392 ADAPTATIONS TO MEDIA AND SUBSTRATES 



Carl Hubbs has pointed out that the acanthopterygians, by and large, 

 are adapted for a shallow-water, shore existence. They have spread far 

 and wide into fresh waters, but their marine representatives have mostly 

 stayed in the littoral zone, on the continental shelves. The malacopter- 

 ygians, Hubbs emphasizes, are characteristically pelagic. Abundant in 

 fresh waters and over the continental shelf, they have also been able to 

 go out into the open ocean, whereas the acanthopterygians are tied to 

 the bottom. The soft-rayed fishes have retained the primitive connection 

 of the air-bladder with the throat, and can thus reduce their buoyancy 

 quickly when they wish to descend for a considerable distance. A few, 

 e.g., Arapaima, still use it for what was probably its original function — 

 that of a lung. In the acanthopterygians, the gas-bladder is a blind pouch 

 and is employed variously as a slow-acting hydrostatic organ, as an ear- 

 trumpet, or as a resonator for vocalization. Many of these bottom-bound 

 fishes — the darters, for example — have lost it entirely. 



The differentiation of the malacopterygians and acanthopterygians 

 into originally pelagic and demersal types, respectively, did not remain 

 at all rigid. Littoral forms belonging to both divisions learned to live 

 beyond the edge of the continental shelf, farther and farther down the 

 continental slopes and into the deep water of the bathyal zone. Some 

 even went out onto the ocean floor, where the depth of the water ranges 

 mostly between two and three miles. These inhabitants of the abyssal 

 zone constitute the deep-sea benthos, the bottom fauna. Many families 

 of fishes are represented in the abyssal portion of the benthos, some of 

 them having no members elsewhere. For the most part, the abyssal fishes 

 are archaic. 



The benthonic fishes are a minority in the whole deep-sea fish pop- 

 ulation. A number of pelagic malacopterygians have sunk lower and 

 lower to become bathypelagic, and a few have even gone all the way 

 to the ocean floor to become a part of the benthonic fauna. Both the 

 bathypelagic and abyssal faunas have received new additions from time 

 to time, and will no doubt continue to do so. 



The benthos (but not the richer bathypelagic fauna) contains elasmo- 

 branchs as well as teleostean species; and of course at one time the only 

 bathybic fishes were elasmobranchs. A number of rays and sharks, and all 

 of the bizarre chimaeras, live on the continental slopes and on the ocean 

 floor. Specimens of the weird luminous shark, Etmopterus (=Spinax) 

 niger, have been taken at various levels between 100 meters and 3000 



