neritic biochore is above the continental shelf and 

 is commonly 16-240 km (10-150 mi) wide. The 

 oceanic biochore is subdivided vertically with the 

 boundary between the epi- and mesopelagic zones, 

 depending on the extent of effective light penetration. 



or inactive and attached to surfaces when the tide is 

 in (Faure-Fremiet 1951). These rhythms in inter- 

 tidal organisms may persist for days, even when the 

 organisms are placed experimentally under constant 

 conditions (Brown 1959). 



Tides 



Substratum 



The level of water in the ocean rises and falls 

 usually twice each day or at an interval of 12 hours 

 and 26 minutes. In some parts of the world the tides 

 are less regular or there may be but one daily. Flood- 

 tide is the period in which the level is rising and 

 covering more and more of the shoreline ; ebb-tide is 

 the period in which the waters are receding. In the 

 open sea the change in water level is less than a 

 meter, but the change may be much more than this on 

 the shore, depending on its configuration. The Bay 

 of Fundy opens broadly to the sea and tapers to a 

 narrow head landward, and tides may be 6 to 10 or 

 even 15 meters. On the other hand, when bodies of 

 water have only a relatively narrow connection with 

 the sea, as does the Gulf of Mexico with the Atlantic, 

 the range in water level is less than 30 centimeters. 

 Even lakes have a tide, but it is hardly perceptible 

 except in the larger lakes where it may amount to 

 a few centimeters. 



Tides are caused by the attraction of the moon 

 and, to a lesser extent, the sun. When the sun's at- 

 traction is added to that of the moon, as occurs twice 

 each month at times of full moon and new moon, 

 the fluctuations of the tides are unusually high and 

 unusually low. These are called spring tides. When 

 the tidal influences of sun and moon are opposed as 

 happens twice each month, the tides have the least 

 amount of flow and ebb and are called neap tides. 



Tides have their greatest effect on animals on the 

 seashore, because of the associated pounding of waves 

 and the alternate submergence in water and exposure 

 to the air. However, the organisms appear well ad- 

 justed to this rhythmic submergence and exposure 

 (Flattely and Watson 1922, Korringa 1947). For 

 instance, as the stones on which the chiton occurs 

 become exposed, the animals react positively to grav- 

 ity and negatively to strong light, and move down- 

 wards. They travel at maximum speed while the 

 stone is still moist and become aggregated on the 

 damp lower sides of the stones. When the stone again 

 becomes immersed by the returning tide, the animals 

 lose their geotatic orientation, and, since illumination 

 becomes more or less equal on all sides of the stones, 

 they move about at random until they reach the upper 

 surfaces again. 



On the other hand some ciliated and flagellated 

 protozoans and diatoms in inter-tidal habitats are 

 active only when the tide is out and become encysted 



The pounding action of waves on rocky shore 

 may have tremendous force, estimated in one instance 

 at 15,000 kg/cm-. Animals occupying exposed rocky 

 shores in the surf belt must be strongly protected and 

 firmly attached (Flattely and Walton 1922). The 

 conical-shaped limpets present a minimum of surface 

 to the waves. Barnacles are protected by heavy shells 

 and grow fast to the rocks, snails and chitons hold 

 themselves by powerful suction apparatus on their 

 feet, mussels like Mytiltis have a glandular byssus, 

 while some species of sea urchins bore shallow craters 

 into the rock. Advantage is taken of nooks, crannies, 

 and spaces underneath stones and rocks (Glynne- 

 Williams and Hobart 1952). Large depressions in 

 the rock retain water at ebb tide to form tidal pools 

 and thus may contain the more delicate species be- 

 cause of the protection they afford. The various sea 

 weeds absorb some of the wave shock for the animals 

 living with them. The shape, form, and size of corals, 

 sponges, and other colonial types are affected by the 

 amount of wave action to which the animals are ex- 

 posed. A shell of the Mytihis mussel may weigh 58 

 g where the animal is exposed to a heavy surf but 

 only 26.5 g in more quiet waters. 



Sandy beaches occur only where the force of 

 waves is reduced by being spread over a more gentle 

 slope. Even here, especially during storms, the sand 

 makes a very unstable substratum and not many 

 animals except mollusks and some of the echinoderms 

 can keep from being smothered or buried. Mud but- 

 toms occur only in relatively quiet waters. Burrows 

 made in mud hold their form better than in sand, so 

 larger populations of animals can occur in mud. 



The sea-floor at greater depths is covered with a 

 variety of sediments. Terrigenous deposits of min- 

 eral and organic matter derived from the land and 

 from the littoral and neritic biochores are relatively 

 rich in nutrient substances and extend into the 

 bathyal zone. All other deposits on the sea-bottom 

 are pelagic, being derived, in part, from the skeletons 

 of dead plankton and other organisms. In the long 

 slow journey of these dead organisms to the bottom 

 of the sea, much of the organic matter decomposes, 

 releasing carbon dioxide, nitrates, phosphates, and 

 the many other elements in the composition of the 

 protoplasm. Even various amounts of skeletal mate- 

 rial may dissolve, but enough of the organisms reach 

 the bottom to create a substratum of loose flocculent 



352 Geographic distribution of communities 



