ENVIRONMENTAL RELATIONS OF MODIFICATION COMPOSITIONS OF CERTAIN CARBONATE SECRETING MARINE INVERTEBRATES

Studies by numerous investigators, both incidental and direct, on the modification composition of skeletal carbonate in recent marine organisms have established that the stable modification calcite and the metastable aragonite constitute the commonly secreted constituents, singly or in combination [1-6]. Mostly going hand-inhand with studies of skeletal micro-architecture, the purpose of the investigations has been to determine the distributional representation of the two modifications through the spectrum of earbonate synthesis with reference to the successive levels of organization. Undertaken for obvious reasons along taxonomic lines, random selection of species secured from biologic collections served in general for the analyses. The locations of specimen derivation and relation to ecologic aspects were in almost all instances of no concern, the compositional determinations serving primarily to add to the taxonomic diagnosis of the species examined. Data accumulated over a period of time on the mineralogic composition of random selected species within most classes from the family level up, in some cases as high as the phyllum level, as in the echinodermata, showed mostly identical modification composition. From this the concept evolved that with the exception of the two molluscan classes, the gastropods and pelecypods, comparatively uniform modification synthesis is the rule, the skeletons within taxonomically deformed limits consisting either entirely of calcite or aragonite. In the two molluscan classes, however, considerable complexity is involved. Calcite and aragonite occur there not only singly in skeletons as in the other classes, but both minerals may partake in the composition of single shells. In such instances the two modifications form microarchitecturally separate elements. As far as the distribution is concerned, the modification composition was found to differ in certain families and in some instances noted between species within a genus [3]. It is perhaps for this reason that these two molluscan classes have been investigated more thoroughly from the statistical point of view than any others. By now there exist diagnoses of the modification composition for all known carbonate secreting groups, though to judge from the literature controversies are in evidence in a few instances. They involve the entire phyllum bryozoa [2,3,7,8,9], the serpulidae [2,8,9,10,11] among the polychaete worms, and recently certain molluscan species [3,6]. Analysis of the reasons for these particular uncertainties show that, frequently, later determinations of random selected representatives of these groups differed from those made earlier, leading to questioning of the accuracy of determinations of one or another investigator. This is perhaps best illustrated by Stolkowski's [6] recent disagreement as to the composition of an individual of the same species of Haliotis determined by him to consist entirely of aragonite, as contrasted to the earlier diagnosis by Boggild [2] stating that a volumetrically small calcitic layer is intercalated between the well-developed aragonitic ones. The basic philosophy involved, though not stated in these words, seems to pertain to the generally held concept that the modification synthesis products on the species level, and in most categories for the entire groups diagnosed, are constant. Thus, the mineralogic composition was inferred to be rigidly, that is genetically fixed. This perhaps explains the reason why the locations of respective sample derivations remained beyond consideration in the evaluation of the differences in modification determinations. This very aspect would involve the possibility of ecologic controls. This question was actually considered by Boggild [3]. His conclusion however, based on admittedly limited data, was that ecologic variables were not effective. Since then, no further consideration has been given to the possibility of ecologic relations to the modification composition in carbonate skeletons. It is the purpose of the following presentation to show that within certain classes the mineralogy of skeletal materials is environment-dependent. The survey on which this study is based pertains principally to the analysis of one of the groups open to question in the literature, the serpulidae, and considers examples similarly found by the writer among the gastropods and pelecypods only as far as they contribute to the elucidation of the problem.