178 MARINE ANIMALS 



The "benthos," as the animal life of the bottom may be called, is 

 composed of animals which are wholly or for the most part dependent 

 on the bottom for support. Thus the squid Sepia belongs to the benthos, 

 although it can swim for a long time; the mullet (Mullus barbatus) 

 belongs to it because it depends on this section for its food. One might 

 hesitate about the classification of the skillfully swimming labrid 

 fishes, which frequently rest on the bottom to sleep,* or Labrus males 

 which watch over the eggs that are placed on the bottom, though their 

 food is taken from the open water. 



Within the benthal the different kinds of shore material produce 

 important differences in the environmental factors influencing its life. 

 The substratum may be loose or solid, with every sort of transition. 

 Loose bottom may be coarse or fine gravel, sand, or mud. In water 

 subject to motion, such a bottom moves with the water above it. Solid 

 shore material, i.e., rock coast, is found only on steep slopes and in 

 actively moving water, consequently mostly near the surface, occa- 

 sionally at greater depths, up to 1000 m. In quieter water, sediments 

 accumulate and maintain themselves even in the face of slow movement 

 of the water. 



Other differences in the life of the benthal depend on the motion of 

 the water and lighting. Both light and motion diminish with depth, but 

 the decrease in light is uniform, while that of motion is subject to 

 various irregularities and special conditions imposed by cosmic and 

 terrestrial influences. 



Subdivision of the benthal. — The sea bottom is accordingly 

 divided vertically into two zones: the lighted sea bottom, littoral 

 benthal, or more briefly the littoral; and the lightless abyssal benthal. 

 The littoral zone may be divided into two subdivisions, the littoral 

 proper, or eulittoral, extending to the limit of more abundant vegeta- 

 tion at 40 to 60 m., and the sublittoral from this point to the edge of 

 the continental shelf, which drops off into the abyssal depths at about 

 the 100-fathom (200-m.) line. We follow Pruvot 1 in not delimiting 

 further zones in the abyssal benthal. Thermal differences appear to 

 offer the best basis for such a division if one were to be attempted. 



Division of the littoral according to motion of the water is into 

 three vertical zones. The first is that of the tides, bounded above and 

 below by high- and low-tide marks. The second zone is that of wave 

 motion, and below that is the third zone of quiet water. The depth of 

 these zones varies with locality. On the west coast of Europe the dif- 



* Coris, Julis, Crenilabrus. 



