The reproductive physiology of yellowtail flounder, Pleuronectes ferrugineus, with an emphasis on sperm physiology

Clearwater, Susan Jane (1996) The reproductive physiology of yellowtail flounder, Pleuronectes ferrugineus, with an emphasis on sperm physiology. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Male and female yellowtail flounder Pleuronectes ferrugineus were individually identified, monitored and maintained in captivity for two reproductive seasons during which time both sexes produced viable gametes. Female spawners had elevated plasma levels of 17β-estradiol (E₂) in November indicating vitellogenesis had commenced; plasma E₂ continued to increase until the beginning of the spawning season in June. Plasma testosterone (T) increased just prior to the spawning season, peaked after plasma E₂ and decreased by the end of the spawning season in August. -- Despite the year-round presence of milt with motile sperm, male yellowtail flounder showed a distinct seasonal change in plasma 11-ketotestosterone (11KT). 11KT peaked at the beginning of the spawning season when milt volumes were highest but decreased while milt volumes were still elevated. Plasma T showed a less distinct seasonal cycle than 11KT and peaked later in the season. -- Compared to control treated fish, gonadotropin releasing hormone-analogue (GnRHa) treatment delivered either by microspheres or cholesterol pellets successfully increased sperm production and milt volume in mature male yellowtail flounder during the spawning season. Sperm production remained high for 4 weeks after implantation of GnRHa and then decreased in all treatments (including the controls), indicating milt volume increased due to an initial increase in sperm production and remained high due to milt hydration. Plasma levels of T, 11KT and 17α,20β-dihydroxy-4-pregnen-3-one showed no clear pattern of response to GnRHa treatment. GnRHa treatment did not have a negative effect on sperm fertilizing ability, percentage hatch or larval appearance. Sperm motility and seminal plasma pH was increased by GnRHa treatment. -- Several aspects of yellowtail flounder sperm physiology were examined. Sperm were activated in buffer with an osmolality greater than 367 mOsm (pH 8.2-8.8) and diluted seawater with an osmolality greater than 387 mOsm (pH 7.6-8.2). Percentage sperm activated and swim times increased as osmolality increased up to approximately 800 mOsm. Changing seawater pH between 4.8 and 9.0 had minimal effect on sperm motility and no optimal pH was observed. Spectrophotometry was developed as a method to rapidly measure the sperm concentration of yellowtail flounder milt (1.26 × 10¹⁰ ± 0.10 × 10¹⁰ cells ml⁻¹ ). Cool storage (4°C) of milt diluted 10 fold in buffer with or without antibiotic successfully maintained sperm fertility and motility for seven days and had no adverse affect on egg hatching rates or larval appearance. Rapid dilution of urine contaminated milt in buffer, mitigates the negative effects of urine contamination on sperm motility and fertility.

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