22 UNDERWATER GUIDE TO MARINE LIFE 



and molluscs. Furthermore, the sublittoral soft bottoms are frequently covered 

 with lush growths of eelgrass or one of the marine algae which, though rootless, 

 can obtain a foothold in sand or mud. This adds another batch of animals such 

 as sea urchins, starfishes, sea horses, pipefishes, eels, killifishes, and even more 

 worms, molluscs, and crustaceans. The presence of plants markedly increases 

 not only the diversity, but also the abundance of life due to the added oxygen 

 and debris which they bring. Here also, a sand-mud mixture supports the 

 greatest amount of life. As might be expected, coarse sand or gravel supports 

 the least. The forms that live in pure mud are very delicate and usually of 

 fairly deep water, since fine mud particles do not come to rest on the bottom 

 until quiet seas are reached near the outer "mud-line" of the continental shelf. 



Special Zones 



Three zones, all of them littoral and tropical, must be given special attention 

 because of their unique characteristics. They are (1) a special type of muddy 

 shore, the mangrove zone, (2) a special type of plant community, the Sargasso 

 Sea, and (3) a special type of rocky zone, the coral reef. 



THE MANGROVE ZONE 



World-wide in muddy, estuarine flats of the tropics grow mangrove trees, 

 especially the red mangrove, Rhizofhora mangle, whose branching and support- 

 ing aerial and stiltlike roots form a swampy tangle in which the fauna of the 

 sea meets that of the land. Crabs, tree oysters, snails, small tropical fishes, 

 snappers, tarpon, and other brackish-water species meet raccoons, land snails, 

 land crabs, and even some fresh-water fishes. Actually the beginning of man- 

 groves usually means the end of the shallow sea for in the roots of the plants, 

 debris and coral, sand or mud are trapped and a land-building process is begun. 

 In fact, as the debris gets thicker and drier, the mangroves themselves die out. 



THE SARGASSO SEA 



In the great eddy of central Atlantic waters from Berm.uda to the south and 

 west toward the Bahamas, Sargassnm, or sargasso weed, the only floating 

 brown alga, is found in fairly large quantities to form a distinctive community 

 of its own. This weed breaks off from its anchorage on rocks in the Caribbean, 

 especially during hurricanes, and is carried to this sea by ocean currents such 

 as the Gulf Stream. There it lives and grows, but it probably does not repro- 

 duce there, until it dies and disintegrates. The total mass of this algae has been 

 greatly overestimated. Clumps of it do become quite large, according to how 

 much gets tangled up together, but it is not a threat to navigation as is some- 

 times stated. Actually, the area of the Sargasso Sea would be quite sterile of 

 life were it not for this weed, for, though this is the area where the world's 

 clearest waters are found, it is also an area of sinking water where nutrients 

 are rather sparse. 



Sargasso weed is brown and leafy (Chapter 5) so most of the animals that 

 live in it are brown and bear leaflike appendages as concealing coloration and 

 form. The weed forms a substrate, a sort of pseudobenthos, for a rather restricted 



