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, deve lop hollow, outward flexions of the lateral surface of their carapace that are called nodes. While the potential positions on the shell surface of suc h nodes are normally fixed, their number and relative strength are variable . This phenomenon, which is called variable noding, is best known in Cyprid eis torosa (Jones, 1850) but it actually features in several more species o f Cyprideis and other cytherideinid genera, both Recent and fossil. Alterna tely explaining it as ecophenotypic or genotypic, nearly all primary source s in the literature associate variable noding with low environmental salini ty. Culturing results confirm that the phenomenon reflects a direct physiol ogical response rather than a genotypic adaptation. From the cultures, it a lso appears, however, that there is yet another factor active, in addition to and interfering with environmental salinity. This factor is provisionall y called 'factor X'. The experiments suggest that factor X may represent ei ther the pH or the CO2 content of the ambient water, and hence be directly related to dissolved CaCO3. With two interfering factors, variable noding c an possibly be described by the cusp model of Catastrophe Theory with salin ity and factor X as the controls and noding capability as the resultant rea ction surface. Lacking sufficient calibration, the model must remain qualit ative for the time being. Further experimentation and careful observation i n the field should allow its quantification, thus clearing the path for pal aeoecological application.