J. zyxwvutsrqp Fish Biol. zyxwvuts (1975) 7,519-525 zyxwvuts Induced polyploidy in zyxw TiEapia alcrea (Steindachner) by means of temperature shock treatment zy R. J. VALENTI Montauk, New York 11954, U.S.A. New York Ocean Science Laboratory, Department zyxw of ichthyology, (Accepted 24 July 1974) A technique for increasing ploidy in the blue tilapia, Tilupia nurea, was investigated for the purpose of determining its effect on standard length. Gametes zyxw of T. aweu were stripped from ripe adults and eggs were fertilized with donor sperm from several males. A description of the extra-oral incubation system used for the embryos is provided. This system resulted in a maximum of 90% successful hatch. Normal egg development was interrupted by immersion of newly fertilized eggs (15 min post-insemination) in cold or hot water for various durations. The reliability of these techniques for inducing polyploidy was examined using increased volume of erythrocyte nuclei as a criteria for success. A cold shock of from 32' C (ambient) to 11" C for a 1 h duration resulted in the largest incidence of poly- ploidy (75 %). Polyploid fish were found to be larger than diploid siblings when measured at zyxwvuts 14 weeks of age. I. INTRODUCTION Winkler (1916) first introduced the term polyploidy to designate individuals M ith three or more complete sets of chromosomes in place of the usual two sets. Numerous accounts of an increase in ploidy have been investigated in plants, where polyploidy is normally a frequent occurrence and can be induced by various experimental procedures. These include the use of chemicals such as colchicine and sulphydryl compounds, or by physical means such as temperature shock treatments and centrifugation (Blakeslee & Avery, 1937; Galinsky, 1956; Sensen, 1959; Levan, 1938). Among the bisexual vertebrates pnlyploidy is a rarity (Austaurov. 1965) but several excellent reports of normal or induced polyploidy do exist in amphibians (Becak ef al., 1957; Book, 1941 ; Briggs, 1947; DeJong, 1957; Fankhauser, 1941, 1945; Fankhauser & Griffiths, 1939; Fankhauser & Humphrey, 1942; Fankhauser & Watson, 1942; Pennock, 1945; Sanada, 1951) and mammals (Beatty, 1957; Beatty & Fischberg, 1949, 1952; Edwards, 1954, 1958a, b). In fish induced polyploidization has been attempted with several species, the plaice, Pleuronectes platessa and its hybrid with the flounder, Ptatichthys fIesus (Purdom, 1973); hybrids between Sulmo sular and S. trutta (Svardson, 1945); Gasterosteus aculeatus (Swarup, 1959a, b); Oryzias melastigma (Sriraniulu, 1962) ; Gambusia afinis holbrookii (Roberts, 1965) ; Lebistes reticulatus (Squire, 1968), and Betta splendeiis (Squire, 1968). It is a general phenomenon that nuclear size increases in proportion with an increase in chromosome number while the nucleo-cytoplasmic ratio is maintained (Fank- hauser, 1945; Swarup, 1959b). If a constant number of cells is maintained for each organ of the body, then one could expect animals with an increased ploidy to be larger than their diploid counterparts. In plants there is usually an increase in final 519