THE EFFECTS OF TEMPERATURE AND PHOTOPERIOD 

 ON REPRODUCTIVE CYCLING IN THE ESTUARINE GOBIID FISH, 



GILLICHTHYS MIRABILIS 



Victor L. De Vlaming^ 



ABSTRACT 



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; long 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 

 gonadal 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- 



* Department of Zoology, University of California, 

 Berkeley, Calif.; present address: Department of Bi- 

 ology, Wehr Life Sciences Building, Marquette Univer- 

 sity. Milwaukee, WI 53233. 



Manuscript accepted May 1972. 



FISHERY BULLETIN: VOL. 70, NO. 4, 1972. 



mental factors to the reproductive cycles of 

 gobies has not received experimental consider- 

 ation. Moreover, experimental work with the 

 environmental control of teleost reproductive 

 cycles has been confined to fewer than 20 species 

 representing only 8 families. 



Photoperiod and temperature are presumed 

 to be the most important factors (i.e., the most 

 studied) influencing the neuroendocrine centers 

 that control gonadotropin secretion in teleosts 

 (de Vlaming, 1972a). The experimental condi- 

 tions employed in a majority of the previous 

 studies, however, are diverse and the results 

 contradictory. In fact, most of the experimental 

 work was too poorly controlled and too brief 

 in duration to allow proper assessment of the 

 role of the environment in synchronizing fish 

 reproduction (de Vlaming, 1972a). 



The subject of the present study is the long- 

 jaw goby, Gillichthys mirabilis. It is distributed 



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