446 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



the shores of Norway to the north, and is particularly abundant off the shores 

 of England. It is comparatively rare in the Mediterranean Sea. M. phaseolina 

 has a very wide vertical range ; it is found from the littoral to shallow depths 

 in the Atlantic Ocean, mostly to 100 m, and on hard or rocky sea-floors. 

 In the Black Sea it is most abundantly developed at depths of 65 to 100 m, 

 living only on soft mud floors, not rising above 40 m and not sinking down 

 below 167 m (L. Yakubova, 1948). Almost everywhere in the Black Sea M. 

 phaseolina is a dominant form at depths below 60 m, with a few hundred 

 specimens per m 2 . 



Of other organisms commonly thriving in phaseolin mud one can point to 

 the molluscs Cardium simile, Syndesmya alba and Trophonopsis breviatus; 

 the sponge Suberites domuncula; the actinia Cehanthus cestitus and Cyliste 

 vicuata ; the worms Terebellides stromi, Melinna palmata and Nephthys cir- 

 rosa ; the crustacean Crangon crangon ; a number of amphipods ; the echino- 

 derms Amphiura florifera and Cucumaria orientalis ; the tunicates Ctenicella 

 appendiculata, Eugyra adriatica, and Ciona intestinalis. Modiola phaseolina, 

 Melinna palmata, Cerianthus vestitus and Amphiura florifera are predominant 

 among all these forms. 



Sea- weeds become rare at 80 to 90 m ; below this lies the pseudo-abyssal 

 and its only group is the fauna of the phaseolin ooze. 



The filter-feeding phenomenon. The bottom-living population of the Black 

 Sea is characterized by a strong development of filter-feeding phenomena. 

 Accumulations of bivalves (typical filter-feeders) and among them the ones 

 of greatest mass — sea-mussel, oyster, Mytilaster and Modiola {Modiola 

 adriatica and M. phaseolina) — form a wide ring from the water's edge to the 

 limit of inhabited depth. The capacity of filter-feeders is huge, and the upper 

 column of sea water permanently exposed to their action is freed from micro- 

 sestonic suspension. The effect of the filter organisms on the bottom soils of 

 the Black Sea is just as important. Ooze deposits are a result of their fecal 

 pellets. In this way the apparently contradictory fact of large accumulations 

 of sea-mussels being adapted to soft-deposit zones can be reconciled. Soft soils 

 occur in quiet zones, and the sea-mussel usually inhabits well-washed areas of 

 the sea-bed. Evidently the mussel-shell deposit areas, and partly those of 

 phaseolin ooze, are by no means quiet zones ; moreover, the soft ooze here 

 may be formed by the molluscs themselves and may have a biogenic character. 

 Without the filter organisms the oozes would not have been deposited in 

 masses in these zones. However, this is so far only a hypothesis, which needs 

 to be proved. 



Fauna zonation. L. Yakubova (1935) used the qualitative distribution of the 

 Black Sea benthos as a basis for the classification of the fauna according to 

 three coastal zones (Fig. 214). 



I. The eastern half of the Sea, from the southern coast of the Crimea, along 

 the Caucasian coast and the eastern part of the coast of Anatolia. Yakubova 

 considers the fauna of this area as the most typical of the Black Sea at present. 



II. The southwestern zone, open to the influence of the more saline waters 



