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).

Brachionus plicatilis (Müller, 1786) is a monogonont rotifer that has long been considered as an ecological generalist with a cosmopolitan distribution in inland and coastal marine habitats (Walker, 1981). Among rotifers, B. plicatilis is probably one of the best-studied taxa. It has been widely used as a model for physiological and ecological studies [for examples see (

Mechanisms underlying competitive interactions are important in understanding the structure of planktonic communities, particularly the coexistence of similar species. Here we present experimental results of exploitative competition among three sympatric sibling species of the Brachionus plicatilis complex for two differently-sized species of food microalgae. These three rotifer species are to be found in seasonal succession in brackish ponds on the Spanish Mediterranean coast; they can, however, co-occur for long periods. The functional and numerical responses of the three Brachionus species to both food microalgae, although similar, differed. Neither rotifer population growth nor grazing rates showed a clear positive correlation between rotifer and alga sizes. Tilman's models were applied to these results in order to predict the outcome of competition in two rotifer, two resources systems. For testing predictions and addressing the possibility of rotifer coexistence, we performed pairwise competition experiments in semicontinuous cultures, which introduces periodic disturbance as an additional factor. Results confirmed the pattern expected from Tilman's models regarding the competitive superiority of each rotifer species, when food composition was biased toward one of the two resources. This shows the relevance of the food threshold concept to predict the superior competitor. However, coexistence of rotifers was found in conditions in which Tilman's models had predicted an unstable equilibrium, with the winning species depending on the initial conditions. We hypothesize that variance in food availability mediated coexistence. Our results suggest that both food partitioning and disturbance are important in explaining the coexistence of these sibling species in nature.

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The relevance of predation as a factor mediating the competitive interaction among ecologically very similar species is investigated by experimentally analyzing the effect of the copepod predator Diacyclops bicuspidatus odessanus on three sibling rotifer species belonging to the Brachionus plicatilis species complex. These rotifer species are similar in shape but show notable differences in body size. Predator and prey species co-occur in brackish waterbodies close to the Mediterranean coast of Spain. First, we characterized differential vulnerability of rotifers to predation. A consistent tendency of higher predation rates on smaller prey (i.e., smaller species and younger individuals) was observed. Analysis of predation showed that predator contact rate did not differ significantly among prey species, but that attack, capture probabilities, and handling time did. Second, we performed population dynamics experiments with two prey species competing for a single resource at different levels of predation. Predation extended coexistence of the competing rotifers, whereas the inferior competitor was excluded in the absence of the predator. In some pairwise experiments, we found that the greater the predation level, the larger the relative increase in density of the inferior competitor. Our results suggest that predation can affect the dynamics of very similar competing species in natural aquatic communities, promoting coexistence.

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Continental copepods have been derived from several independent invasive events from the sea, but the subsequent evolutionary processes that account for the current diversity in lacustrine environments are virtually unknown. Salinity is highly variable among lakes and constitutes a source of divergent selection driving potential reproductive isolation. We studied four populations of the calanoid copepod Leptodiaptomus cf. sicilis inhabiting four neighboring lakes with a common history (since the Late Pleistocene) located in the Oriental Basin, Mexico; one lake is shallow and varies in salinity periodically (1.4–10 g L-1), while three are deep and permanent, with constant salinity (0.5, 1.1 and 6.5 g L-1, respectively). We hypothesized that (1) these populations belong to a different species than L. sicilis sensu stricto and (2) are experiencing ecologically based divergence due to salinity differences. We assessed morphological and molecular (mtDNA) COI variation, as well as fitness differences and tests of reproductive isolation. Although relationships of the Mexican populations with L. sicilis s.s. could not be elucidated, we identified a clear pattern of divergent selection driven by salinity conditions. The four populations can still be considered a single biological species (sexual recognition and hybridization are still possible in laboratory conditions), but they have diverged into at least three different phenotypes: two locally adapted, specialized in the lakes of constant salinity (saline vs. freshwater), and an intermediate generalist phenotype inhabiting the temporary lake with fluctuating salinity. The specialized phenotypes are poorly suited as migrants, so prezygotic isolation due to immigrant inviability is highly probable. This implication was supported by molecular evidence that showed restricted gene flow, persistence of founder events, and a pattern of allopatric fragmentation. This study showed how ecologically based divergent selection may explain diversification patterns in lacustrine copepods.

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