30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 28 



Plankton is mainly diatoms, and detritus is mainly bacteria and micro- 

 scopic plant material being used for food by bacteria. Seaweeds per se 

 are little used as food, but when they are broken down to micro- 

 scopic or nearly microscopic size and are decomposing they, along with 

 the great bulk of bacteria feeding on them, form detritus. Practically 

 all bottom-dwelling animals feed on detritus. Detritus even forms 

 coatings on the surfaces of sand grains. Therefore any mud- or sand- 

 ingesting animals live on detritus, and many are equipped with spe- 

 cial devices for straining this from bottom surfaces with a minimum 

 intake of mud or sand. The more finely divided the sediment, the 

 greater the surface, and, since marine bacteria live only on surfaces, 

 the greater the opportunity for bacteria to exist. Therefore it can be 

 said that in general mud is richer in detritus than sand. The richness 

 of the detritus depends on the settling out from the water of organic 

 material and, therefore, detritus is richer close to shore, especially in 

 the vicinity of river mouths. The suspended material in the muddy 

 water of the ocean bottom is mainly detritus, which is constantly being 

 stirred up by the movements of animals, by currents, or the activities 

 of the detritus feeders themselves. Near shore it is churned up by 

 the surf. Even some vertebrates, such as the shovel-nosed shark or 

 bat stingray, are indirectly dependent on detritus for food, as they 

 dig for and feed on clams and worms that feed directly on detritus. 



As there are no seaweeds to speak of in the region of Barrow, the 

 writer wondered where the detritus came from, but in the fall of 1949 

 this question was answered by the entrance of great quantities of 

 tundra plants into the ocean waters from the eroding shores. Any 

 plant material will form food for marine bacteria and, therefore, will 

 produce detritus. Within a month enough plant material to supply 

 detritus for several years was washed into the ocean and drifted over 

 a great area. There is certainly an overabundance of detritus in the 

 Arctic off Barrow, and the food of Arctic animals is far in excess of 

 their needs. 



The feeding habits of Arctic invertebrates, with examples of ani- 

 mals living on the different types of food, may be summarized as 

 follows : 



All free-moving invertebrates surrounded entirely by water above 

 the bottom are plankton feeders, directly or indirectly. Conspicuous 

 among these are jellyfishes, ctenophores, and certain amphipods. 



The majority of the bottom dwellers are detritus feeders, directly 

 or indirectly. These include echiuroid and sipunculid worms; certain 

 polychaete worms such as Arenicola, cirratulids, and terebellids ; 

 clams, bryozoans, and Foraminifera. 



