Effects of temperature and sex genotype on meristic counts of the gonochoristic cyprinodontid fish Rivulus cylindraceus Poey.

Abstract

Eggs were reared at sustained temperatures of 19 °C, 25 °C, and 31 °C, Thermal response curves were declivous (vertebrae), aclivous(caudal and anal fin rays),of inverted-V shape {pectoral and dorsal rays), and nondescript (pelvic rays). Thermally induced vertebral differences in Rivulus cylindraceus, as in previously studied gonochoristic fishes, were far smaller than in a homozygous clone (uniparental, isogenic line) of selfing, hermaphroditic R. marmoratus Poey. Mean counts of males were higher for all meristic trails at 19 °C and for vertebrae and rays of pectoral and caudal fins at all three temperatures, those of females were higher for dorsal, anal, and pelvic fins at 25 °C and 31 °C. Differences in means between sexes were significant statistically for pectoral and anal rays at 19 °C, pectoral rays at 25 °C, and dorsal rays at 31 °C. Among three genetically different homozygous clones of R. marmoratus, mean meristic counts were neither significantly different nor consistently higher between males and hermaphrodites. The switch mechanism determining 'sex type' (male versus hermaphrodite) in homogametic R. marmoratus is thermal, and meristic variability is thermally influenced. Meristic variability in heterogametic R. cylindraceus is also thermally influenced but the switch mechanism determining sex is chromosomal, with apparent side effects on meristic counts.