ZONES OF THE SEA-WHERE THEY LIVE 



25 



As has been pointed out, the coral reef is the most gaudy and varied realm of 

 life on earth. The animals live so close to each other spatially that they often 

 develop symbiotic dependencies on one another. The pearlfish and the sea 

 cucumber, the conchfish and the conch, and the zooxanthellae and the coral 

 polyps are only a few examples of this. 



Coral itself "blooms" at night, the polyps mostly keeping to their limestone 

 chambers by day. In fact, the whole reef teems with nocturnal animals, such 

 as big-eyes, squirrel fish, and others that are more active at night than by day. 

 But even by day the coral reef is a bustling community. So ideal is the protection 

 offered among coral branches and caves that many animals never leave the reefs. 

 This protection also seems to allow them to be more brilliantly colored than 

 elsewhere, but it must be admitted that the reason for the exceptionally gaudy 

 colors of reef animals is not known. Possibly, the colors of the animals have 

 evolved to match the brightness of the lisht and the clearness of the waters. Com- 



o O 



petition for space on the reef is keen and is probably another reason for the 

 diversity of life, since the animals attempt to adapt to new situations quickly 

 (Chapter 5). 



Most of the life of the reef is benthic or nekto-benthic since almost all of the 

 species, including the large groupers, make their homes in reefs or feed near 

 the bottom. The great majority of benthic groups are represented on reefs 

 as are the great majority of animal phyla. About the only group which is not 

 well represented on reefs is the large algae, chicflv the brown algae (phylum 

 Phaeophyta). Reefs are largely an animal realm and, indeed, very little matter 

 of nonanimal origin is found there. 



FRINGING REEF 



BARRIER REEF 



ATOLL 



Fig. 5. The three kinds of reefs Qadapted from Darwin^. The top row of diagrams 

 represents cross-sectional views of reefs; the bottom row, views from, above. Each type 

 of reef may he exposed at low tide and may even form, the foundation for land 

 formation. Since a reef grows fastest at its outer, seaward border, land masses which 

 are sinking will draw the coral downward. The coral will grow to reach the surface 

 only at its seaward edge and a lagoon will be formed. 



