THE BENTHAL 205 



lated wave-beaten rock affords different habitat conditions on its 

 various sides. The face toward the open sea receives the full shock of 

 the waves. The opposite side is washed by the water as it streams back- 

 ward, foaming and oxygenated, but without destructive force, and 

 hence favors numerous animals which require moving water. The outer 

 face has only a few forms, the barnacle Chthamalus stellatus perhaps, 

 and the common mussel, while the inner abounds with animal life. The 

 same is true of groups of rocks and of larger islands similarly situated. 

 The small island of Batz near Roscoff (Brittany) (Fig. 37) illustrates 

 these conditions very satisfactorily. Its northwest coast is covered 

 with barnacles and mussels, which are supplemented by Haliotis be- 

 low low tide mark; its south coast, in quiet water very rich in oxygen, 

 supports a flourishing animal life of different forms. 40 Hollows, cracks, 

 and any protected sites on wave-beaten rocks afford a foothold to an 

 assemblage of sessile animals and their free-living companions. 



The severe conditions, which reduce the numbers of species in 

 unfavorable sites to a few, eliminate the competition of other forms, 

 so that those few flourish greatly. Mobius 41 estimated the number of 

 barnacles {Balanus crenatus) on a buoy at 10,000 per sq. m. Petersen 42 

 counted 3580 larger and 95 smaller mussels per sq. m.; these mussels 

 form a connected strip half a meter wide exposed at low tide, on long 

 stretches of the Scandinavian coast. 



A supratidal stage is present also on rock coasts, with specialized 

 inhabitants. The spray forms small pools in hollows in the rock above 

 high tide mark. In calm weather this water is warmed by the sun and 

 concentrated by evaporation, while continued rains at other times 

 reduce the salinity. Only especially euryhaline and eurythermal ani- 

 mals are able to survive in these pools. A few rotifers, a few copepods 

 and isopods, a few snails, and midge larvae {Chironomus frauenfeldi) 

 are found in them. The conditions in such pools are the more unfavor- 

 able, the higher above tide mark they occur. Besides Littorina, small 

 and active isopods of the genus Ligia occur on the spray-covered 

 rocks above the water line. 



Oysters and pearl oysters, which are of commercial importance, are 

 generally more or less abundant on rocky coasts. The oysters, Ostrea 

 edulis and numerous other species, require solid bottom on which to 

 attach their shells. It is sufficient for them, to be sure, if stones or 

 mollusk shells lie on the surface of the more solid sand or mud. The 

 Australian Ostrea glomerate/, even attaches itself to living snails 

 {Potamides ebeninus) ; 4 or 5 individuals often fasten themselves to 

 the same snail. With the growth of the oysters, their weight becomes 

 too great, the snail is forced into the substratum, and dies. Oysters 



