THE EFFECTS OF TEMPERATURE AND PHOTOPERIOD ON REPRODUCTIVE CYCLING IN THE ESTUARINE GOBlID FISH, GILLICHTHYS MIRABILlS

Investigations were undertaken at several different times during the year to examine the effects of various photoperiods and constant-temperature regimes on reproductive function in the longjaw goby, Gillichthys mirabilis, with the intent of evaluating the influence of these factors in regulation of the annual sexual cycle. Testicular regression occurs at any time during the year when fish are exposed to constant temperature of 24°C and above, independent of photoperiod. Similar results were obtained with female fish, but 22°C is the thermal threshold. It is concluded that the gonadal regression ob­ served in the Alviso population of this species during the summer months is a conse­ quence of increasing temperature. At high temperatures, the transformation of sper­ matogonia to spermatocytes is blocked, and in females vitellogenesis is inhibited. The degree of gonadal regression is temperature-dependent. Gonadal recrudescence is de­ pendent on low temperatures (10°-20°C) and will not occur if fish are exposed to high temperatures (24°C or above) regardless of photoperiod. At low temperatures, short photoperiods accelerate recrudescence. Between January and June spermatogenesis and oogenesis are maintained at temperatures between 10° and 21°C; lorig photoperiods are more effective in this respect, but not essential. Termination of the reproductive season in this species is not endogenously timed. Regression is not "obligatory" since gOllltdal involution does not occur at the "normal" time if fish are exposed to temper­ atures of 20°C or below. The survival of any species in a seasonally chang­ ing environment is dependent on the develop­ ment of mechanisms that permit it to adjust physiological functions to changes in the envi­ ronment. Studies of reproductive timing and how the environment influences this timing are of importance in understanding the ecology of any species. Compared with the wealth of information available on the systematics, ethology, and physiology of fishes, there is little knowledge concerning how external factors regulate their reproduction. Some investigations have been undertaken to elucidate the role of environmental factors in regulating the reproductive cycles of various teleosts. The relationship of environ