EXPERIMENTAL FISH EMBRYOLOGY 367 



5. Specific gravity : An increase in the density of the medium increases the respira- 



tory and metabolic rates. Since fishes can tolerate a greater density range than 

 can parasites, this fact is often used in pro'phy lactic and therapeutic measures. 



6. Flow and aeration of vater : Chlorine and nitrogen are toxic in certain concentra- 



tions, and even oxygen may be too concentrated and cause gas embolisms. There la 

 an optimum level, different for different fish (e.g., very low for the Betta) 

 which must be maintained. 



7. Metabolic waste products : Water in which fish have lived for a time is known as 



"conditioned water" and presumably such water is better for that particular 

 species than for others, due, it is thought, to the concentration of certain 

 species specific beneficial metabolites. 



8. Diet : Many fish diseases are due to" vitamin deficiency, particularly when fish 



are kept in captivity and fed artificial diets. Liver and renal damage are prob- 

 ably the major ailments encountered. 



9. Handlinj^ : Directly or Indirectly handling may be the cause of the majority of 



deaths. Abrasions are sites for infections. Even with the greatest care, moving 



established pairs or colonies from one tank to another usually means the loss of 



some specimens. New fish, regardless of size, are often attacked by the perma- 

 nent residents of a particular aquarium. 



10. Parasitism : Many ecto-parasites show no host specificity, hence there is great 

 virulence (see Nigrelli & Atz, I9U3: Zoologica 27:1). 



BEFERENCES : 



"The Aquarium Journal" - San Francisco Aquarium Society ($5.00 per year), 

 "The Aquarium" - Innes Publishing Co., Philadelphia, Pa. ($2.25 P©r year). 



FISH SUITABLE FOR LABORATORY EXPERIMENTATION 



There is an ever increasing number of fish species which are being adapted to labora- 

 tory conditions so that they will breed normally and thereby provide suitable material for 

 embryological studies. The following list consists of good breeders, providing large num- 

 bers of eggs or live bearers with many young at a time. 



EGG LAYERS : 



A. Eggs dropped or attached to vegetation . 



1. Oryzias latipes , the Japanese Medaka. Hardy fish which provides eggs almost 



daily, each female giving I-80 fertilized eggs each morning. Development 

 slower than that of tropicals, described elsewhere. 



2. Danio rerlo . the zebra fish, provides eggs daily but eats them almost immedi- 



ately If they are not removed. Development described elsewhere in this out- 

 line. Pearl, Gold, and Giant Danlos also good, all from India. 



5. Albonlubes . the White Cloud Mountain Fish. Dependable breeders. 



k. Anoptichthys Jordanii . The White Blind Give Tetra which breeds easily under 

 aquarium conditions and produces large quantities of eggs. 



5- Barbus conchonius . the Rosy Barb. Other species are the Clown (Breverettl), 

 Gold dwarf ( B. gelius), and Pygny (B. phutunis) Barbs are from India. Breed- 

 ing directions for Barbs similar to those for Danlo. 



6. Chriopeops goodei . the Blue Dace. Excellent breeder. 



7. Hemi chromus . the Jewel Fish. Will produce about UOO eggs every "^-h weeks If 



congenial pairs are kept together. Attaches eggs to bottom of container, 

 or to pieces of crockery which may be removed with eggs for study. Large 

 fish, voracious eater, viability of embryos high. 



8. Eggs supplied for experimental purposes by Commissioner of Fisheries, U. S. 



Department of Interior, Washington, D. C. 

 Salmon, Trout, and Whltefish 



