Phylogenetic Implications of Calcium Carbonate Mineralogy in the Stylasteridae (Cnidaria: Hydrozoa)

The carbonate mineralogy of the calcified hydrozoan family Stylasteridae is controlled largely by phylogenetic rather than environmental factors. In a comprehensive study— the first of its kind—of the mineralogy of almost an entire family of calcareous organisms, this finding is well documented. X-ray diffraction analyses of 24 of 25 genera indicate that the skeletons of most species analyzed (54 species in 20 genera) were aragonite; only 7 species in 4 genera were calcite. In addition, several species contained coexisting carbonate polymorphs: 7 calcitic species with traces of aragonite; 1 aragonitic species with traces of calcite; and two species with variable percentages of both polymorphs, which is dependent on the distance from the growing tip. In the last two species, staining and progressive X-ray diffraction analyses showed that the ontogenetically earlier polymorph was calcite, the later aragonite. Mole % magnesium carbonate in calcite ranged from 6.5-10.0, but had no correlation to any known variable. Likewise, no correlation was found between polymorph type and any morphological or environmental variable, except for a generalized temperature effect, wherein genera with calcitic corolla are restricted to waters colder than 13° C; aragonitic coralla occur at temperatures from -1.5° to 30° C. However, there is a close correlation of polymorph type to the generic phylogeny of the family: the calcite polymorph clustered in the center of the cladogram, aragonite occurring on both ends, including the out-groups. Because the correlation of polymorph types to the cladogram was not perfect, it prompted a re-examination of the placement of some of the stylasterid species and genera. Based on its mineralogy, a Tertiary calcitic stylasterid would probably belong to the Errinopsis-Errina-Errinopora species complex and have occurred in water less than 13° C; on the other hand. Tertiary aragonitic stylasterids were broadly distributed. The adaptive value of calcium, carbonate polymorphs in Stylasteridae is still poorly understood.