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Reproductive biology and endocrinology of the greenback flounder Rhombosolea tapirina (Gunther, 1862)

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Barnett, Carolyn Wendy (1998) Reproductive biology and endocrinology of the greenback flounder Rhombosolea tapirina (Gunther, 1862). PhD thesis, University of Tasmania.

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Abstract

The greenback flounder Rhombosolea tapirina (Pleuronectidae) is distributed around
Southern Australia and New Zealand. There is growing interest in developing an
aquaculture industry for greenback flounder in Tasmania and pilot aquaculture schemes
are already underway, however, current programs are limited by unreliable egg
production. This study provided baseline information on reproductive events under
normal reproductive conditions in wild greenback flounder, the stress response of
greenback flounder to common husbandry and laboratory aquaculture practices,
developed induced ovulation protocols, examined the mechanism of action of hormone
treatment and assessed in vitro effectiveness of putative maturational steroids on
ovarian tissue.

Oocyte size frequency distributions showed that female greenback flounder are
multiple ovulators and have group synchronous oocyte development. Plasma and
ovarian levels of testosterone (T) and 17β-estradiol (E2) were elevated in association
with vitellogenesis. Plasma levels of 17α20β-dihydroxy-4-pregnen-3-one (17,20βP),
and 17,20βP-sulphate but not 17,20βP-glucuronide were significantly elevated in
ovulated females, whereas ovarian levels of 17,20βP were elevated in association with
final oocyte maturation (FOM) and ovulation.
Changes in macroscopic testis stage were characterised by few changes in proportions
of gamete types between gonad stages, indicating low milt production, even in mature
fish. Plasma levels of T were elevated in association with spermatogenesis, but not
spermiation. No significant changes in plasma 11-ketotestosterone (11KT ), 17,203P,
17,20βP-sulphate and 17,20β-glucuronide were detected with change in gonad stage
of males. The latency of the plasma cortisol response to stress was approximately 10 min.
Plasma levels of cortisol were significantly higher in wild fish sampled after capture,
confinement and transport, and some routine husbandry practices had the capacity to
stimulate elevated cortisol levels for up to 48 h. Hematocrit (Hct) did not change
significantly in response to stress, suggesting that either Hct is not effected by stress in
this species, or changes in Hct were not detected within the sampling protocol. After exercise, muscle lactate did not significantly change, however, plasma lactate and
muscle [11+] significantly increased and plasma [H+] significantly decreased, indicating
that muscle and blood physiology of greenback flounder do change in response to
exercise, but unlike other flatfish, there was little evidence for in situ glycogenesis
within white muscle tissue.
Female greenback flounder were induced to repeat ovulate using a range of exogenous
hormone treatments. Des Gly10 [D-Ala6] LHRH ethylamide (LHRH-a) and human
chorionic gonadotropin (hCG), significantly increased the number of ovulations above
control levels. Co-administration of LHRH-a and the dopamine antagonist PIM
provided no significant advantage over treatment with LHRH-a alone. Dopamine
appeared to have an inconsistent effect on reproductive function in greenback flounder.
Co-treatment of LHRH-a + T significantly enhanced the ovulatory effects of LHRH-a,
suggesting steroid feedback enhances pituitary responsiveness to GnRH.
In vitro bioassays indicated that ovarian fragments required pre-treatment with hCG
before they were receptive to steroids. In most cases, the maturational response to
steroids after priming with hCG exceeded the maturational response to hCG treatment
alone. Ovarian fragments were receptive to all steroids at all concentrations tested.
There was considerable inconsistency in maturational responses to each steroid and
each dose, however, 20a and 20β- hydroxylated steroids were most effective at
inducing maturation, and 5-pregnene and 5β-pregnane steroids were least effective.

Item Type: Thesis (PhD)
Keywords: Fishes, Rhombosolea tapirina, Fishes
Copyright Holders: The Author
Additional Information:

Thesis (PhD )--University of Tasmania, 1998. Includes bibliographical references

Date Deposited: 25 Nov 2014 00:48
Last Modified: 11 Mar 2016 05:55
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