Net samples were preserved immediately after collection in a 4% borax-buffered formaldehyde-seawater solution. A total of 245 samples from 24 stations were analysed. The crustacean zooplankton was identified in the laboratory under a stereoscopic microscope. Representatives of taxa belonging to Copepoda, Cladocera and Cirripedia, and the developmental stages (nauplii, copepodites I–V, mature males

and females) were identified. The epizoic and parasitic protozoans on crustaceans were also identified, and the degree of infestation and the location of protozoans on various body parts were investigated. Three different ranges of infestation Alpelisib were arbitrarily distinguished: up to 13, from 13 to 12, and more than 12 the body surface. Analysis of the plankton material revealed the presence of Copepoda (Calanoida: Acartia longiremis  , Acartia bifilosa  , Acartia

tonsa  , Temora longicornis  , Gemcitabine Centropages hamatus  , Eurytemora   sp. Pseudocalanus   sp. and representatives of Harpacticoida – typical zoobenthic copepods), Cladocera (Bosmina   sp. Evadne nordmanii  , Pleopsis polyphemoides  , Podon   sp. and freshwater organisms) and Cirripedia larvae (Balanus improvisus  ). The parasites attached to the crustacean bodies were classified as the genus Ellobiopsis   (Myzozoa, Ellobiopsida) ( Figures 1A–C). The epizoic protozoans observed on crustaceans of the Gulf of Gdańsk belong to Peritricha (Vorticellidae). Fossariinae Ciliated epibionts were divided into two categories: Peritricha type I – individual organisms or tufts of organisms (like the genus Vorticella  ) and Peritricha type II – clearly branched colonies (like the genus

Zoothamnium  ) ( Figure 1D–F). Such discrimination was introduced owing to the deformation of the body of organisms observed in the preserved material. Epibionts and parasites were noted on various crustacean taxa.Calanoida (Copepoda) overgrown with ciliated Protozoa (Peritricha types I and II) were observed, as were body deformations related to the presence of the parasite Ellobiopsis   ( Figure 1A, D, E) ( Table 2 and Table 3). These organisms were found at all research stations and in all research periods, and constituted from 4% (2006) to 16% (1998) of all Copepoda ( Table 2). The prevalence of Peritricha type II was from 0.8% to 13% of the total population of each taxa (max. infestation in Acartia   spp. in 1998), and that of Ellobiopsis   was 2–11% (max. infestation in Temora longicornis   in 1999). Representatives of Peritricha type I (cf. Vorticella  ) were less frequently noted on copepods – 0.1–9.2% of the population were infested ( Table 2). The dominant taxa of Copepoda of the Gulf of Gdańsk were the most commonly attacked ( Table 3) – Acartia   spp. (up to 54% of all infested calanoids), Temora longicornis   (26–49% of all infested calanoids) and Centropages hamatus   (10.5–13% of all infested calanoids).