Variable noding in Cyprideis torosa (Ostracoda,Crustacea): an overview,experimental results and a model from Catastrophe Theory

Various anomalohaline ostracod species will, under certain conditions, develop hollow, outward flexions of the lateral surface of their carapace that are called nodes. While the potential positions on the shell surface of such nodes are normally fixed, their number and relative strength are variable. This phenomenon, which is called variable noding, is best known in Cyprideis torosa (Jones, 1850) but it actually features in several more species of Cyprideis and other cytherideinid genera, both Recent and fossil. Alternately explaining it as ecophenotypic or genotypic, nearly all primary sources in the literature associate variable noding with low environmental salinity. Culturing results confirm that the phenomenon reflects a direct physiological response rather than a genotypic adaptation. From the cultures, it also appears, however, that there is yet another factor active, in addition to and interfering with environmental salinity. This factor is provisionally called `factor X'. The experiments suggest that factor X may represent either the pH or the CO₂ content of the ambient water, and hence be directly related to dissolved CaCO₃. With two interfering factors, variable noding can possibly be described by the cusp model of Catastrophe Theory with salinity and factor X as the controls and noding capability as the resultant reaction surface. Lacking sufficient calibration, the model must remain qualitative for the time being. Further experimentation and careful observation in the field should allow its quantification, thus clearing the path for palaeoecological application.