age/size at maturity (Schreibman
and Kallman, 1978). A sex linked gene called
P-locus
is known to primarily
control the age/size at maturity. On maturity, body growth of poeciliid
males is almost arrested; consequently, poeciliid males are usually 60-80%
the length of the female and are more slender (Bisazza, 1997). Therefore,
the age/size at maturity of poeciliids has attracted a lot attention to assess
the role of the social environment on
P-locus.
Kolluru and Reznick (1996)
designed two long term experiments to assess the inheritance of age and
size of males at maturity. In the poeciliid,
Phallichthys quadripunctatus
males
attain sexual maturity either at a small size of 15 mm body length or 19 mm
size. Briefl y, large males sired early and delayed maturing sons; likewise,
the smaller males too sired delayed and early maturing sons. Hence the
size at maturity in this poeciliid is more effectively controlled by social
environmental factors than
P. locus
.
Temperature is a very important factor in determining growth and
maturation. For instance, the European cod
Gadus morhua
matures at the age
of 3+ in the relatively warmer British waters but in colder Barents Sea within
Arctic Circle, it begins to mature at the age of seven years (see Wooton, 1998).
In this context, a recent publication by Bernet et al. (2009) is interesting. In a
commendable description, they provide the chronological sequence of events
in gonad differentiation upto maturation of females and males as functions
of age and size with due consideration to temperature in the whitefi sh
Coregonus lavaretus.
From Fig. 21, selected events in sexual differentiation are
