The phenotypic plasticity in Chinese populations of Daphnia similoides sinensis: recurvate helmeted forms are associated with the presence of predators

Morphological species delineation within the cladoceran genus Daphnia is often blurred by phenotypic plasticity, frequently associated with predator presence. We focused on phenotypic variation and genetic relatedness of nine Chinese populations of Daphnia (Ctenodaphnia) similoides sinensis. The typical form of this taxon inhabits fishless pools, a habitat characteristic for most Ctenodaphnia species. However, we demonstrate that D. similoides sinensis also occurs in large lakes where it coexists with multiple predators (planktivorous fish, Cyclops and Leptodora). Individuals from such lakes differ substantially phenotypically (to the extent they have been considered a distinct undescribed species), being significantly smaller and exhibiting prominent recurvate helmets. Genetic variation of D. similoides sinensis, based on mitochondrial cytochrome c oxidase subunit 1 sequences, was low; all Chinese populations studied, regardless of phenotype or environment, apparently recently expanded from a single mitochondrial lineage. This is consistent with other Daphnia species in which body and helmet size are phenotypically plastic traits responding to predator presence. The general phenotype of lake populations of D. similoides sinensis is strikingly similar to North American Daphnia retrocurva, an unrelated species of the subgenus Daphnia. We presume that similar predation pressures in lake habitats triggered convergent evolution of body shapes in both species.