As the surf weakens, the population of the sandy beaches becomes 

 significantly richer, particularly in the lower portion. The leading 

 groups are usually the bivalves (Veneridae, Cardiidae, hbctridae, 

 Tellinidae, Myidae) and polychaetes ( Ci rratul idae, Opheliidae, 

 Maldanidae, Terebel 1 idae, Phyl 1 odocidae , Glyceridae, Eunicidae, 

 Capi tell idae). Among the gastropods, both predators and scavengers are 

 common, particularly Naticidae ( Natica , Tectonatica , Acrybia , Pol inices , 

 etc.). There are also large numbers of crustaceans: in the upper 

 intertidal zone, the crabs and amphipods-tal itrides, in the middle and 

 lower intertidal zone--the sand shrimp Crangonidae, decapod crustaceans 

 Upogebia and Callianassa , Squill idae, the isopods Sphaeromatidae, less 

 frequently Cirolanidae. Macrophytes begin to appear here, particularly 

 in the tropics--Caul erp a , Udotea , Halimeda , etc. 



In temperate waters, most of the algae do not use sand as the 

 substrate, but rather shells, rocks or pebbles, while some species, such 

 as Cladophora fracta in the White Sea, simply rest on the sand, but are 

 not attached to it. 



Still further weakening of the surf, observed in partially closed 

 bays, lagoons and estuaries, is accompanied by silting of the sand and 

 freshening of the water. In countries with arid climates, lagoons 

 become brackish. For estuarine and lagoon types of intertidal zone, the 

 Potamogetonaceae and Hydrocharidaceae dre more common than algae. Among 

 the genera of the first family, Zostera is most widespread, while 

 Posidonia grows in the Mediterranean, and in the tropics-- Cymodocea , 

 Halodule and Al thenia . Representatives of the second family, such as 

 Halophila , Thalassia and Enhalus , grow only in the tropical seas. The 

 infauna in stands of eel grass consist primarily of the same groups as 

 are found on sandy beaches with weak surf, but a unique fauna settles on 

 the leaves of the grass, containing both common phytophilic forms 

 (Trochidae, Turbinidae of the gastropods, Vilasina of the bivalves, 

 etc.) and species which are specially adapted for life on the leaves of 

 marine grasses, e.g., the isopod Idotea rotunda ta , mollusks such as the 

 limpets Siphonacmea oblongata and Col 1 isel la angusta . 



In addition to the sea grasses, mangroves Are also found in the 

 tropics. These are ancient plants, which have adapted to life along the 

 sea shore. Mangrove stands are low (up to 5-10 m) evergreen groves, 

 rising from silty shores protected from the surf. The adaptation of the 

 mangroves to life in an amphibiotic medium on muddy, semiliquid silt 

 under conditions of physiologic dryness has been to develop stiltlike 

 roots which can survive in the air, pulpy leaves with watery stoma, 

 through which the excess of salts is excreted, reservoirs of fresh water 

 in old leaves and air-carying tissues in the fruit, allowing it to 

 float. Mangrove stands are common in the tropics and extend into the 

 subtropics from 35°N to 37°S. Nbst common are representatives of the 

 genera Avicennia , Rhizophora , Sonneratia , Laguncularia , Bruguiera and 

 Ceriops . Mangroves are the pioneers in the colonization of silty 

 estuaries by land plants and the advance of the shore into the sea. 

 Populating the brackish silts, suitable only for a very few halophiles, 

 they facilitate consolidation of the bed and its gradual transformation 

 to soil. In this way, they gradually create the prerequisites for the 

 formation of a tropical forest. The distribution of mangrove stands is 



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