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ABSTRACT Prorocentrumcf.balticumwas the provisional designation assigned to strains of a small, pelagic, mixoplanktonic dinoflagellate found to produce carbon‐rich mucilage‐based prey capture devices, termed “mucospheres.” Here we characterize the morphology and phylogeny of the strains, describe them asProrocentrum insidiosumsp. nov., and discuss common morphological features among the six species of the phylogenetically definedP. cordatumgroup. Cells ofP. insidiosumsp. nov. were round to slightly ovate in lateral view, 12–16 μm long and 8–15 μm deep, and laterally compressed. Scanning electron microscopy revealed the thecal plates were densely ornamented with short spines and there were two size classes of pores irregularly distributed across both plates, and a row of two to four large round pores in apical‐ventral position on the right thecal plate. The periflagellar area consisted of eight platelets, and there were two prominent wing‐like apical projections in the form of a double layered curved structure on platelet 1 with additional projections on most other platelets except platelet 4.Prorocentrum insidiosumsp. nov. is distinct from all genetically represented species within the genus and possesses a unique combination of morphological features differentiating it from other protologues of smallProrocentrumspecies. 

Prorocentrum comprises a diverse group of bloom-forming dinophytes with a worldwide distribution. Although photosynthetic, mixoplanktonic phagotrophy has also been described. Recently, the small P. cf. balticum was shown to use a remarkable feeding strategy by crafting globular mucus traps to capture and immobilize potential prey. Here we present evidence showing that two additional related species, the recently described P. pervagatum and the cosmopolitan bloom-forming P. cordatum, also produce large (80–120 µm) mucus traps supporting their mixoplanktonic activity. Prey are captured within the traps either through passive entanglement upon contact with the outside surface, or through active water movement created by rotating Prorocentrum cells eddying particles to the inside surface where trapped live prey cells became immobilized. Entrapment in mucus assisted deployment into the prey of a peduncle extruded from the apical area of the Prorocentrum cell. Phagotrophy by P. pervagatum supported faster growth compared to unfed controls and time series quantification of food vacuoles revealed ingestion rates of ca. 10–12 Teleaulax prey cells day−1. Model calculations show clear advantages of deploying a mucus trap for increasing prey encounter rates. This study demonstrates that the large size and immobilization properties of mucus traps successfully increase the availability of prey for small Prorocentrum species, whose peduncle feeding mode impedes consumption of actively moving prey, and that this strategy is common among certain clades of small planktonic Prorocentrum species.