INFLUENCE OF THE INCUBATION-TEMPERATURE ON THE MALE-FEMALE RATIO OF ATIPA EGGS, HOPLOSTERNUM-LITTORALE HANCOCK (1828)

Sixteen batches of eggs collected immediately after spawning from five atipa (Hoplosternum littorale, (Hancock, 1828), Siluriformes, Callichthyidae) nests have been incubated at temperatures ranging from 22.5 to 32 degrees C. After a growing period of 8 months allowing all the fishes to express totally all secondary sexual characteristics, the sex of 9756 fishes was determined. The sexing criteria based on the color and length of the first ray of the pectoral fins are very reliable, and may lead to an error for only 0.3% of the fishes. For the 16 batches, the male sex ratio (ratio between the number of males and the total number of fishes) varies from 46.4 to 98.5% and is always greater than 50% with one exception. An overall analysis of the results shows no marked correlation (r(2) = 0.312) between the proportions of males and the thermic conditions during incubation of eggs. But within each set of eggs from the same nest, this ratio systematically increases with increasing temperatures. The very low probability (0.0015) of such an event in the 15 batches of eggs from four independent nests suggests both a strong influence of the incubation temperature on the final proportion of males, and an important parental effect on the thermosensitivity of the offspring. The analysis shows clearly that a differential mortality between the sexes cannot account for the shifts towards such high proportions of males in the sex ratios. Thermosensitivity would be expressed early in this species, in the 3 days following spawning. The validity of these experimental results with regard to the natural environment as well as the potential adaptative advantage of such a mechanism for the species are discussed.