BRITTLE STARS BID A FAREWELL TO ARMS: TESTING THE LINK BETWEEN ECHINODERM REGENERATION RATES AND SEAWATER CHEMISTRY

Do seawater Mg/Ca ratios affect regeneration rates in echinoderms? Mg/Ca ratios have shifted throughout the Phanerozoic as a result of fluctuating rates of seafloor spreading. These changing Mg/Ca ratios control whether calcite or aragonite is the dominant CaCO₃ precipitant in the oceans. The high-Mg calcite endoskeleton of echinoderms is capable of regeneration after instances of sub-lethal predation, thereby increasing the likelihood of individual survival. This study aims to determine whether echinoderm regeneration rates are linked to the geochemical environment. If Mg/Ca ratios affect rates of regeneration, then secular changes in seawater chemistry may be a driver of macroevolutionary patterns in echinoderms by altering growth rates or other physiological parameters.

The extant ophiuroid Ophioderma cinereum (Muller and Troschel, 1842) was used to test whether seawater Mg/Ca ratios are causally linked to arm regeneration rates. Three 10-gallon tanks containing four ophiuroid specimens each were used in three experimental treatments totaling 12 ophiuroids per treatment. Treatments contained low-Mg seawater (molar Mg/Ca ~1.7) to approximate “calcite seas” and intermediate seawater (molar Mg/Ca ~2.5) from the boundary field between aragonite and calcite precipitation. High-Mg seawater (molar Mg/Ca ~5.2) approximating modern “aragonite seas” was used as the control. Sub-lethal predation was simulated by removing one arm from each specimen at the onset of the experiment. Regenerated arms were photographed biweekly and the length of each arm was measured in Adobe Illustrator to calculate regeneration rates. Empirical data collected from this experiment may provide insight into the link between taxonomic survivorship of CaCO₃ skeletonized taxa and secular changes in seawater chemistry.