THE BLACK SEA 403 



radiolarians, siphonophores, pteropods, molluscs and salps, and, of some 

 typical larvae of bottom-living organisms. 



The researches of V. Nikitin (1926, 1928, 1929, 1930, 1939, 1941), A. Kus- 

 morskaya (1950, 1954, 1955) and I. Galadzhiev (1948) on the Karkinit Bay are 

 the most comprehensive investigations of the zooplankton of the open parts 

 of the Black Sea. 



The main forms of zooplankton of the open parts of the Black Sea comprise 

 Noctiluca miliaris among the Cystoflagellata ; Cyttarocylis helix, C. ehrenbergi, 

 Tintinnopsis campanula. T. ventricosa, T. tubulosa, Tintinnus mediterranea 

 and T. subulatus among the Tintinnoidea ; Amelia aurita and Pilema pulmo 

 among the true Medusae; the ctenophore Pleurobrachia pileus; the following 

 Copepoda: Oithona nana, O. similus, Paracalanus parvus, Acartia clausi, 

 Calanus helgolandicus, Pseudocalanus elongatus, Centropages kroeyeri; the 

 Cladocerans Evadne nordmanni, E. spinifera, Podon polyphemoides ; Sa- 

 gitta euxina among the Chaetognatha and Oikopleura dioica among the Appen- 

 dicularia. Moreover, in the off-shore regions the Hydromedusae Rathkea 

 octopunctata and Sarsia tubulosa, the Copepoda Pontella mediterranea, Ano- 

 malocera patersoni, the Penilla avirostris and the Chaetognath Sagitta setosa 

 are just as abundant. The relatively large isopod crustacean Idothea algirica is 

 found everywhere in the plankton, at times in large numbers. 



Apart from the above-mentioned forms, eggs and larvae of various pelagic 

 and botton invertebrates and fish are mixed with the coastal plankton, 

 especially in the summer. Among them the most abundant are anchovy ova, 

 the larvae of Lamellibranchiata and the eggs and larvae of various Copepoda. 



Vertical distribution of plankton. Several groups can be distinguished in the 

 Black Sea plankton by the character of their vertical distribution. 



S.ome forms are distributed alike in winter and summer. The greatest mass 

 of them is usually adapted to a depth of 15 to 50 m. Their vertical distribution 

 is only slightly affected by variations of temperature and light, observed 

 throughout the seasons. These forms include Oithona nana, the most abundant 

 Copepoda, Acartia clausi, Paracalanus parvus and Oikopleura dioica. Idothea 

 algirica and Noctiluca miliaris are similar in distribution but the numbers of 

 the latter fluctuate considerably during the year ; it is very scarce in the winter 

 and multiplies intensively in summer. 



The next group is represented by cold water stenothermal forms found in 

 winter at all depths ; in summer they sink to the greater depths. This group 

 includes Calanus helgolandicus, Pseudocalanus elongatus, Oithona similis, 

 Sagitta euxina and Pleurobrachia pileus. Throughout the whole of the cold 

 period of the year (December to April) they are found from the surface to 

 the lower limit of plankton distribution. With the spring warming up of the 

 upper layer of water they sink down, disappearing gradually from the upper- 

 most 50 m layer. At the end of November, with the autumn fall in temperature, 

 they move into the upper waters, remaining there until the beginning of May. . 

 This migration takes place only in the uppermost 50 to 60 m layer, since 

 below this the hydrological conditions are comparatively constant and there 

 is little change in the distribution of the cold water forms throughout the year. 



