Meristic variation in a clone of the cyprinodont fish Rivulus marmoratus related to temperature history of the parents and of the embryos

Abstract

Parents from one clone of the self-fertilizing cyprinodont fish Rivulus marmoratus were held at 25 or 30 °C, and their offspring were subjected to either a sustained temperature of 25 or 30 °C or to a temperature break (abrupt transfer in either direction between 25 and 30 °C) at various developmental stages. Effects of parental temperature before fertilization on meristic counts of offspring were determined both by comparing offspring produced either soon or long after parents had been transferred to a new temperature, and also by examining meristic responses of developing embryos to temperature breaks. Both line of evidence indicate that a parental temperature of 30 °C produces fewer vertebrae (0.31), pectoral rays (0.54), and caudal rays (1.11) than does one of 25 °C, in offspring reared under comparable temperature regimes. Neither line provides clear evidence of an effect of parental temperature on number of anal or dorsal rays in offspring. Responses of all five meristic series to temperature breaks in either direction were extralimitary (beyond the counts produced by sustained incubation at either temperature) and were satisfactorily fitted by an "atroposic" model described previously. Embryos transferred to fresh water during development tended to have higher meristic counts than those with sustained rearing in brackish water, but counts among embryos transferred to fresh water at different developmental stages differed significantly only for caudal rays. Only dorsal ray numbers differed significantly among embryos retained within parents for different times after fertilization. Previous studies claiming uniquely high thermolability of vertebral number in R. marmoratus are re-evaluated; thermolability of vertebral number in R. marmoratus is concluded to be within the range reported for other species.