epresentati' 



iter fishes (Sver. 



e/o/. 1942). 



The bathy- and abyssopclagic communities are 

 considered as one. The fishes are slender and dark- 

 colored. Pelagic invertebrates include a few endemic 

 species of radiolarians, jellyfish, ctenophores, nemer- 

 tinians, ostracods, copepods, amphipods, euphausi- 

 aceans, mysidaceans, shrimps, and squids. Red color 

 is more common than at intermediate depths. Benthic 

 animals commonly have flat bodies, very long legs, 

 or other means of distributing their vi'eight over the 

 loose, flocculent ooze. Many species rise above the 

 ooze on stalks. The fragile glass sponges, long- 

 stemmed crinoids, and long-legged crabs are possible 

 only in very quiet waters that occur at great depths. 

 Skeletons of all animals are fragile because of the 

 difficulty of forming lime at low temeperatures. 

 Abundance decreases with depth, but even at 8300 

 m in the hadal zone, some twenty species have been 

 found, chiefly holothurians, polychaetes, and sea ane- 

 mones. 



Bioluminescence is exceptionally well developed 

 among deep-sea forms. In some invertebrates, light- 

 producing organs are scattered over the body. In 

 other invertebrates and in pelagic fishes, there are 

 special luminescent organs. It is estimated that two- 

 thirds of the bathypelagic fish species and over 96 

 per cent of the individuals are luminous. Although 

 several species of organisms occurring in surface 

 waters are luminous, bioluminescence is more highly 

 developed in the twilight zone, between 300 and 800 

 m, and occurs at still greater depths in the complete 

 absence of natural light. The adaptive significance of 



bioluminescence is highly speculative. It may serve, 

 in part, for attracting and seeing prey. Luminescent 

 display may also serve for species and sex recogni- 

 tion as does color in many surface animals. Joined 

 with this bioluminescence is often the development 

 of large eyes and special structures to permit vision 

 at the very low light intensities that are produced. 

 Perhaps as compensation for the difficulties of vision 

 is the extensive development of antennae on some 

 crustaceans and the very long rays in the fins of some 

 fish which may serve for contact reception. In those 

 fish where the eyes are small there is a reciprocally 

 large development of olfactory organs. 



The benthos and pelagic forms of the greater 

 depths are doubtless derived from intermediate-depth 

 forms, and these in turn from forms occurring on the 

 continental shelf. Species have come to live in the 

 deeper waters only as they became progressively 

 adapted to this rigorous habitat. Relatively few forms 

 have reached the hadal zone. 



The deep-sea habitat has existed relatively un- 

 changed since very early geological time except for 

 the increasing deposition of bottom sediments and 

 for some fluctuations in temperature. This uniform 

 habitat has allowed some very ancient forms to per- 

 sist to the present time. The recently discovered 

 coelacanth fish Latimeria. the mollusk Ncopilina. and 

 certain crustaceans belong to taxonomic groups that 

 supposedly became extinct many millions of years 

 ago. The examination of deep-sea bottom cores will 

 doubtless give us information as to what kinds of 

 animals were present in past ages. Determination of 

 ratios of different oxygen isotopes and of different 

 minerals in the composition of the fossil skeletons in 

 these cores may make possible the determination of 

 water temperatures and salinities at the time these 

 fossil organisms were living (Ladd 1959). 



Food chains 



As in aquatic and terrestrial communities, bac- 

 teria in the sea are largely responsible for the final 

 decomposition of excreta and dead bodies to make 

 their essential nutrients available for reabsorption by 

 the green phytoplankton (Ketchum 1947, Harvey 

 1955). 



Nitrogen and phosphorus are least concentrated 

 near the surface of the ocean, since this is the stratum 

 in which they are most rapidly absorbed by the photo- 

 plankton. Excreta and dead organisms sink during 

 the process of decomposition, so nitrogen regenera- 

 tion is most evident at depths of 500 to 1500 m. 



The organic matter that remains undissolved ac- 

 cumulates on the sea bottom. Numerous species of 

 invertebrates depend on it for food and on the bac- 

 teria that it contains (ZoBell 1952). The deep-sea 



360 Geographic distribution of communities 



