A FAREWELL TO ARMS: TESTING THE EFFECT OF SEAWATER CHEMISTRY ON ECHINODERM REGENERATION RATES USING BRITTLE STARS

Secular changes in ocean chemistry may have affected diversification patterns in marine invertebrates throughout the Phanerozoic. Fluctuating rates of seafloor spreading have shifted Mg/Ca ratios that control the dominant CaCO₃ precipitant in the oceans: calcite or aragonite. Echinoderms are marine animals that possess a high-Mg calcite endoskeleton capable of regenerating after instances of sub-lethal predation. Regeneration is important because it increases the likelihood of individual survival yet the rate at which regeneration takes place may be linked to the geochemical environment. If Mg/Ca ratios affect rates of regeneration then secular changes in seawater chemistry may have produced macroevolutionary effects in echinoderms and other CaCO₃skeletonized taxa by altering growth rates or other physiological parameters. Because sub-lethal predation pressure can be held constant in a laboratory setting, echinoderms are an ideal taxon for conducting experiments to investigate the relationship between regeneration rates and seawater chemistry.

We tested for covariation between Mg/Ca ratios of seawater and regeneration rates using the extant ophiuroid Ophioderma appressum (Say, 1825). Three 10-gallon tanks containing five ophiuroid specimens each were used in three experimental treatments totaling 15 ophiuroids per treatment. Treatments contained high-Mg seawater (molar Mg/Ca ~5.2) and low-Mg seawater (molar Mg/Ca ~1.0). Seawater with molar Mg/Ca ~2.5 was used as a control. Regeneration was induced by removing one arm from each specimen at the onset of the experiment. The length of regenerated arms were measured biweekly and used to calculate regeneration rates. These data may provide insight for exploring links between taxonomic survivorship of CaCO₃ skeletonized taxa and secular changes in seawater chemistry.