196 MARINE ANIMALS 



Vast numbers of small green turbellarian worms Convoluta roscof- 

 fensis are exposed at low tide along the coast of Normandy, England, 

 and the Channel Islands, and form dark green bands, often for many 

 kilometers. By count, the numbers of these worms per square meter 

 may be as high as 3,350,000. Light is required for the symbiotic algae 

 contained in them. 19 The annelids Arenicola, Nereis, Terebella and 

 Echiurus, and the hemichordate Balanoglossus inhabit the intertidal 

 beach, as do many sorts of crustaceans. Shallow tide pools retain 

 shrimps, and various other crustaceans, especially hermit crabs. 



A number of species of the area exposed by the tides are so adapted 

 to its special conditions that they are typically found in it; they are 

 stenobathic, and confined to shallow water. This applies to the above- 

 mentioned Convoluta; the worm Arenicola; and the snails Melampus 

 and the abundant Littorina litorea. 



Where a sand beach is freely exposed to surf, so that the sand is 

 continually stirred up and redeposited and the detritus particles washed 

 out, the conditions for plant and animal life are very unfavorable. 

 Animal life is then usually wanting in the upper levels, and only ap- 

 pears in considerable amount when the wave motion is reduced, at 

 considerable depths. The Californian pismo clam lives in this habitat. 



Life of the spray zone. — Marine life, finally, extends into a zone 

 above high tide mark, where only spray and waves reach it, i.e., the 

 supratidal zone. The marine animals that live in this situation have 

 adopted an air-breathing existence, and must be protected from drying 

 out by a strong shell. Small quick-jumping amphipods (Orchestia) 

 live beneath the seaweed which is often thrown up on the beach. Some 

 of the related forms even extend further inland, like Talitrus locusta. 

 Small snails are attached to the seaweed leaves. In the tropics the 

 supratidal stage is characterized above all by the crabs Grapsus and 

 Ocypoda which are air-breathers, and the widely distributed hermit 

 crab Coenobita. 



Estuaries. — -Estuaries form a special biotope in the littoral zone. 20 

 Change and mixing of fresh and salt water and periodic stirring up by 

 the tides produce special conditions. A river mouth at low tide has a 

 median channel; the shores on each side are steep as high as ordinary 

 tides reach, and more sloping above mean high tide, in the region of 

 exceptional tides. On these slopes is deposited a sticky and often very 

 dense mud in which a special fauna exists, composed of a mixture of 

 fresh-water and marine animals. The list of marine animals is ex- 

 tremely small; it includes, in the northeastern Atlantic, the soft-shelled 

 clam, Mya, the annelid Nereis cultrifera, and besides a few Gam- 

 maridae, the ever-present beach crab, Carcinides maenas. (See also 



