The environmental control of oocyte development in the striped mullet, Mugil cephalus

Abstract

The environmental control of oocyte development in the striped mullet, Mugil cephalus, was investigated in 3 studies. The first examined individual variation in oocyte development under natural conditions. Data on oocyte stage, oocyte growth rates, and fecundity were collected during three reproductive seasons. Females initiated the cortical vesicle stage in September-October and vitellogenesis in October-December. The average clutch required 65 days to complete vitellogenesis and had an oocyte growth rate of 6.5 μm/day. Approximately 60% of the females initiated a single clutch while 40% initiated 2 or more clutches per season. Clutch fecundity ranged between 418,000-3,008,200 eggs and was related to female length and weight. The second study examined the effects of four combinations of photoperiod and temperature (short/cold, short/warm, long/cold, and long/warm) on oocyte development. Trials were conducted with females during the refractory period, and with females that had primary growth stage, cortical vesicle stage, and vitellogenic stage oocytes. In general, the short/cold combination stimulated, while the long/warm combination inhibited oocyte development. Short photoperiod stimulated the onset of the cortical vesicle stage in cold or warm temperatures while cold temperature stimulated the onset of vitellogenesis under short or long photoperiod. Histological examination of oocytes found that the migration of the yolk nucleus of Balbiani's vitelline body occurred in the short/cold, but not in the long/warm combination. This is the earliest event during oocyte development in any animal shown to be influenced by environmental factors. A hypothesis is also presented to explain differences in M cephalus reproductive seasons throughout the world. The third study examined the effects of melatonin (MLT) on oocyte development, The first of two experiments tested the effects of MLT implants under either natural, inhibitory, or stimulatory photoperiod. The second experiment examined the effects of afternoon administration of MLT feed. While both of these techniques are used to advance the reproductive cycles of mammals, neither implants nor MLT feed affected oocyte development in M. cephalus. The results of these studies have led to the development of hatchery techniques to spawn this species throughout the year.