ABYSSAL BENTHAL AND PELAGIAL 255 



several meters long, is an exception. The reason may lie in the scarcity 

 of food. The relatively gigantic size of a few deep-sea forms, in com- 

 parison with their relatives in shallow water (see p. 159), is the more 

 conspicuous. Long duration of growth due to postponement of sexual 

 maturity conditioned in turn by low temperature has been suggested 

 as an explanation. 



The density of the animal population on or near the bottom in 

 deeper water is unknown. There is a definite, positive correlation be- 

 tween the amount of life at these depths and the development of 

 phytoplankton at the surface. The ultimate source of food for these 

 deep-dwelling animals appears to be from (a) surface organisms or 

 their excreta which sink or (6) the utilization of dissolved substances. 

 The plankton organisms cannot reach the bottom at great depth in sig- 

 nificant amounts, but the excreta and bodies of large animals must pro- 

 vide a considerable amount of food. Calculations of the number of 

 whales dying in Antarctic waters support this contention. In a column 

 of water taken at 5400-m. depth off Bermuda, Krogh found that the 

 concentration of nitrogen and carbon was constant from surface to 

 bottom at 0.244 and 2.35 gm. per cu. m., respectively. These quanti- 

 ties are approximately 300 times the average amounts in marine 

 organisms per cubic meter. The theory of Putter that organisms take 

 such dissolved nutriment directly from the water may hold for smaller 

 forms such as bacteria and protozoans and perhaps for sponges. The 

 small number of bacteria in sea water argues against their having a 

 high food value; those in the bottom ooze may be present in high 

 enough numbers to have significance as food for protozoans which are 

 also able to utilize dissolved substances, at least to some extent, and 

 to feed upon other minute particles. These protozoans, together with 

 the particles falling from upper levels, probably constitute the base of 

 the food pyramid in the deep waters. 14 



Bioluminescence. — The production of light by deep-sea animals 

 is to be regarded as an adaptation to the absence of sunlight. Light 

 production is by no means confined to the animals of the deep sea, for 

 pelagic animals of diverse groups give off light in sufficient quantity to 

 produce the bioluminescence of the surface of the sea. The protozoan 

 Noctiluca, with Pyrocystis in the tropics, the jellyfishes Cyanea and 

 Pelagia, the ctenophores Beroe and Cestus, copepods among crusta- 

 ceans, Phyllirhoe among snails, and Pyrosoma among tunicates are 

 examples. Animals that live in the dark may show no tendency toward 

 light production. The cave animals do not exhibit a single light- 

 producing form, although there are a number of bioluminescent forms 

 among terrestrial animals, including an earthworm and the various 



