FISH FOR EXPERIMENTATION 355 



3. Paradise : Brown and Albino. These need not be kept in isolation until they show 

 signs of bubble-nest building, and breeding. Early embryos develop rapidly 

 with high viability, but mortality great after about 2 weeks when there is a 

 change of diet. Many will go through to sexual maturity. 



Strippers: (See Fundulus below) 



b. Live Bearers 



With these fish the eggs are fertilized within the female where they develop in a chamber, 

 without any maternal connections, sometimes beyond a stage comparable to the hatching 

 stage of egg laying fish embryos. The young lie folded head to tail and are born singly. 

 Females may be induced to drop their young prematurely by excessive handling such as 

 transferring to new tanks, etc. 



The gestation period is irregular since it depends upon the temperature of the environ- 

 ment of these cold blooded forms. Psychological factors may also enter into the picture, 

 females dropping their young so early in their development that they cannot survive, or 

 retaining them too long. For most live bearers the best temperature is about 75 F. , 

 although some species can stand as much as 10° C. lower or higher. 



The young must be caught in traps, or provided with hiding places in vegetation, or the 

 adults must be removed because there is a tendency for them to eat their young almost 

 immediately. Since the young are born at an advanced stage, these fish are the easiest 

 to raise in large numbers. Infusorian food is too small for them and they nnust be put on 

 a diet of chopped white worms (Enchytrae) or small Daphnia. The adults are good eaters, 

 accepting almost anything whether at the surface, dropping, or on the bottom. Species 

 should be segregated because there is frequent interbreeding among live bearers, par- 

 ticularly between the Swordtails and Platys. Live bearers are generally omnivorous. 



From the standpoint of early embryology these fish are not so satisfactory, but there 

 are numerous experimental approaches that are yet to be made on these forms. 



1. Lebistes reticulatus , the common Guppy. The Gold Guppy and the Black Tail 

 Guppy are very good. 



2. Gambusia affinis , breed at 5-10 week intervals, called Mosquito fish from 

 southern U. S. Is pugnacious and cannibalistic. 



3. Xiphophorus, the Swordtails. Red or Green Swordtails very good. These may 

 be hybridized with the Platys. (Species helleri most common.) 



4. Xiphophorus maculatus, the multicolored "Platy" brought to the U.S. from Mex- 

 ico by Dr. Myron Gordon and distributed widely by him. The mutants Red, Blue, 

 and Gold Crescent are all excellent. Female may drop as many as 60 young at 



a time, at monthly intervals. P. variatus is found in at least 20 patterns. 



5. Xiphophorus hellerii, the Mexican Swordtail. 



6. Mollienesia latipinna, the Sailfin Molly. The Perma Black Molly is very com- 

 mon and entirely satisfactory. Breeds at 5-10 week intervals. Predominantly 

 herbivorous. Require large aquarium and heavy aquatic greens in which the 

 young can hide during growth. 



c. Breeding of the Betta 



These are bubble nest builders which should be raised in isolation except for the breed- 

 ing periods. Originally these fish were found in extremely stagnant pools and they seem 

 to survive best when there is no vegetation and they are confined in small covered con- 

 tainers. Pint sized mayonnaise jars or inany kinds of plastic containers two-thirds 

 filled with conditioned tap water and covered (but cover not screwed on) seem to be 



