218 Ross F. NiGRELLI 



general characteristics : (1) they are phylogenetically primitive, 

 (2) they are sluggish in their movements, (3) they are bottom 

 inhabitants or live in fairly shallow waters, (4) they have 

 accessory respiratory devices, (5) they can aestivate or 

 hibernate, (6) they live in regions of extreme sunlight, (7) they 

 are adapted to live in environments with low oxygen concen- 

 tration (less than 5 p.p.m.), and (8) they are adapted to 

 environments with extreme fluctuations in temperatures and 

 salinities. The significance of some of these factors may not 

 be apparent. For example, there seems to be some correlation 

 between longer daylight and decreased respiratory rates, 

 especially in fishes kept above 15°. Also, fishes that live 

 on some coral reefs and banks no doubt are acclimatized to 

 great changes in temperature and salinities. Certain "reef" 

 fishes can even survive in fresh water, provided the calcium 

 content is high. Thus, Breder (1934) reports 12 typically 

 marine species that he found living in a freshwater lake (Lake 

 Forsyth) on Andros Island, British West Indies. Further, 

 experience has shown that many marine tropical species 

 survive longer in captivity if the sea water is reduced in 

 salinity to around 30 parts per thousand. 



Although there is much evidence that fishes (teleosts) 

 undergo reproductive and actuarial senescence, there is very 

 little information in the literature relative to pathological 

 changes associated with ageing. Rasquin and Hafter (1951) 

 and Hafter (1952) reported age changes in the testes and 

 thymus of the teleost, Astyanax mexicanus. From the 

 clinical records of the New York Aquarium, in addition to 

 gonad atrophy we have found the following pathological 

 manifestations to be most frequently associated with ageing: 

 cirrhosis and fatty changes in the liver and kidneys, haemo- 

 chromatosis, hypochromic anaemia, degeneration of the 

 mucus-producing glands of the skin, hyperostosis of the 

 haemal arch bones (see Breder, 1952) and other vertebral 

 abnormalities. The haemochromatosis and anaemia are the 

 direct result of changes in the kidney and spleen, important 



