202 MARINE ANIMALS 



Lo Bianco 31 says that the Secca di Chiaja is rather rich in animals 

 which are elsewhere rare, and the somewhat deeper-lying Scogli coral- 

 liferi in the same area harbor a different association of animals. These 

 differences are the more marked with greater geographic separation, 

 especially below the laminarian zone. The sessile animal life, and its 

 accompanying free-moving forms, is especially rich in deep sounds 

 where a strong current keeps the bottom free from mud. Kukenthal 32 

 reports such a wealth of life from the West Indian waters near St. 

 Thomas: "at times our great trawl was filled with hundred weight 

 catches of tunicates, gorgonids, hydroids, sponges, and free-swimming 

 forms." Schaudinn 33 reports similar conditions in the straits east of 

 Spitzbergen. 



The force of the surf is greater on rocky coasts than on sand 

 beaches. This results in restriction of the fauna, which is greater the 

 more open the coast and the stronger the tides. In the surf-beaten belt, 

 and especially between tide marks, the growth of algae is sparing or 

 absent. The animals exposed to the surf are all protected against being 

 torn loose and crushed against the rock by special adaptations. They 

 are mainly animals with a solid shell, especially mollusks and 

 barnacles. Barnacles of the genus Balanus extend to the highest level, 

 reaching the uppermost high tide mark. In localities like St. Malo, 

 where the tides are especially high, they may be attached to the rocks 

 many meters above the low tide mark, where they are reached by the 

 high tide, and bathed in sea water for a few hours only twice a month. 

 They are exposed to the summer sun and the cold of winter, but re- 

 main alive, protecting themselves against complete desiccation by the 

 water retained within their shells. At a slightly lower level, snails of 

 the genus Littorina are interspersed among the barnacles ; Nerita holds 

 a similar position in the Indian Ocean. Although these snails belong 

 to different suborders of the gasteropods, their similarity of form, pro- 

 duced by convergent adaptation to similar conditions, is astonishing 

 (Fig. 35). Below these appear limpets {Patella) and chitons (Chiton), 

 with Haliotis, Murex, and Trochus at a still lower level. All these 

 mollusks have a broad foot surface, with which they attach themselves 

 to the rocks. The adhesion of Patella is supplemented by a glandular 

 secretion, and resists a pull of 3.7 kg. per sq. cm. 34 In addition, their 

 shells are pressed tightly to their supporting surface ; where the surface 

 is uneven, the shells of Patella acquire a corresponding irregularity at 

 their edges 35 (Fig. 36), so that a given snail must retain its exact sta- 

 tion, to which it returns after short excursions in search of its algal 

 food. At such a permanent station the rock surface is smoothed, doubt- 

 less by the corrosive action of the secretion of the foot. Instances are 



