Life of the Cla?ns 



HOW THEY FEED 



35 



Normally one does not think of clams and oysters as being very active 

 feeders and certainly, in comparison with the voracious methods of fish 

 and squid, the bivalves are rather peaceful eaters. Yet in their characteristic 

 way they are highly efficient and, in proportion to their size, possess a large 

 and varied menu. Most clams feed on minute plants and, in a relatively short 

 time, can filter from the sea water an extraordinary number of living diatoms 

 and dinoflagellates — microscopic, swimming plants — and protozoa of the 

 ocean. A few genera, such as the small Cuspidaria and Foromya clams, are 

 carnivorous and feed upon small living or dead animals, usually crustaceans 

 and annelid worms. 



Figure i i. Extended animals of some bivalves, showing various types of siphons. 



a, Mya arenaria Linne; b, Tellina agilis Stimpson; c, Tagelus plebeius Solander; 



d, Eiisis directus Conrad. (From A. E. Verrill 1873.) 



The bivalves fall into two general classes of feeders — suspension feeders 

 which merely pump water through their mantle cavity and thus obtain free- 

 swimming or suspended creatures from the water; or deposit feeders which 

 suck up food from the muddy bottom with their long, mobile inhalant si- 

 phons. Among the suspension feeders are the oysters, scallops, venus clams, 

 cockles, the shipworms and many others. They may or may not possess 

 siphons, but when present these are generally short. The deposit feeders 

 include such forms as Tellina, Macoma and Abra which all have long siphons. 



Whether food is taken in through the inhalant siphon as in the tellins or 

 through a slit in the mantle as in the scallops, it must pass over the gills. These 

 filament-like organs are covered by a thin sheet of mucus. Food passing 

 through the gills becomes ensnarled in the mucus which is transported by 

 water currents and myriads of tiny, hair-like cilia. Mucus is constantly be- 



