Cohort cultures of Tubifex tubifex forms

By means of cohort cultures at different temperatures, the life cycles of two sympatric forms of T. tubifex have been compared: the normal T. tubifex tubifex and the blanchardi forms. Although the two forms have certain basic similarities, the blanchardi form tolerated low temperatures less well, requiring a longer time to develop to maturity and producing fewer ova. The threshold temperature for development appeared to be around 8 °C, notably higher than for the normal form around 0 °C. Moreover, for the blanchardi form, resistance was lower and mortality higher for embryos and throughout the life cycle. Cross-breeding tests were carried out, along with cultures of individuals serving as controls for partheno-genetic activity. In cross-breeding, pairs produced progeny similar to the normal form only, most likely derived not from eggs fertilized by the partner, but from parthenogenetic eggs. As a matter of fact, only the normal form was able to reproduce through parthenogenesis. Thus, the two forms have remarkable differences, indicating genetic differences, and they may be considered not as forms, but rather as distinct species.