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Meristic variation in the medaka (oryzias latipes) produced by temperature and by chemicals affecting metabolism Ali, Mohammed Youssouf


Meristic characters in offspring from 27 pairs of medaka were investigated with respect to some factors known to alter metabolism. Temperature, light, thyroxine, thiourea, dinitrophenol, urethan and salinity were tested. Egg size, egg density, nature of rearing containers, quality of successive day's egg batches from the same parent, mechanical shock of developing eggs and pricking the chorion of eggs were also studied as possible factors producing meristic variations. Mean vertebral counts showed a V-shaped relation to temperature in 9 out of 15 replications, inverse relation in 2, and no consistent relation in 4. Pectoral fin ray counts were inversely related to temperature. Degree and direction of change of other fin rays with temperature varied between genotypes. Vertebral counts were not affected by variations in light intensities or duration; fin ray counts were altered but their reaction lacked uniformity. Mean total vertebral counts of 8 out of 11 replications were altered in thyroxine solution, but magnitude and direction of change differed between genotypes. When eggs were hatched in thyroxine solution, pectoral ray counts were lowered. Exposure of larvae to thyroxine produced significant decrease in pectoral, anal, dorsal, and caudal fin ray counts. Rearing eggs to hatching in thiourea produced significant increase in mean total vertebral counts in only 2 of 11 replications. Pectoral and anal fin ray counts increased, but total caudal rays decreased, in samples from treated eggs as well as from larvae treated in thiourea after hatching. Rearing of eggs to hatching in dinitrophenol, urethan, or sea water resulted in an increase in mean vertebral counts. Pectoral rays increased in lower concentrations of dinitrophenol, or in urethan, but were unaffected in sea water. Anal, dorsal, and total caudal rays were not altered in sea water, but variable effects resulted from dinitrophenol or urethan. No correlation was found between meristic counts and egg size. Vertebral and pectoral ray counts seemed to follow those of the father; paternal influence was very pronounced in inheritance of pectoral rays. Vertebral, pectoral and dorsal ray counts were not affected by other extraneous factors tested. Effect of egg density of anal and total caudal rays was variable. Final fixation of total vertebrae occurred at the embryonic stage when eye pigmentation commenced and pectoral buds had appeared. Other characters remained sensitive to environmental influence even after hatching The relation of metabolism to meristic characters, and evident parallels between effects of the several factors used are discussed.

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