60 The Substratum 



The limitation of growth or of distribution by the inadequacy of the 

 substratum in respect to these needs will be discussed in subsequent 

 sections of this chapter dealing with the different types of substratum. 

 The substratum may also have importance in various special ways. 

 For example, the hue and pattern of many animals protect them from 

 detection by enemies because they blend with the background (Cott, 

 1940). The color and texture of the substratum are thus essential 

 considerations in the operation of protective coloration in nature. 



The larvae of many sessile organisms will not continue their de- 

 velopment unless they find a suitable substratum. This fact has many 

 important practical applications as for instance in oyster culture. For 

 years oystermen have realized that clean, hard surfaces must be avail- 

 able in the spawning areas if the oyster larvae, or "spat," are to make 

 a successful "set" each year. To insure the presence of a suitable 

 substratum the oystermen dump overboard whole boatloads of empty 

 shells or other material at a time when ecological conditions are such 

 that the oysters of the region are about to spawn. This specially pro- 

 vided substratum is known as cultch, and its presence in sufficient 

 abundance at the critical moment for the attachment of the larvae 

 is necessary for successful oyster culture. 



Attainment of the Substratum 



The attainment of a proper substratum is crucially important in the 

 lives of most plants and animals, and special methods meeting this 

 need have developed during the course of evolution. One obvious 

 and common procedure is the broadcasting of such great numbers of 

 seeds, spores, or larvae in the attaching stage that some of them will 

 eventually "fall on fertile soil." The majority of terrestrial plants 

 follow this method, and, although only an extremely small fraction 

 of the seeds are ordinarily carried to a substratum suitable for growth 

 to maturity, enough seedlings usually become established to per- 

 petuate the species. 



The tremendous numbers of animal larvae in the attaching stage 

 are often an index of the critical nature of the attaching process. A 

 graphic illustration of the intensity of the reaction for the species to 

 attain a proper substratum was found in the study of the settling of 

 barnacle larvae in experiments conducted in Biscayne Bay, Florida. 

 Glass plates 20 x 25 cm in size were placed in the water each day, and 

 the number of barnacle cyprids which attached to them was deter- 

 mined. On one occasion the count showed that 3860 cyprids had 

 settled on one glass plate during the previous 24 hours! When it is 



