GEOCHEMICAL SIGNATURES OF SAND DOLAR TESTS (MELLITA TENUIS) FROM TWO DISPARATE ENVIRONMENTS ALONG THE GULF COAST OF FLORIDA

Whether the incorporation of minor and trace elements into the echinoderm skeleton is primarily controlled by metabolic processes or shaped by environmental parameters is a matter of debate. Although laboratory experiments can provide important insights, interpreting their outcomes may be hampered by potential pitfalls related to short experimental time-scales and artificial settings. Contrasting live-collected conspecific specimens from different environments (e.g., fully marine vs brackish settings) may provide natural experimental settings to assess the effect of environmental factors on skeletal mineralogy in situ. Here we report elemental concentrations of the skeletons of the sand dollar Mellita tenuis collected from different salinity regimes in Cedar Key area, along the Florida’s Gulf coast. Our preliminary data show that, notwithstanding environmental settings, the bulk skeletal Mg/Ca ratios in the analysed specimens were comparable. These salinity-invariant ratios suggest that M. tenuis is able to exert a significant biological control over Mg incorporation. In contrast, bulk Ba/Ca ratios exhibit notable differences between the two sites, indicating that accumulation of Ba can be shaped primarily by salinity. Our initial results also show that the skeletal Mg/Ca ratio may vary between different types of stereom within a single ossicle: the inner galleried or labyrinthic stereom typically exhibits lower Mg/Ca ratio than outer imperforate stereom layer that forms tubercles. We suggest that the higher Mg content in the outer tubercles relative to that of the underlying stereom may be an adapation to enhance their mechanical strength. However, elevated Mg content is expected to increase skeletal solubility, which may be detrimental, when epidermis is degraded within acidified sediment. On the other hand, the higher density of the outer stereom relative to that of the underlying stereom appears to mitigate the potential effect of increased skeletal solubility due to elevated Mg content. These results suggest that geochemical signatures of echinoids may provide useful information about environmental and intrinsic biological processes, with potenial application to the fossil record of this important group of invertebrates. [The study funded in part by NCN grant 2020/37/B/ST10/01460].