Abstract Mummichogs
(
Fundulus heteroclitus) were exposed to various
regimens of temperature and photoperiod in order to develop a
routine husbandry procedure so as to have throughout the year
a population of reproductively healthy fish with a good
quantity and quality of prematuration follicles (1.2–1.5 mm)
suitable for carrying out homologous bioassays of
F.
heteroclitus gonadotropin. During the fall and winter
months, wild fish in the field all had regressed ovaries
(gonadosomatic index <1). On the other hand,
laboratory-maintained fish, with or without cold temperature
(15°C) and short photoperiod (10 h light/day) pretreatment,
generally had sexually mature ovaries (GSI>10) when
maintained on a warm temperature (25°C) and long photoperiod
(14 h light) protocol. Ovarian follicles retrieved from
laboratory fish were responsive to
F. heteroclitus
pituitary extract stimulation, and underwent germinal vesicle
breakdown normally
in vitro. Hence these ovarian
follicles served well as a bioassay for
F. heteroclitus
gonadotropin even outside of the normal breeding season. The
pituitary glands retrieved from laboratory fish in winter also
retained high gonadotropic potencies, in terms of maturational
and steroidogenic activities. Our results thus demonstrated
that active gametogenesis in laboratory-maintained fish can be
extended five months beyond the end of the normal breeding
season. Apparently,
F. heteroclitus in Florida is
potentially a continuous breeder when under favorable
conditions, but has a reproductive quiescent period imposed
upon it by some environmental stressor(s). Although the design
of the present experiments did not determine the relative
importance of nutritional factors, temperature, and
photoperiod on the annual reproductive cycle of
F.
heteroclitus, there are indications that diatary factors
may play a much more dominant role in the reproductive cycle
than previously recognized.