EXPERIMENTAL FISH EMBRYOLOGY 



575 



MATERIALS : 



Biological : Pairs of mature Medakas ( Oryzlaa latipes). 



Plant material for aeration: Nltella, Utrlcularla, Elodea, Cabomta. 

 Food: Living Tutlfex or Enchytrae. 



Dried shrimp, dried Daphnia, small-grained mixed dry fiah food. 



Technical : Aquaria: 1, 5, 10 gallon capacities. 



Fish bowls: Ten cent store variety for pairs of fishes. Wire covers. 

 Celluloid cups for feeding living worms to fish. 

 Feeding ring for dried food. 



Aeration: Stream of air huhbles (not necessary if plants are abundant). 

 Artificial lighting: Gooseneck lamps with 25 watt bulbs if daylight is 

 Inadequate or Irregular. 



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. 



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

 aquarium, or 50-50 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 overheat- 



ing with lamp. 



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



will eat its ovn eggs, larvae, or even the fry. 



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



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 laboratory 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 ovlposition of several females (see Discussion below). On the morning following 

 several days of regular ovlposition, anesthetize a female in I/5OOO MS 222 in aquarium 

 water from 1-2 hours prior to the time of anticipated ovlposition. (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. Onxy conipletely mature eggs will be liberated 

 from their follicles, a process which takes from 20-6o nilnutes. (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 ovlposition, and 

 Is accomplished by the male using his anal and dorsal fins to hold the female while spread- 

 ing milt over the eggs with his vibrating pectoral fins. This occurs after a characteris- 

 tic courtship on the bottom of the tank. It may be possible to delay ovlposition by keep- 

 ing the male away from the egg-laying female until arrival at the laboratory in the morn- 

 ing. 



