Mineralogical Variation in Shells of the Blackfoot Abalone, Haliotis iris (Mollusca: Gastropoda: Haliotidae), in Southern New Zealand

The New Zealand blackfoot abalone, Haliotis iris Gmelin, is among the few gastropods that precipitate both calcite and aragonite in their shells. The location, composition, and thickness of these mineral layers may affect color, luster, and strength of the shell, which is locally important in jewelry manufacture. Skeletal mineralogy and shell structure of H. iris from three southern New Zealand locations were determined using X-ray diffractometry, scanning electron micrography, and mineral staining. In H. iris an outer calcitic layer is separated from an inner aragonitic surface by both calcified and noncalcified organic layers running longitudinally through the shell. Skeletal mineralogy within individual shells varies from 29 to 98% aragonite, with older shell having significantly higher aragonite content than young sections. Variation within populations ranges from 40 to 98% aragonite, and among three populations from 34 to 98% aragonite. Shell thickness, too, varies within individual shells from 0.2 to 4.2 mm, with a significant positive relationship with age. Withinpopulation variation in shell thickness ranges from 2.1 to 5.4 mm, with no significant difference in shell thickness variation among populations. The high degree of variability within and among individual shells suggests that it is essential to test replicate samples from individual mollusk shells, especially when they have complex bimineral structure. MOLLUSKS HAVE BEEN secreting mineral skeletons for some 600 million yr. Skeletons may function in detoxification, temporary storage of vital ions, gravity perception, light reflection, navigation, structure, and/or protection from predation (Litivin et al. 1997, Weiner and Addadi 1997). Mollusk shells are usually composed of about 95% CaC03 (Belcher et al. 1996, Schaffer et al. 1997) precipitated as one or both of the major crystalline polymorphs: the more thermodynamically stable trigonal calcite and metastable 1 This research was supported by a New Zealand Ph.D. Graduates in Industry Fellowship (GRIF) from the Foundation for Research, Science, and Technology (FoRST). Manuscript accepted 6 May 2003. 2 Department of Marine Science, University of Otago, P.O. Box 56, Dunedin, New Zealand. 3 Corresponding author (phone: +64 (3) 479 8306; fax: +64 (3) 4798336; E-mail: [email protected]). Pacific Science (2004), vol. 58, no. 1:47-64 © 2004 by University of Hawai'i Press