Diatom teratologies have intrigued scientists since the XIXth century, with respect to their causes and origins. These deformities, mainly observed in long-term cultures or under high levels of pollution, were poorly considered, until they were recently found to be potential indicators of toxic impairment of freshwaters. However, very little is known about their ecology. In this study, the growth and fitness of morphologically distinct descendants of the same cell line of Gomphonema gracile (teratological vs. non teratological forms) were compared over a typical growth cycle. Contrary to expectations, teratological populations grew slightly faster, at a rate of 0.47 ± 0.03 divisions.day-1, versus 0.41 ± 0.04 for the normal morphotype. They had similar physiological performances as non-teratological forms. They did not differ in their movement velocities, but the trajectory of teratological forms was more linear, likely as a consequence of their elongated outline. Under the same culture conditions, no competitive exclusion of one phenotype over the other was demonstrated on the time scale of an exponential growth cycle (9 days). Moreover, the deformities were faithfully reproduced over time, and no evidence of decreased viability in teratological forms was provided. These new insights call into question the common hypothesis that deformed diatoms are altered individuals produced by unfavorable conditions and thus highlight ecosystem dysfunction. They call for further investigations of their ecology.Read less <