munity are sedimentation, pollution, competition, 

 disease, and predation. The interaction of these 

 five positive and five negative factors acting 

 simultaneously on a community determines its 

 utiUzable productivity. 



POSITIVE FACTORS OF 

 ENVIRONMENT 



CHARACTER OF BOTTOM 



Oysters may grow equally well on a hard, rocky 

 bottom or on semihard mud firm enough to sup- 

 port their weight. Shifting sand and soft mud 

 are the only types of bottoms which are totally 

 unsuitable for oyster communities. With the 

 exception of these extreme conditions oysters 

 adapt themselves to a great variety of bottoms. 

 They thrive well on shore rocks and underwater 

 structures which are left exposed at low tide. The 

 controlling factor in this situation is the climate, 

 since no oyster can survive several hours exposure 

 to below freezing temperature, and, therefore, 

 none are found growing near the surface in the 

 latitudes where in winter they may be killed at 

 low tide, or frozen in the ice and carried away by 

 tidal currents. The degree of softness and insta- 

 bility of the bottom can be quantitatively meas- 

 ured by penetrometers, and by determining the 

 amount of bottom material transported by cur- 

 rents of different velocities. The depth of sinking 

 into a mud layer of a probe of known dunensions 

 imder constant weight can be used as a measure 

 of the relative softness of sediment. The deter- 

 mination must be made in situ because the removal 

 of a sample and handling in the laboratoxy changes 

 the consistency of mud. A penetrometer to eval- 

 uate certain physical properties of marine sedi- 

 ments in situ has been designed by Miller (1961). 

 The instrument is a conical probe which is driven 

 at low, constant speed into the bottom; resistance 

 to penetration as the prol^e sinks mto the bottom 

 is recorded graphically. An instrument answering 

 these specifications and constructed at the Uni- 

 versity of Rhode Island has been used in research 

 at the Narragansett Marine Laboratory at Kings- 

 ton, R.I., and has proved reliable in providing 

 information about compactness, degree of plas- 

 ticity, allowable bearing loads, and other proper- 

 ties of sediments. No reliable method has yet 

 been developed for measuring the resistance of 

 sediment to water current in the sea. 



A soft muddy bottom may be artificially im- 

 proved by planting oyster or clam shells to attain 

 the desired firmness. Other materials, such as 

 gravel and slag from blast iron furnaces, have 

 been tried experimentally but are less satisfactory, 

 primarily because of the greater weight and higher 

 cost. At present the reinforcement of oyster 

 bottoms by shells remains the principal practical 

 method used on a large scale for the improvement 

 of oyster bottoms or for the establishment of new 

 ones. 



Soft nmddy bottoms may be gradually con- 

 verted by the oysters themselves into oyster 

 banks or reefs because of an innate ability of 

 larvae to choose a substratum upon which to 

 settle. This ability is probably common to most 

 species of bottom invertebrates having free- 

 swimming larvae (Verwey, 1949). The process 

 begins with the attachment of several larvae to 

 a single shell or other hard object lying on the 

 surface of the mud. Other larvae attach to 

 those that have already settled, and soon a cluster 

 of oysters is formed on the surface of the mud 

 (fig. 365). 



Dead oyster shells dropping from clusters pro- 

 vide additional surfaces, and the reef begins to 

 grow horizontally and vertically. The process 

 is t3rpical for the tidal flats of South Carolina 

 and Georgia where successive phases can be 

 easily observed. Oysters grown on mud have 

 long, slender shells. 



The suitabihty of bottom to an oyster com- 

 munity may be expressed by an arbitrary scale 

 from to 10, according to the relative softness 

 and stability. Bottom conditions fully unsuit- 

 able for the formation of any oyster community 

 may be designated as zero. The zero value of 

 any positive factor denotes conditions under 

 which the community cannot e.xist, regardless 

 of the values of all other factors of the environ- 

 ment. The zero value of bottom factor refers 

 either to extremely soft mud not capable of sup- 

 porting the weight of an empty shell or to shifting 

 sand; both conditions are unsuitable for oysters. 

 Marginal conditions are indicated by 1 , and opti- 

 mum conditions by 10. The highest value of 

 bottom factor may be assigned to firm and stable 

 bottoms such as rocks and hard or sticky mud. 

 The value of 1 is assigned to the soft muddy 

 bottoms of the South Atlantic States and Texas. 



FACTORS AFFECTING OYSTEH POPULATIONS 



399 



