570 EXPERIMENTAL FISH EMBRYOLOGY 



Betta fry are extremely difficult to carry through the first three weeks of develop- 

 ment. This may be In part due to injxu-ies sustained when the male parent rescues them and 

 returns them to the hubble nest after they hatch. The food for such fry should be Protozoa 

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

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



THE EARLY DEVELOPMENT OF BRACHYDANIO RERIO, THE ZEBRA FISH 



Brachydanla rerio is a CJyprinld 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-I8 months. A general description of the conditions necessary for continuous production 

 of eggs will be given (see papers by Eoosen-Eunge) . 



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 2h x h x k inches will prove 

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

 tanks measuring 2U x 18 x 10 inches may be used for 10-12 pairs of fish. The temperature 

 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 protected from the 

 fish which normally hunt out and eat their own eggs. Aeration should be provided if pos- 

 sible. 



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 

 environment 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 mm. 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 in- 

 tercepting it with a flask of water. 



