230 THE BIOLOGY OF MARINE ANIMALS 



It is interesting to test the calculated values against actual measurements 

 of food stuffs available. One ml of 2 will burn about 0-8 mg of organic 

 matter. For an animal filtering 15 l./ml 2 , this is equivalent, in terms of 

 energy consumption, to 0-05 mg of organic matter per litre. Allowing 

 two-thirds of the energy absorbed for growth, we calculate the total food 

 requirements for growth and respiration as about 0- 1 5 mg of organic matter 

 per litre. Only part of the organic matter ingested is utilized, however; 

 in Calcmus finmarchicus, for example, it is estimated that half the nitrogen 

 ingested is lost in the faeces. Actual amounts of organic matter, in detritus 

 and phytoplankton from different waters, range from 0-14-2-8 mg/1. 

 These values appear to be of about the order necessary to satisfy the 

 nutritional requirements of the filter-feeding animals concerned. All the 

 calculations presented involve many variables, which make them rather 

 unsatisfactory (44, 52, 59). 



Filtering Devices in Vertebrates 



Despite great disparity in size, certain adult vertebrates utilize filtering 

 methods of feeding fundamentally not unlike those already described in 

 certain invertebrate forms. In all these cases, whether they concern fish, 

 fowl or whale, there is some form of filter or sieve which strains off plank- 

 ton from the sea water. These animals really by-pass one or several in- 

 termediate links in the food chain by feeding on planktonic crustaceans; 

 because of their size and food requirements they must clear large volumes 

 of water to obtain sufficient food organisms, and at least for feeding 

 purposes they are restricted to areas of high planktonic density. 



Fish. Various pelagic fish are plankton feeders. Huge basking sharks 

 Cetorhinus and whale sharks Rhineodon feed exclusively upon plankton. 

 These creatures are provided with numerous closely set gill-rakers, which 

 strain off the myriads of small crustaceans from the water which enters 

 the mouth (74). 



Among teleosts, mackerel and herring possess large thin gill-rakers 

 which project across the pharyngeal openings and prevent the escape of 

 planktonic organisms. Copepods predominate in the food of young herring. 

 Adults feed largely on a non-crustacean diet in spring (mostly sand eels) 

 and shift to a crustacean diet in summer. 



Birds. Although many marine birds feed on plankton {see p. 246), 

 there is only one group which is anatomically specialized for sifting out 

 floating animals, namely prions or whale birds (Pachyptila). In certain 

 species the upper mandible bears two rows of comb-like lamellae, strik- 

 ingly analogous to the baleen plates of whalebone whales (Fig. 5.23). 

 The resemblance is heightened by the presence of a large fleshy tongue. 

 Whale birds are denizens of subantarctic waters. They feed from the surface 

 on Crustacea by submerging the head, and scooping up food with the 

 laminated bill (78). 



Whalebone Whales (Mystacoceti). These animals show highly specialized 

 adaptations for securing plankton. The filtering apparatus consists of 



