Mitonuclear discordance across taxa is increasingly recognized as posing a major challenge to species delimitation based on DNA sequence data. Integrative taxonomy has been proposed as a promising framework to help address this problem. However, we still lack compelling empirical evidence scrutinizing the efficacy of integrative taxonomy in relation to, for instance, complex introgression scenarios involving many species. Here, we report remarkably widespread mitonuclear discordance between about 15 mitochondrial and 4 nuclear Brachionus calyciflorus groups identified using different species delimitation approaches. Using coalescent-, Bayesian admixture-, and allele sharing-based methods with DNA sequence or microsatellite data, we provide strong evidence in support of hybridization as a driver of the observed discordance. We then describe our combined molecular, morphological, and ecological approaches to resolving phylogenetic conflict and inferring species boundaries. Species delimitations based on the ITS1 and 28S nuclear DNA markers proved a more reliable predictor of morphological variation than delimitations using the mitochondrial COI gene. A short-term competition experiment further revealed systematic differences in the competitive ability between two of the nuclear-delimited species under six different growth conditions, independent of COI delimitations; hybrids were also observed. In light of these findings, we discuss the failure of the COI marker to estimate morphological stasis and morphological plasticity in the B. calyciflorus complex. By using B. calyciflorus as a representative case, we demonstrate the potential of integrative taxonomy to guide species delimitation in the presence of mitonuclear phylogenetic conflicts.

The discovery and exploration of cryptic species have been profoundly expedited thanks to developments in molecular biology and phylogenetics. In this study, we apply a reverse taxonomy approach to the Brachionus calyciflorus species complex, a commonly studied freshwater monogonont rotifer. By combining phylogenetic, morphometric and morphological analyses, we confirm the existence of four cryptic species that have been recently suggested by a molecular study. Based on these results and according to an exhaustive review of the taxonomic literature, we name each of these four species and provide their taxonomic description alongside a diagnostic key.

The purpose of this paper was to study the plankton food web structure in Lake Koronia during an extremely dense bloom of Prymnesium parvum. The Prymnesium bloom fulfilled the main criteria of an ecosystem disruptive algal bloom, since it was a massive persistent, monospecific bloom which disrupted the dynamics of the plankton community. It appeared after an N-fixing cyanobacterial bloom, when a decrease in temperature and low nitrogen conditions may have caused a positive feedback, which allowed the replacement of cyanobacteria by the invasive Prymnesium. The initiation of the bloom was further facilitated by the disruption of zooplankton grazing, with the biomass of the primary grazers, rotifers and cladocerans, dropping to undetectable levels. Furthermore, the rest of the phytoplankton responded to the onset of the bloom by an 82% decrease. Cyanobacteria and diatoms exhibited the highest decrease in biomass (95 and 99%, respectively) followed by chlorophytes (55% decrease). This enabled the uncontrolled increase of the P. parvum population to numbers close to the highest abundance ever recorded. At that point, the grazing community was reinstated and the system became replete with nutrients with an N:P ratio of 10:1. This coincided with the beginning of the bloom termination, which was followed by an increase in the biomass of competing phytoplankton.