On the Unique Isotropic Aragonitic Tube Microstructure of some Serpulids (Polychaeta, Annelida) Olev Vinn* Department of Geology, Institute of Ecology and Earth Sciences, University of Tartu, Ravila 14A, 50411 Tartu, Estonia ABSTRACT A Scanning electron microscopy study of peculiar serpulid tube microstructures was carried out to determine whether this structure is unique to serpulids and to understand its formation and evolution. The iso- tropic aragonitic tube microstructure of some serpulids, termed spherulitic irregularly oriented prismatic structure (SIOP), is unknown in other animal skeletons. After nuclea- tion, initial spherulitic prismatic aragonite crystals pre- sumably grow in length and width at one end, collide with the surrounding growing crystal fans, and then continue intertwined growth until the secreted carbonate-charged mucus is completely crystallized. Spherulitic irregularly oriented prismati structure presumably evolved either from the IOP or homogeneous rounded crystal microstructures. J. Morphol. 274:478–482, 2013. Ó 2012 Wiley Periodicals, Inc. KEY WORDS: Serpulidae; tube microstructure; biomineralization; aragonite INTRODUCTION Invertebrates commonly use biomineralization to build their skeletons. Serpulid tubeworms secrete tubes of calcium carbonate mixed with organic ma- trix. Their biomineralization system is the most advanced among the annelids (Vinn et al., 2008a). Serpulids possess diverse skeletal microstructures and are comparable in this aspect with other major groups of biomineralizing invertebrates (Vinn et al., 2008a). Serpulid tubes can be arago- nitic, calcitic or comprise a mixture of these poly- morphs (bimineralic). The mineral composition of the aragonite-dominated bimineralic calcareous tube produced by juvenile serpulids is strongly de- pendent on the aragonite saturation state (Chan et al., 2012). In addition to the tubes, some serpul- ids secrete calcareous opercula (Vinn and ten Hove, 2011). There are 14 skeletal microstructures hitherto known in serpulids (Vinn et al., 2008a, c; Vinn and ten Hove, 2011). Serpulid skeletal micro- structures are in general similar to those found in a variety of invertebrate phyla. However, serpulids possess a smaller number of skeletal microstruc- tures than molluscs and bryozoans. Their diversity of skeletal structures is comparable to that of cni- darians and arthropods (Vinn and ten Hove, 2011). There is one unique microstructure, hitherto described in Ditrupa Berkeley, 1835, termed regu- larly ridged prismatic structure by Vinn et al. 2008c. In this structure, calcitic prisms are inter- locked by a zipper-like system (Vinn et al., 2008c; Supporting information Appendix 1). The orientation of crystals in serpulid tubes is presumably controlled by the organic matrix (Vinn et al., 2009). In serpulids, carboxylated and sulph- ated polysaccharides constitute most of the organic matrix soluble in EDTA. Based on a study of Hydroides dianthus Verrill, 1873, proteins form a minority component of the soluble organic matrix (Tanur et al., 2010). These authors described the most abundant amino acids in the soluble organic matrix of H. dianthus as follows: aspartic acid (8.4 mol%), glutamic acid (10.2 mol%), glycine (14.5 mol%), and proline (21.4 mol%). The presence of Asp, Glu, and Gly is consistent with studies of vari- ous calcareous biologically controlled mineralizing organisms such as molluscs and corals (Tanur et al., 2010). These authors found that the fibre structures in H. dianthus could be partially com- posed of polysaccharides in addition to the collagen fibrils. It is also possible that organic sheet struc- tures observed via SEM in H. dianthus could be composed of polysaccharides (Tanur et al., 2010). Both polysaccharides and collagen have been found to influence the process of biomineralization. Sul- fated and carboxylated polysaccharides have the capability to bind cations, which can play a role in the nucleation process (Addadi and Weiner, 1992). Also the serpulid opercular plate is secreted by an organic matrix-mediated system where the outer cuticle and two calcified layers are formed by a single layer of epithelial cells (Bubel, 1983). The organic components of the opercular plate play an important role in the organization of the mineral components (Bubel, 1983). *Correspondence to: Olev Vinn, Department of Geology, Institute of Ecology and Earth Sciences, University of Tartu, Ravila 14A, 50411 Tartu, Estonia. E-mail: olev.vinn@ut.ee Received 12 October 2012; Revised 29 October 2012; Accepted 4 November 2012 Published online 20 December 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/jmor.20112 JOURNAL OF MORPHOLOGY 274:478–482 (2013) Ó 2012 WILEY PERIODICALS, INC.