Understanding patterns and processes in biological diversity is a critical task given current and rapid environmental change. Such knowledge is even more essential when the taxa under consideration are important ecological and evolutionary models. One of these cases is the monogonont rotifer cryptic species complex Brachionus plicatilis, which is by far the most extensively studied group of rotifers, is widely used in aquaculture, and is known to host a large amount of unresolved diversity. Here we collate a dataset of previously available and newly generated sequences of COI and ITS1 for 1273 isolates of the B. plicatilis complex and apply three approaches in DNA taxonomy (i.e. ABGD, PTP, and GMYC) to identify and provide support for the existence of 15 species within the complex. We used these results to explore phylogenetic signal in morphometric and ecological traits, and to understand correlation among the traits using phylogenetic comparative models. Our results support niche conservatism for some traits (e.g. body length) and phylogenetic plasticity for others (e.g. genome size).

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Four geographic strains of B. calyciflorus are investigated regarding their genetic similarity and ability to cross-mate. DNA sequence analysis of the mitochondrial cox1 gene (694 bp) and the nuclear ribosomal ITS region (735 bp) showed that the Florida and Georgia strains were very similar to each other (0.3% sequence divergence for the 1429 bp) and different from the Texas and Australia strains ($7% and 9% sequence divergence for the 1429 bp, respectively). Consistent with this genetic relatedness, cross-copulation occurred only between the Florida and Georgia strains. Thus, B. calyciflorus is a complex of cryptic species. While the Florida, Texas and Australia strains were reproductively isolated from one another, most combinations of cross-strain mating tests showed intense and prolonged male circling behavior following male-female encounters. This suggests that precopulatory male circling and copulation are two separate behaviors that may be controlled by different female chemicals and male coronal receptors. In some cross-strain mating tests, females regularly retracted their corona when circled by a male, indicating that they can recognize ÔforeignÕ males and actively interfere with copulation.

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