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Serpulids are the only polychaetes in which all species secrete calcium carbonate tubes. Some serpulid lineages, such as species in the genus Spirobranchus, possess a second kind of hard-calcified part—the operculum. The tube and operculum microstructure and mineralogy are highly variable among serpulid species, but this type of information is still scarce for some species. Furthermore, comparing the microstructure and mineralogical composition of species from warm and temperate waters helps to determine which are more susceptible to the effects of climate change. Therefore, the aim of this study is to determine and describe the microstructure and mineralogical composition of the tubes and opercula of three Spirobranchus species from warm and temperate waters of the Mexican Pacific. The microstructures of the two hard structures were analyzed with a scanning microscope and complemented by X-ray computed nanotomography; the mineralogy was determined by X-ray diffractometry. The tubes of the three species had two layers—inner and outer—and five microstructures were found in the tubes. Numerous pores cover the operculum of all three species and have an irregularly oriented prismatic (IOP) structure. The tubes of the three species were bimineralic but had different percentages of calcite and aragonite, whereas the opercula were only composed of aragonite. Given the mineralogical characteristics of S. spinosus and because its distribution is only in temperate waters, we may conclude it could probably be the main species negatively affected according to the climate change scenarios predicted for the end of this century. [ABSTRACT FROM AUTHOR]