Detailed controls on biomineralization in an adult echinoderm: skeletal carbonate mineralogy of the New Zealand sand dollar (Fellaster zelandiae)

Abstract

Fellaster zelandiae, a sand dollar endemic to Aotearoa New Zealand, follows other echinoderms in producing Mg-calcite. Their skeletons, however, show mineralogical variation at different levels of scale: nanostructure, body part, individual, and population. Atomic-level imaging highlighted differences in skeletal ultrastructure with varying levels of consistency in elemental composition. Teeth, the deepest internal skeletal structures in the individual and vital for feeding, showed the greatest compositional variation at the nanoscale, whereas tests and spines were both more consistent in Mg concentrations. Mg incorporation is, approximately, a function of proximity to seawater, with levels highest in layers further away from the marine environment. Body part variation within individuals of a populations was relatively low (Maximum SD_x̄ = ± 0.19 wt% MgCO₃, n = 9) while average variation was ± 0.14 wt% MgCO₃ (n = 670), reflecting genetic variability. Population variation across a range of latitudes indicated both well-known and novel environmental influences. Skeletal mineralogy in a population in Tauranga, North Island, New Zealand at 38°S (mean = 8.5, SD = 0.07, n = 50) is significantly different (p < 0.0001) from a population in Timaru, South Island, New Zealand at 44°S (mean = 8.2, SD = 0.07, n = 62). Populations across the country showed that external parts (spines) were most affected by temperature and classical environmental factors, while internal parts (Aristotle’s lanterns) were not swayed by abiotic factors. Intermediate structures (tests) were unexpectedly influenced by wave energy, where increases in Mg content among populations was correlated to higher wave-energy beaches. While intrinsic, phylogenetic, and extrinsic factors can individually influence skeletal carbonate mineralogy, these data show that accounting for the cumulative individual- and population-level factors affecting mineralogy provides an extremely nuanced understanding of biomineralization within a single species.