FISH FOR EXPERIMENTATION 363 



METHOD: 



Precautions: 



1. See general precautions in introduction to Fish studies. 



2. Keep aquaria covered, the fish may jump out. 



3. Do not use chlorinated tap water until it has been properly conditioned. 



4. Avoid crowding: A pair of fish in a single aquarium, a dozen in a 5-gallon 

 aquarium, or 30-50 in a 10-gallon tank is a good proportion. 



5. Optimum temperature about 25 C. to 27 C. 



6. Northern daylight best, but artificial lighting is satisfactory, avoid over- 

 heating with lamp. 



7. Remove eggs from the aquarium, or pick eggs off of the female, since the 

 Medaka will eat its own eggs, larvae, or even the fry. 



8. Light, food, temperature and pH are controlling factors in breeding, impor- 

 tant in that order. 



Controls: This exercise is one of observation rather than experimentation hence 



there is no need for a control. It must be remembered, however, that labora- 

 tory conditions may not represent conditions of the normal environment of the 

 fish. If, however, fish can be raised to sexual maturity under a certain set of 

 laboratory conditions, it may be assumed that these conditions closely simulate 

 those in nature. 



OVULATION, AND FERTILIZATION: 



Oryzias latipes should be secured fresh from the dealer and kept for 2-3 weeks under 

 uniform conditions and close observation in order to select and have available females 

 that may be depended upon to provide fertile eggs almost daily. 



Since Oryzias eggs are ovulated shortly before being laid, determine the approximate 

 time of oviposition of several females (see Discussion below). On the morning following 

 several days of regular oviposition, anesthetize a female in 1/3000 MS 222 in aquarium 

 water from 1-2 hours prior to the time of anticipated oviposition. (The lighting condi- 

 tions will regulate this quite satisfactorily. ) Pin the female, belly down, in a Permo- 

 plast operating dish filled with aquarium water, and remove the dorsal and lateral body 

 wall. This will expose the dorsal side of the ovary through which ripe eggs may be seen. 

 Observe the eggs in situ, or excise the entire ovary and place in normal aquarium water 

 and observe under binocular magnification. Only completely mature eggs will be liber- 

 ated from their follicles, a process which takes from 20-60 minutes. (Compare the 

 process with that previously observed in the other cold-blooded form, e. g. , the frog. ) 



The fertilization of the egg occurs at the time of or shortly after oviposition, and is ac- 

 complished by the male using his anal and dorsal fins to hold the female while spreading 

 milt over the eggs with his vibrating pectoral fins. This occurs after a characteristic 

 courtship on the bottom of the tank. It may be possible to delay oviposition by keeping 

 the male away from the egg-laying female until arrival at the laboratory in the morning. 



EARLY DEVELOPMENT OF ORYZIAS LATIPES: 



In an aquarium containing several dozen fish you will find eggs (1-80 per female) every 

 morning. Since the eggs are laid shortly after daylight these observations must be made 

 very early in the day. 



Using a dip-net, catch a female which has a cluster of eggs attached and transfer it to a 

 finger bowl of aquarium water. Pick off the eggs either with a wide-mouthed pipette or a 

 pair of forceps. Eggs which appear to be opaque within 10 minutes after removal are 

 very likely to be dead. Separate the eggs so that there are about 10-20 per finger bowl. 

 Keep no vegetation with the eggs. 



