FISH FOR EXPERIMENTATION 357 



Betta fry are extremely difficult to carry through the first three weeks of development. 

 This may be in part due to injuries sustained when the male parent rescues them and 

 returns them to the bubble nest after they hatch. The food for such fry should be Proto- 

 zoa (Infusoria) cultured in a separate tank and added to the fry tank along with a pitch of 

 the smallest grain fish fry food. This food will allow the Protozoa to propagate within 

 the fry tank, so that it will not be necessary to add fresh Protozoa more than twice a 

 week. The fry tank temperature should be 80° F. There should be adequate light (75 

 watt bulb within 6-8 inches of the top of the tank) but no direct sunlight. The best light 

 is north skylight. 



After about 2 weeks, add to the previous diet the smallest pieces of chopped white worms 

 (Enchytrea), but do not overfeed because the subsequent contamination would be fatal to 

 the fish. Breeding activity will first be seen in fish about 6 months of age. 



d. The Zebra Fish, Brachydanio Rerio 



THE EARLY DEVELOPMENT OF BRACHYDANIO RERIO, 

 THE ZEBRA FISH 



Brachydania rerio is a Cyprinid fish from India which can be raised in fresh water in 

 the laboratory. The young will spawn at from 5-7 months, the most fertile period being 

 9-18 months. A general description of the conditions necessary for continuous produc- 

 tion of eggs will be given (see papers by Roosen-Runge, Hisoaka &t Legault). 



The zebra fish tank should be long and narrow to allow the fish ample room to dart back 

 and forth since they are very active. A tank measuring 24 x 4 x 4 inches will prove sat- 

 isfactory. Place about 12 specimens in such a tank, 7 or 8 of which are males. Larger 

 tanks measuring 24 x 1 8 x 10 inches may be used for 10-12 pairs of fish. The tempera- 

 ture of the water should be regulated at 27° C. although the eggs will develop normally 

 even at 25° C. The bottom of the tank should be covered with marbles, or with smooth 

 stones, or even a matting of plant material among which the eggs will drop and be pro- 

 tected from the fish which normally hunt out and eat their own eggs. Eggs can be col- 

 lected by using a porous nylon net for the bottom so that the eggs can drop through. 

 Aeration should be provided if possible. 



When the fish are kept in schools there will be continuous spawning so that from 10-50 

 eggs may be found daily. It is best to catch the eggs as they are laid, using a dip-tube, 

 for the eggs develop very rapidly. The eggs should be transferred to Stenders or to 

 Syracuse dishes which may be covered with cheese cloth and returned to the normal en- 

 vironment of the aquarium for further development. However, since the eggs develop 

 very rapidly it is generally best to observe them in separate dishes as described below. 



Place a thin film of beeswax, paraffin or permoplast on the bottom of a #2 Stender, to 

 the thickness of about 2 min. In the center of this film excavate a depression about the 

 diameter of the zebra fish egg. This opening will allow the reflected microscope light 

 to pass through the egg but will block out all extraneous light, and will at the same time 

 tend to keep the egg oriented. Avoid any heat from the microscope lamp by intercepting 

 it with a flask of water. 



