As the worm grows the cavity it occupies is 

 enlarged by boring to provide for its increased size. 

 Several shell layers are deposited by the oyster 

 over the bUster. The mechanism of boring is not 

 well understood; probably erosion of the shell is 

 accomplished by a combined chemical and me- 

 chanical action. P. ligni is found living in mud 

 tubes on tidal flats or attached to shells and rocks. 

 On several occasions the reproduction of P. ligni 

 on the oyster bottoms of Delaware Bay was so 

 rapid that nearly every live oyster of the aft'ected 

 area was killed by a deposit of mud several inches 

 thick consisting of numberless hve worms and their 

 tubes. The process of gathering mud by Polydora 

 is shown in a photograph (fig. 371) taken of a live 

 worm, which was placed in a glass tubing for 

 observation in the laboratory. 



Oysters themselves accumulate large quantities 

 of organic sediment, which is discarded with the 

 feces. During feeding the oyster discharges fecal 

 ribbons at the ratp of several centimeters per 

 hour. In a sluggish current large cfuantities of 



fecal masses settle in the crevices between the 

 oysters and contaminate the bottom. The situa- 

 tion may become serious enough to cause a decline 

 in the productivity of oyster beds, as has been 

 demonstrated by Japanese biologists (Ito and 

 Imai, 1955). 



Organic material constitutes a major portion 

 of marine muds. The physical properties of a 

 sediment may be of lesser importance to oyster 

 ecology than the comple.x biochemical changes 

 associated with the bacterial decomposition of 

 its organic components that result in the forma- 

 tion of carbon dioxide, ammonia, phosphates, 

 sulfates, and various organic acids. In the case 

 of anaerobic oxidation, methane and hydrogen 

 sulfide are formed (Waksman, 1942; Waksman 

 and Hotchkiss, 1937). The effect of these prod- 

 ucts of decomposition on bottom populations 

 probably is the mam reason for the slower rate 

 of growth for oysters on the bottom than for 

 those which are kept above the bottom on trays 

 or are suspended from rafts and floats (Shaw, 



/ 



Figure 371. — Photograph of live P. ligni collecting mud. The worm i.s contained in a glass tube placed in sea water 

 containing silt in suspension. i\Iud particles along the grooved antennae are transported toward the mouth and 

 have begun to accumulate at the edge of the glass tubing. 



FACTORS AFFECTING OYSTER POPULATIONS 

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