CHEN and MARTINICH: OVULATION IN ZEBRAFISH 



same pond, the inhibiting metabolites cannot be 

 species specific. 



As discussed earlier, many of the freshwater 

 fishes in the tropics spawn only in the rainy season. 

 During this period, there is an addition of flooded 

 lowland suitable for the deposition of eggs, an 

 increase in the dissolved oxygen favorable for 

 embryological development, and an increase of 

 organic and inorganic nutrients which promote 

 growth of food plankton. Rain would also dilute 

 any metabolic wastes accumulated during the dry 

 season. In this context, metabolites may serve as a 

 controlling factor, repressing ovulation until the 

 rainy season when environmental conditions are 

 more favorable for both embryo development and 

 larval growth. 



The results of the present study clearly indicate 

 the stimulatory effect of the male pheromone and 

 the inhibitory effect of metabolites on ovulation in 

 the zebrafish. As gonadotropin is known to be ef- 

 fective in inducing ovulation in fishes, either 

 directly, or via stimulating the synthesis of cor- 

 ticosteroids and/or progesterone (Donaldson 1973; 

 de Vlaming 1974), the action of the ovulating 

 pheromone and the metabolites is probably to ac- 

 tivate or to deactivate the hypothalamus-pitui- 

 tary-gonad axis. A pheromonal facilitation of 

 gonadotropin-induced ovulation has been reported 

 in mouse (Zarrow et al. 1973). Further studies are 

 needed to clarify the route of action of the 

 pheromone and the metabolites. 



Aronson (1965) cited numerous examples in 

 fishes in which gonadal development and sub- 

 sequent spawning were stimulated by either an 

 increase or a decrease in temperature. An increase 

 in temperature has been reported to affect the 

 gonadal response to treatment with gonadotropin 

 in Lepomis ct/aneUus by Kaya (1973) and in 

 GiUichthys mimhilis by de Vlaming (1972c). In 

 the present study, a sudden decrease in tempera- 

 ture alone does not seem important in stimulating 

 ovulation in the zebrafish. 



In the zebrafish, visual or auditory and lateral 

 line stimuli between sexes do not seem important 

 in enhancing ovulation, although some of these 

 factors may be pertinent in eliciting the proper 

 behavior during the actual spawning act. 



The onset of light alone is not sufficient to 

 stimulate ovulation, as demonstrated by the 

 complete failure to strip eggs during the morning 

 hours from females tested directly from their 

 holding compartments (experiment 8). Further- 

 more, these females were stimulated to ovulate 



later that day, after exposure to the male 

 pheromone and fresh tap water. Ovulation and 

 natural spawning were induced regardless of time 

 of day. One of us (Chen) has observed natural 

 spawning of zebrafish to commence at midnight in 

 darkness and continue for hours. These observa- 

 tions conflict with all previous accounts that the 

 onset of light is important to trigger ovulation and 

 spawning in the zebrafish (Legault 1958; Hisaoka 

 and Firlit 1960; Eaton and Farley 1974). 



LITERATURE CITED 



Amouriq, L. 



1965. Origine de la substance dynamogene emise par 

 Lebisfes reticulatus femelle (Poisson Poeciliidae, 

 Cyprinodontiforme). C. R. Acad. Sci. (Paris) 

 260:2334-2335. 

 Aronson, L, R. 



1945. Influence of the stimuli provided by the male cichlid 

 fish, Tilapia macrocephala , on the spawning frequency of 

 the female. Physiol. Zool. 18:403-415. 



1965. Environmental stimuli altering the physiological con- 

 dition of the individual among lower vertebrates. In F. 

 A. Beach (editor), Sex and behavior, p. 290-318. Wiley 

 and Sons, N.Y. 



Brawn, V.M. 



1961. Sound production by the cod (Gadun callarias 

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 Breder, C. M., and D. E. Rosen. 



1966. Modes of reproduction in fishes. Natural History 

 Press, Garden City, N.Y., 941 p. 



Chien.A. K. 



1973. Reproductive behavior of the angelfish PferophyUum 

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 21:457-463. 



DE Vlaming, V. L. 



1972a. Environmental control of teleost reproductive cycles: 

 A brief review. J. Fish Biol. 4:131-140. 



1972b. The effects of temperature and photoperiod on 

 reproductive cycling in the estuarine gobiid fish, 

 GiUichthyii mirahilis. Fish. Bull., U.S. 70:1137-11.52. 



1972c. The role of the endocrine system in temperature- 

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 fish, Gillichthys wirabilis. Comp. Biochem. Physiol. 

 41A:697-713. 



1974. Environmental and endocrine control of teleost 

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Donaldson, E. M. 



1973. Reproductive endocrinology of fishes. Am. Zool. 

 13:909-927. 



Eaton, R. C, and R. D. Farley. 



1974. Spawning cycle and egg production of zebrafish, 

 Brachydanio rerio, in the laboratory. Copeia 

 1974:195-204. 



Egami, N., and M. Nambu. 



1961. Factors initiating mating behavior and oviposition in 

 the fish, Oryzias latipes. J. Fac. Sci. Univ. Tokyo, Sect. 

 IV, Zool. 9:263-278. 



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