26 Thomas G. Thompson and Tsaihwa J. Chow 



A summary of all analytical results is presented phylogeneticaily in Table III. The 

 data given for each column represent the average values obtained on various carbonate- 

 secreting marine organisms as listed in foregoing tables. The strontium-calcium atom 

 ratios are very constant in accordance with the phylogenetic classification. With the 

 exception of Zoantharia (Madreporaria) and Opisthobranchia (Nudibranchia), the 

 atom ratios in calcareous portions of marine organisms are less than that of sea-water, 

 8-9 X 10-'. In these instances and in that of radiolaria (Odum, 1951 a), it is apparent 

 that strontium does play a physiological role in the development of calcareous shells 

 of carbonate-secreting marine organisms. The mechanism of this selectivity presents 

 an interesting physiological problem. Controlled laboratory experiments of growing 

 marine organisms in artificial sea-water free of strontium, and further elaborating 

 Odum's work with waters of varying strontium-calcium atom ratios, would probably 

 yield fundamental information for explaining the role of strontium in marine organisms. 



Ecological Aspects 



1. Habitat: Organisms of various species are found associated together in an 

 ecological niche. All species that have not adjusted themselves physiologically to the 

 existing conditions will be eliminated from a given community. Since the environ- 

 mental conditions influence the life of marine organisms, it is of interest to observe 

 any variations of chemical composition of marine organisms which live in such a 

 community. Various species were collected from two rocky shores near the Hopkins 

 Marine Station, California, at the mid and the low inter-tidal levels. The results of 

 analyses of calcareous portions of these organisms are listed in Table IV. 



The strontium-calcium atom ratios of the organisms collected at the mid-tidal 

 level varied from 1-01 x 10-* (Mytilus califomianus) to 7-91 X 10* (Nuttallina 

 calif ornica). The Mytilus-Mitella-Pisaster which were closely associated in the habitat, 

 showed striking differences in the strontium-calcium atom ratio. The organisms 

 collected at the low inter-tidal level had strontium-calcium atom ratios ranging from 

 1-35 X 10* (Diodora aspera) to 11 X 10-* {Anisodohs nobilis). It appears that the 

 marine organisms which live in the same ecological niche accumulate calcium and 

 strontium in their calcareous shells in decidedly different proportions. On the other 

 hand, a very definite relationship between the atom ratio and the phylogenetic 

 classification of the organisms is indicated. 



Another series of studies was carried out on the specimens of Echinodermata. 

 Species of Echinodermata which lived in different habitats varying from inter-tidal 

 rocky shore to deep-water, muddy substratum were collected near the Carmel- 

 Monterey-Pacific Grove area. All Echinodermata given in Table V showed a remark- 

 able constancy in their strontium-calcium atom ratios ranging from 2-56 to 2-89 X 

 10*. Allowing for the individual variation and for experimental error, the atom ratios 

 can be considered as identical for the whole phylum. Thus it may be concluded that 

 some types of marine organisms will have a constant strontium-calcium atom ratio 

 in their calcareous skeletons regardless of their habitats. 



2. Water temperature: The water temperature as well as the salinity affect the 

 solubility of the calcium and strontium carbonates in sea water. Wattenberg and 

 TiMMERMANN (1936) demonstrated that the solubility of calcium carbonate in sea 

 water increased with increasing salinity and with decreasing temperature. The 



