Raman spectroscopic analysis of the composition of the clam-shrimp carapace (Branchiopoda: Laevicaudata, Spinicaudata, Cyclestherida): a dual calcium phosphate-calcium carbonate composition

Clam shrimps constitute a paraphyletic group of bivalved branchiopod crustaceans that includes orders Laevicaudata, Spinicaudata, and Cyclestherida. The unique mineral content of the carapace, formed by molt retention in Spinicaudata and Cyclestherida, has been variously ascribed to calcium carbonate or calcium phosphate. We analyzed the composition of modern carapaces from one laevicaudatan species, thirteen spinicaudatan species (including cyzicids, leptestheriids, and limnadiids), one cyclestherid species, and two species of the notostracan TriopsSchrank, 1803 (as an outgroup comparison within Branchiopoda) via Raman spectroscopy. The results were surprisingly variable. The outgroup Triops species varied in either having no mineral content to having a slight amount of calcium phosphate. The laevicaudatans likewise had a minor calcium phosphate peak; the leptestheriid spinicaudatan had strong calcium phosphate and calcium carbonate peaks; the limnadiid spinicaudatans were variable, varying from no mineral content to strong dual calcium carbonate and calcium phosphate peaks; the cyzicid spinicaudatans tended to have strong calcium phosphate peaks with some amount of calcium carbonate; and the cyclestherid had no mineral content. The results support the conclusion that spinicaudatans primitively have a dual mineralization system in their carapace that utilizes both calcium phosphate and calcium carbonate, with the calcium phosphate ability being basal. This suggestion of a dual mineralization system is novel in branchiopods and warrants study from material scientists. Better understanding of the distribution of phosphate versus carbonate among modern spinicaudatans will help us understand the evolution of biomineralization in the group. This, in turn, will shed light on the fossilization potential of different fossil clam-shrimp lineages.