organisms and of small fish are probably responsible 

 for the shifts in the position of the deep scattering 

 layer evident in the reverberation of liigh freqnency 

 sound waves sent out from the surface ( Kyring ct al. 

 1948, Backus atid Barnes 1957). 



NKKTO.N 



burrciwing forms as iujatina. Ivjiifauna in the littoral 

 zone decreases in variety toward tlie Poles since it is 

 subjected to cold and ice erosion, but the species 

 composition of infauna remains about the same. 



OCEANIC PLANKTON AM) 

 NEKTON BIOME 



Mollusks, fishes, birds, and mammals make 

 up the nekton of the sea. Mollusks are represented 

 by the squids: fish, by the sharks, flying fish, her- 

 rings, mackerels, as well as many otiiers including 

 lunuerous varieties of small species: and mammals, 

 by the seals, porpoises, dolphins, and whales. The 

 distribution of fish is irregular, but in general they 

 occur more abundantly in neritic waters than in the 

 open ocean. Likewise they are much more numerous 

 in the epipelagic than in lower strata. Most pelagic 

 fish, e.xcejit sharks, possess a swim-bladder useful 

 for maintaining hydrostatic equilibrium at the depth 

 where they occur: those fish that lack one are com- 

 monly bottom forms (Marshall 1954). In Arctic 

 waters, fish are less abundant, and mammals rela- 

 tively more important, than is the case farther south. 

 Birds, like many other marine animals, are more 

 numerous in the neritic hiochore than in the open 

 ocean. In the oceans far from land occur only pen- 

 guins, albatrosses, shearwaters, and petrels, and even 

 these species become more common shoreward. 

 Other marine species in neritic waters are tropic- 

 birds, pelicans, gannets, boobies, cormorants, frigate- 

 birds, ducks, gulls, terns, skimmers, auks, and 

 murres. These marine birds may spend many days 

 or weeks feeding and travelling over the water, but 

 all must search out some shore, cliiT, or isolated island 

 on which to nest. Here they sometimes concentrate 

 in enormous numbers during the nesting season be- 

 cause of the limited number of suitable nesting loca- 

 tions available. 



BENTHOS 



Benthos is of much greater variety in ma- 

 rine than in fresh-water habitats. These animals are 

 very abundant in the littoral zone and decrease in 

 numbers with depth until only scattered individuals 

 are found in the deep ocean trenches. Benthos con- 

 sists of sessile forms, the sponges, barnacles, mus- 

 sels, oysters, crinoids, corals, hydroids, bryozoans, 

 and some worms : creeping forms such as crabs, lob- 

 sters, certain copepods, amphipods, other crustaceans, 

 many protozoans, snails, echinoderms, some bivalves, 

 and some fishes : and burrozvinci forms including most 

 clams, worms, and some crustaceans. Sessile and 

 creeping forms are often grouped as epifauna and the 



This biome is characterized by the pre- 

 dominance of organisms possessing life-forms adapted 

 to keep them afloat. Plankton and nekton predomi- 

 nate, although the deep-sea benthos may also be con- 

 sidered as belonging to this biome. Seasonal aspec- 

 tion may bring drastic changes in species composition, 

 especially in plankton. Dominance, in the sense used 

 for terrestrial communities, probably does not e.xist, 

 e.xcept possibly in the Sargasso Sea where the float- 

 ing vegetation establishes the habitat. The ecosystem 

 is self-contained, however, since energy is derived 

 from the sun and nutrient material continues to re- 

 circulate with little or no dependence on terrestrial 

 resources. 



The Sargassiim community of the Atlantic Ocean 

 is of special interest. The floating Sargassum alga 

 accumulates and is held within a limited area by 

 circular ocean currents. This plant belongs to the in- 

 tertidal zone of the Caribbean islands but is torn 

 loose in large amounts along with attached animals 

 during the hurricane season. It continues to grow 

 thereafter, but does not reproduce. The fauna that it 

 contains is a truly littoral one, rather than pelagic, 

 l)ut because the alga accumulates in fresh amounts 

 as fast as old plants die, the animals reproduce and 

 maintain a continued existence far from any shore. 



Composition and characteristics 



The species composition of this biome varies 

 consistently with depth so that a series of overlapping 

 secondary communities may be recognized (Murray 

 and Hjort 1912, Ekman 1953, Marshall 1954, Bruun 

 in Hedgpeth 1957). 



The epipelagic coimminity or stratal society has 

 the greatest abundance of plankton, nekton, and birds 

 as already described. The aquatic animals are gener- 

 ally colorless, transparent, or of a blue cast. 



In the mesopclagic community the fishes are usu- 

 ally small, laterally compressed, often silvery or 

 grayish in color, with very large or telescopic eyes, 

 and usually provided with luminescent organs. Some 

 velvety black or brown fishes also occur here. Inver- 

 tebrates are reduced in number and variety and tend 

 to be reddish in color. Since red rays do not pene- 

 trate to the depths where these animals live, they are 

 essentially invisible. 



Marine biomes 359 



