THE BLACK SEA 441 



The small bivalves Cardium, Syndesmya and Loripes live in deeper places 

 (22 to 25 m) on fine and dense sand ; while on the bottom live Gebia littoralis ; 

 the crab Portunus holsatus, and a number of fish : Gobius, Blennius, Urano- 

 scopus, Mullus and others. 



At the same depths on coarser sand, with an admixture of shell gravel- 

 lives an abundant fauna of worms : a large number of Turbellaria and poly, 

 chaetes ; the interesting archiannelid Polygordius ponticus ; and a remarkable 

 inhabitant of the Black Sea Branchiostoma (Amphioxus) lanceolata. At times 

 the holothurians Synapta digitata and S. hispida and gerbil are found here in 

 large numbers. 



Some molluscs also are likewise most characteristic of sand bottoms; 

 among the gastropods Nassa reticulata, and among the bivalves Venus gallina, 

 Gouldia minima, Divaricella divaricata, Merethrix rudis, Calyptraea chinensis, 

 Mactra subtruncata, Tapes proclivis, Mytilus galloprovincialis, Cardium exi- 

 guum and others. 



As has been said above, at the head of the bays and inlets the facies of the 

 rocks is gradually more and more reduced, while the sands become covered 

 with mud. Inside all the bays and inlets, in quieter places protected from the 

 waves, growths of Zostera, sheltering a very typical bottom-living fauna, are 

 found everywhere at depths of 5-5 and even down to 9 m. Z. marina lives pre- 

 ferably on silt and silty sand floors, but Z. minor prefers pure sand. Zernov 

 gives the following characteristics of the fauna of Zostera growths : ' A large 

 number of mysids, amphipods, isopods, shrimps, different genera and species 

 of pipefish, Grenilabrus tinea and other fish, the Medusa Cladonema and 

 Sagitta (Spadella) swim among the Zostera leaves ; innumerable Rissoa with 

 Syllids planted on their shells crawl about their leaves, as well as many 

 Tergipes ; masses of various Rhabdocoela and Acoela ; Cerithiolum, which 

 are found there in immense masses, Trochus and other molluscs. At the 

 approach of autumn Zostera becomes covered with bryozoans: Lepralia, 

 Membranipora and the tunicates Didemnidae, which die off in the winter, 

 causing the Zostera to sink under the weight of these accretions. Among 

 the roots of Zostera there hide amphiurae, Stenelais, Lagis, Rhynchobolus, 

 Gebia, Calianassa, Syndesmya, Cardium, Gastrana and other molluscs 

 which live on sand, and in the more muddy places very numerous poly- 

 chaetes, chiefly the two species Nereis cultrifera and N. diversicolor, but 

 also Nephthys, Glycera, Arenicola, Lagis and others. 



Somewhat higher than the Zostera growths, in the silts near sea-level, the 

 same polychaetes which hide under Zostera roots live in large numbers: 

 Arenicola, Nereis, Glycera and others. 



Shell-gravel biocoenosis. At the lower limit of the zone of sand and Zostera 

 growths, where the slightly muddy sand gradually passes into silty sand and 

 sandy mud, there lies along the shore a fairly wide band of the so-called 

 shell gravel — an accumulation of living and dead molluscs, mostly bivalves. 

 Shell gravel is specially well displayed in places where, in Zernov's words, 'the 

 effect of the waves is already too weak to break and powder it to sand, but is 

 still strong enough to carry the main mass of silt particles over them and 



