Consumers 479 



times as much organic matter is present in dissolved form as is repre- 

 sented by the phytoplankton. Piitter claimed to have evidence that 

 this material was used extensively for the nourishment of marine 

 animals. However, investigation of the problem now indicates 

 strongly that, if any dissolved organic matter is available to multi- 

 cellular organisms, it forms an insignificantly small fraction of their 

 total nutritive requirement (Krogh, 1931). We know that some dis- 

 solved organic matter in the water can be used by many kinds of 

 bacteria and other saprophytes, but a large fraction of it is resistant 

 even to bacterial attack, and this portion represents an unrecoverable 

 loss for the oceanic ecosystem. 



Organic detritus appears in the water directly from the disintegra- 

 tion of dead plants and animals and is also stirred up from muddy 

 bottoms. This material occurs in particles of all sizes, and even the 

 most minute may be removed from the water by animals possessing 

 mucous filtering apparatus. Echiuroid worms can obtain material 

 with dimensions as small as 4 m/A (MacGinitie and MacGinitie, 1949), 

 and Daphnia magna is capable of taking in detritus of colloidal size 

 in sufficient quantities for growth (Gellis and Clarke, 1935). De- 

 tritus has been shown to form a significant portion of the food of at 

 least certain filter-feeding bivalves (Coe, 1948), and no doubt it 

 does likewise for many kinds of zooplankton. The nutritional de- 

 pendencies of copepods and other key primary consumers are there- 

 fore very complex, but directly or indirectly their food is ultimately 

 derived from the organic material synthesized by the diatoms as the 

 chief plant producers of the open sea. 



(b) Carnivores. Since the herbivores in the typical community of 

 the open sea are mostly small filter feeders, the primary carnivores- 

 representing the third link in the food chain— are confronted with the 

 problem of obtaining food from prey species that are often less than 

 a centimeter or two in length. Nevertheless, certain sizable fish, 

 such as the mackerel, herring, and shad, are able to feed directly upon 

 the copepods and other primitive crustaceans of the plankton. In- 

 terestingly enough, the largest animals in the sea including the bask- 

 ing shark and the whalebone whales are also able to live upon the 

 tiny zooplankton. The great sheets of frayed whalebone attached to 

 the upper jaws of these whales serve as gigantic sieves (Fig. 13.6). 

 When the whales are feeding, they take in huge mouthfuls of the sea 

 and by means of their massive tongues force the water out through 

 the whalebone. The copepods, euphausiids, and other types of zoo- 

 plankton retained within the mouth cavity are then gulped down. 

 The fact that the food chain of these whales is composed of only three 



