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Background The occurrence of a small brackish water bloom-forming dinoflagellate belonging to the genus Peridiniopsis is reported for the first time from the Shatt Al-Arab River in Iraq. Based on the study of cell morphology, this taxon was attributed to the recently described Peridiniopsis minima, recorded so far only from the Jiulongjiang River, China. Results The description of P. minima was extended to include some previously unreported details of the thecal surface, apical pore complex, and sulcus structure using light and scanning electron microscopy. Cells of this species from Iraq were oval to rhombic in shape and slightly dorso-ventrally compressed, with distinct red eyespot and endosymbiont nucleus. Cells were covered by a thin theca with the Kofoidian plate formula Po, X, 3′, 1a, 6″, 5c, 5 s, 5′′′, 2′′′′. Complementing the original description of the species, five plates were discerned in the composition of the sulcal area, and the presence of small median sulcal plate was confirmed. Conclusion The finding of P. minima in the waters of Shatt Al-Arab River represents a new addition to the Iraqi aquatic microflora. Our observations of this dinoflagellate in brackish waters of the Shatt Al-Arab River system extend the known range of salinity tolerance for this species.

We studied benthic harpacticoid grazing on diatom algae from two sites on the White Sea intertidal sandflat. Diatoms from sediments and from harpacticoid gut contents were sampled in situ, identified and counted, and grazing rates were calculated by two ways: (1) using potential daily ration estimations and (2) from the gut content and gut-residence time data. Paraleptastacus kliei did not contain any diatoms in the guts and presumably fed on other objects (bacteria or flagellates). Two other dominating species studied, Heterolaophonte minuta and Huntemannia jadensis, contained an average of 604 and 222 diatom cells per specimen. Diet composition differed significantly from the natural algal community. Two diatoms of intermediate cell size (Nitzschia palea var. debilis and Navicula sp.) contributed 92–97% of gut content for H. minuta and 58–81% for Hn. jadensis, whereas these diatoms amounted to only about 10% of biomass in native community. Mean consumption rates were estimated as 50–200 μg of wet biomass/day/cm2, so the harpacticoids grazed only between 3 and 11% of the total microalgae biomass per day. The grazing impact on the two preferred diatom populations, however, was much more intensive, 10–30% per day for Navicula sp. and 55–228% for N. palea. Therefore, native harpacticoid populations demonstrate highly selective feeding and could be strongly limited by their food in spite of seemingly plentiful total abundance of microphytobenthos. This disproportionally high grazing pressure upon some species apparently could affect the structure of microalgae communities resulting in low relative abundance of mid-sized forms. We hypothesize that a very dynamic spatio-temporal distribution of epibenthic harpacticoids (short-living micropatches) may be the possible adaptation to such local food limitation.