214 BULLETIN OF THE BUREAU OF FISHERIES. 



The commonest animal forms in Danish waters are classified into three groups 

 according to their mode of feeding: (i) Herbivores, which include certain gastropods, 

 two echinoderms and some Crustacea. (2) Pure detritus eaters, which comprise all 

 the Lamellibranchia, Holothurians, Sipunculidse, Cumacea, Diptera larvae and Ascidiae, 

 two gastropods, Balanoglossus, Amphioxus, ostracods, Bryozoa, Porifera, and Fora- 

 minifera. The great mass of material in the alimentary tracts of these animals is detritus 

 and when analyzed chemically it corresponds to that on the ocean floor. Plankton 

 organisms are only incidentally present. These observations led Blegvad to make the 

 extreme statement: "The living phytoplankton is thus of no importance at all as a food 

 for the bottom fauna." (3) Purely carnivorous animals, including a few Polychseta, 

 some gastropods, some Crustacea, some echinoderms, coelenterates, nemerteans, plana- 

 rians, and pantopods, constitute the last group. Quite a large number of animals are 

 both carnivores and detritus feeders. 



The Danish investigations tend to show the vast importance of detritus as a food 

 for the fauna on the sea bottom. To use Blegvad's words : 



Detritus forms the principal food of nearly all the invertebrate animals of the sea bottom, next in 

 order of importance being plant food from fresh benthos plants. The value of the live phytoplankton 

 in this connection is absolutely minimal, amounting in any case to nothing more than an indirect sig- 

 nificance through the medium of the plankton copepods. 



This view is given some support by the recent researches of Mitchell (19 17), who 

 presents evidence that oysters can utilize fragments of seaweed (Ulva lactuca) as food. 



That detritus is formed so abundantly in the shallower waters of the ocean and con- 

 stitutes such an important source of food supply for most of the bottom-inhabiting 

 animals is of great significance in its bearing on the coming science of sea farming. If 

 the investigators of the Danish biological station are right in their conclusions con- 

 cerning the importance of detritus as food for the benthos fauna, then we shall have to 

 revise our methods of determining the available oyster, mussel, or clam food supply 

 in the waters of a given locality. It also means that the available fields for the cultiva- 

 tion of oysters or other shellfish may be more fertile than we have ever dreamed in the 

 past. The knowledge of the role played by detritus in its relation to the benthos fauna 

 helps us to understand better the phenomenal growth which often takes place in many 

 mollusks. For example, many mussel beds are known to yield on an average 2,000 

 bushels per acre annually, and experiments have shown that 1 bushel of seed clams 

 planted in a barren flat will yield 10 bushels of marketable clams one year later. This 

 serves to show what splendid opportunities for increased food production lie within our 

 reach. Between the plankton organisms and the detritus there is an inexhaustible 

 ultimate food supply which can be quickly and readily converted into a form available 

 for human consumption. A partial solution of the serious problem of increasing the 

 food production of the Nation lies in the appropriation of this vast resource for conver- 

 sion into mussels, clams, and oysters. Mussels planted in protected situations, where 

 the water currents will bring them an abundance of these materials, will produce flesh 

 food at a rate far in excess of any resource on which we have depended in the past. 

 Cultivating the ocean promises to yield the fisherman far greater returns, with less 

 expense of time and energy, than the farmer is able to derive from the land. Each new- 

 discovery in marine biology is making it more clear that for the comfort and economy 

 of the Nation we ought to be doing more in the scientific development of our fisheries. 



