Many protist plankton are mixotrophs, combining phototrophy and phagotrophy. Their role in freshwater and marine ecology has emerged as a major developing feature of plankton research over recent decades. To better aid discussions, we suggest these organisms are termed “mixoplankton”, as “planktonic protist organisms that express, or have potential to express, phototrophy and phagotrophy”. The term “phytoplankton” then describes phototrophic organisms incapable of phagotrophy. “Protozooplankton” describes phagotrophic protists that do not engage in acquired phototrophy. The complexity of the changes to the conceptual base of the plankton trophic web caused by inclusion of mixoplanktonic activities are such that we suggest that the restructured description is termed the “mixoplankton paradigm”. Implications and opportunities for revision of survey and fieldwork, of laboratory experiments and of simulation modelling are considered. The main challenges are not only with taxonomic and functional identifications, and with measuring rates of potentially competing processes within single cells, but with decades of inertia built around the traditional paradigm that assumes a separation of trophic processes between different organisms. In keeping with the synergistic nature of cooperative photo- and phagotrophy in mixoplankton, a comprehensive multidisciplinary approach will be required to tackle the task ahead.

Plankton metabarcoding is increasingly implemented in marine ecosystem assessments and is more cost-efficient and less time-consuming than monitoring based on microscopy (morphological). 18S rRNA gene is the most widely used marker for groups’ and species’ detection and classification within marine eukaryotic microorganisms. These datasets have commonly relied on the acquisition of organismal abundances directly from the number of DNA sequences (i.e. reads). Besides the inherent technical biases in metabarcoding, the largely varying 18S rRNA gene copy numbers (GCN) among marine protists (ranging from tens to thousands) is one of the most important biological biases for species quantification. In this work, we present a gene copy number correction factor (CF) for four marine planktonic groups: Bacillariophyta, Dinoflagellata, Ciliophora miscellaneous and flagellated cells. On the basis of the theoretical assumption that ‘1 read’ is equivalent to ‘1 GCN’, we used the GCN median values per plankton group to calculate the corrected cell number and biomass relative abundances. The species-specific absolute GCN per cell were obtained from various studies published in the literature. We contributed to the development of a species-specific 18S rRNA GCN database proposed by previous authors. To assess the efficiency of the correction factor we compared the metabarcoding, morphological and corrected relative abundances (in cell number and biomass) of 15 surface water samples collected in the Belgian Coastal Zone. Results showed that the application of the correction factor over metabarcoding results enables us to significantly improve the estimates of cell abundances for Dinoflagellata, Ciliophora and flagellated cells, but not for Bacillariophyta. This is likely to due to large biovolume plasticity in diatoms not corresponding to genome size and gene copy numbers. C-biomass relative abundance estimations directly from amplicon reads were only improved for Dinoflagellata and Ciliophora. The method is still facing biases related to the low number of species GCN assessed. Nevertheless, the increase of species in the GCN database may lead to the refinement of the proposed correction factor.

The larvae of C. martini, C. intertinctus, and the Botanic Gardens species can be distinguished from other Victorian "blood worms" by the structure of the hypostomial plate and the presence of only one pair of blood gills. It is also the case that the larvae of any one species differ from those of the other two in the average length of the fullgrown larva, the length of the blood gills, and the presence of a feeding brush in C. martini. In the adult form the Botanic Gardens species is taxonomically identical with C. intertinctus. All three species possess the usual four salivary gland chromosomes and each species is quite polymorphic. Twelve inversions are known from C. intertinctus, six from the Botanic Gardens species, and nine from C. martini, which is unusual in being polymorphic for three duplications or deficiencies. It is suggested that the similarity between C. intertinctus and the Botanic Gardens species is due to convergence because of their similar ecology.

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Chromosome maps for C. oppositus, which are intended to serve as a standard for the genus Chironomus in the Australasian region, are presented. This species possesses the usual four salivary gland chromosomes and the seven arms, named A-G by Keyl (1962), can be recognized. This species has the combination AE, BF, CD, G, corresponding to the pseudothummi group of Europe. The relationship of C. oppositus to European species of the pseudothummi group is not definite as an independent origin of this arm combination is possible. The species is quite polymorphic, 13 inversions having been recognized. The limits of these inversions, with preliminary data on their distributions, are given.