CARBON AND OXYGEN ISOTOPIC VARIABILITY WITHIN THE SKELETON OF THE SEA URCHIN ECHINOMETRA LUCUNTER

Sea urchins are composed of many skeletal elements that all grow at different rates during the development of the organism. These elements are all made of high-Mg calcite that is precipitated in disequilibrium with the organism’s surroundings. The 𝛿¹³C and 𝛿¹⁸O of calcite within the skeleton of the sea urchin Echinometra lucunter was analyzed to determine isotopic patterns across different parts of the organism. Desired parts of the skeleton were dissected from the specimen and any remaining organic matter visible on those parts was removed. The samples were reacted with phosphoric acid and the resulting gas analyzed with an ion ratio mass spectrometer. The results measured 𝛿¹³C and 𝛿¹⁸O values varying up to several per mil (-4.7 to +0.6 and -1.1 to +1.0 respectively) across the animal and separated into three distinct groupings: elements within the lantern (the jaw apparatus), spines, and plates of the body wall. Within those groupings, there were significant differences in isotopic composition related to the location of plates on the body of the sea urchin and location of samples taken along spines. The relationship between isotopic composition and specific part of the sea urchin is likely related to both the carbon and oxygen sources and the rate at which each of these parts are formed during the growth of the organism. The obtained results suggest a strong vital effect on isotopic composition during the secretion of calcite and a complex relationship between isotope composition and the rate at which these individual parts grow. Further evidence of these relationships among echinoderms as a group could highlight the difficulty of using echinoderms as proxies in paleoenvironment studies.