Body size variability across habitats in the Brachionus plicatilis cryptic species complex

The body size response to temperature is one of the most recognizable but still poorly understood ecological phenomena. Other covarying environmental factors are frequently invoked as either affecting the strength of that response or even driving this pattern. We tested the body size response in five species representing the Brachionus plicatilis cryptic species complex, inhabiting 10 brackish ponds with different environmental characteristics. Principal Component Analysis selected salinity and the oxygen concentration as the most important factors, while temperature and pH were less influential. Path analysis showed a positive interclonal effect of pH on body size. At the interspecific level, the size response was species and factor dependent. Under the lack of a thermo-oxygenic relationship, the expected negative response of size to temperature disappeared, but a positive response of size to oxygen remained. Our results confirm the driving role of oxygen in determining the size-to-temperature patterns observed in the field.