Abstract
The sea mussel, Mytilus calif ornianus, like other lamellibranchs, derives its nourishment from the microorganisms and finely divided detritus which it swallows. 1 It has been suggested that bacteria may constitute a very important part of the food of such plankton-and detritus-feeders. 2 , 3 M. calif ornianus was selected for experimentation along this line because of its abundance in the vicinity, its convenient size and the ease with which it can be maintained in the laboratory.
Specimens measuring 6-9 cm. in length and weighing 20-50 gm. were scraped clean of organisms attached to their shells and placed singly in liter jars of aerated sea water in the dark. After 2 weeks' inanition a bacterial suspension was added to each jar. A red coccus and a spore-forming bacillus which were morphologically and culturally distinctive enough to be readily recognized were used. They were grown on nutrient agar in Blake bottles and washed in 2 changes of saline by centrifuging. The mussels removed most of the bacteria from suspension within 2 hours as indicated by the clearing of the water, 4 whereas in control jars lacking mussels the water remained turbid for several hours. Three hours after the addition of about 200 million bacteria per cc, plate counts revealed the presence of less than ten thousand bacteria per cc. in jars containing mussels. That many of these bacteria were actually ingested and not merely clumped or rejected as pseudo-feces 4 was proved by
microscopic and cultural demonstration of bacteria in the stomachs, especially on the head of the style, of dissected mussels soon after receiving a bacterial meal.
The disappearance of bacteria from the digestive tract within 6 hours after the bacterial meal and their paucity in the animal's feces were regarded as evidence that the bacteria were digested by the mussels. Further proof was obtained by demonstrating the occurrence of digestive enzymes which lyse bacteria.
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