Naturwissenschaften (2009) 96:111–122 DOI 10.1007/s00114-008-0461-1 ORIGINAL PAPER Nacre and false nacre (foliated aragonite) in extant monoplacophorans (=Tryblidiida: Mollusca) Antonio G. Checa & Joaquín Ramírez-Rico & Alicia González-Segura & Antonio Sánchez-Navas Received: 7 February 2008 /Revised: 3 September 2008 /Accepted: 20 September 2008 / Published online: 9 October 2008 # Springer-Verlag 2008 Abstract Extant monoplacophorans (Tryblidiida, Mol- c-axis in this lamellar material is perpendicular to the lusca) have traditionally been reported as having an internal surface of laths and the a-axis is parallel to their long nacreous layer, thus representing the ancestral molluscan dimension. The differences between taxa relate to the condition. The examination of this layer in three species of frequency of twins, which is much higher in Micropilina. Neopilinidae (Rokopella euglypta, Veleropilina zografi, and In general, the material is well ordered, particularly towards Micropilina arntzi) reveals that only V. zografi secretes an the margin, where lamellae pile up at a small step size, internal layer of true nacre, which occupies only part of the which is most likely due to processes of crystal competi- internal shell surface. The rest of the internal surface of V. tion. Given its morphological resemblance to the foliated zografi and the whole internal surfaces of the other two calcite of bivalves, we propose the name foliated aragonite species examined are covered by a material consisting of for this previously undescribed biomaterial secreted by lath-like, instead of brick-like, crystals, which are arranged monoplacophorans. We conclude that the foliated aragonite into lamellae. In all cases examined, the crystallographic probably lacks preformed interlamellar membranes and is therefore not a variant of nacre. A review of the existing Electronic supplementary material The online version of this article literature reveals that previous reports of nacre in the group (doi:10.1007/s00114-008-0461-1) contains supplementary material, were instead of the aragonitic foliated layer and that our which is available to authorized users. report of nacre in V. zografi is the first undisputed evidence A. G. Checa (*) of nacre in monoplacophorans. From the evolutionary Departamento de Estratigrafía y Paleontología, viewpoint, the foliated aragonite could easily have been Facultad de Ciencias, Universidad de Granada, derived from nacre. Assuming that nacre represents the Avenida Fuentenueva s/n, ancestral condition, as in other molluscan classes, it has 18071 Granada, Spain e-mail: [email protected] been replaced by foliated aragonite along the tryblidiidan lineage, although the fossil record does not presently J. Ramírez-Rico provide evidence as to when this replacement took place. Departamento de Física de la Materia Condensada, Facultad de Física, Universidad de Sevilla, Avenida Reina Mercedes s/n, Keywords Monoplacophorans . Mollusca . 41012 Seville, Spain Biomineralization . Microstructure . Nacre . Foliated aragonite . Evolution A. González-Segura Centro de Instrumentación Científica, Universidad de Granada, Paseo del Profesor Juan Ossorio s/n, 18071 Granada, Spain Introduction A. Sánchez-Navas Due to the scarcity of material, relatively few studies have Departamento de Mineralogía y Petrología, Facultad de Ciencias, been done on the shells of present-day monoplacophorans Universidad de Granada, Avenida Fuentenueva s/n, (which constitute the Order Tryblidiida of the Class 18071 Granada, Spain Tergomya; Peel 1991). The shell structure of Neopilina 112 Naturwissenschaften (2009) 96:111–122 galatheae Lemche, 1957, was first briefly described by Our main goal is to characterize it both from the mor- Lemche and Wingstrand (1959). Their study was followed phological and crystallographic viewpoints in order to by the more detailed accounts of Schmidt (1959), Erben et compare it with the nacre secreted by other molluscan al. (1968), and Meenakshi et al. (1970) on the same and on classes. The information obtained will be put into a general congeneric species (Neopilina ewingi, Neopilina bacescui, evolutionary framework. Neopilina bruuni). The first reference to nacre in Trybli- diida is that of Schmidt (1959), who observed that the fractured internal layer of Neopilina was lamellar, brick Material and methods wall like, and aragonitic (as the rest of the shell), thus apparently displaying the basic characteristics of nacre. Material This opinion was later shared by Erben et al. (1968), and, from then on, all subsequent authors have expressed little We have examined three empty shells of Rokopella doubt that the “nacre” of monoplacophorans is a true nacre. euglypta (Dautzenberg and Fischer 1897) and two of According to previous knowledge, the shells of Neo- Veleropilina zografi (Dautzenberg and Fischer 1896). pilina, Laevipilina,andVema are prismatonacreous Locality: Atlantis Bank, NO of “le Suroli”, Seamount 2, (Schmidt 1959; Erben et al. 1968; Urgorri et al. 2005; DW 261, Coordinates 34° 22.40’ N, 30° 2780’ W. Depth Warén and Gofas 1996), whereas in Micropilina, the outer 1,340 m. Collected by Serge Gofas, Bernard Métivier, and shell layer was not prismatic (Cruz et al. 2003). Thus, it Anders Warén (1993–02–03). Repository: Muséum Nation- seems that the shell of monoplacophorans retained the al d’Histoire Naturelle Paris. ancestral condition found in many bivalves (palaeotaxo- Two specimens of Micropilina arntzi (Warén and Hain donts, palaeoheterodonts, pterioids, and many anomalodes- 1992) were studied in addition. Fixation 96% EtOH. matans), most Vetigastropoda and Nautilus. Expedition name: R/V Polarstern ANT IX/3 1991, Station Interestingly, very little attention has been paid to the no. 180. Locality: Antarctica, Lazarev Sea, Coordinates 69° “nacreous” part of the monoplacophoran shell and very few 57.5’ S, 6° 20.0’ E. Collected by Stefan Hain (1991 February descriptions and images can be found in the literature. 23). Depth 280–298 m. Repository: Swedish Museum of Meenakshi et al. (1970) and Cruz et al. (2003) report how Natural History (SMNH), Type no. 4858. monoplacophoran “nacre” is terrace like. Meenakshi et al. (1970) recognized a brick wall structure as well as the Electron microscopy existence of interlamellar and intercrystalline membranes, as in typical nacre. In fact, they compared the shells of Scanning electron microscopy (SEM) observation was several species of Neopilina to those of prismatonacreous carried out of both fractured specimens and of the shells’ bivalves. Chateigner et al. (2000) applied X-ray diffraction interiors. Samples were usually observed intact, although in and concluded that the monoplacophoran nacre-like layer some we removed the organic matter (with 5% NaOCl from has a fiber texture, with the c-axis as fiber axis. 5 to 15 min). Samples were coated with carbon (Hitachi Despite the valuable data contained in the previous UHS evaporator) for variable pressure SEM (Leo 1430-VP) studies, they do not provide a general overview of how this or field emission SEM (FESEM) observation (Leo Gemini particular “nacre” is organized in the different parts of the 1530). shell and what is its crystallographic arrangement. High-resolution transmission electron microscopy (TEM; The study of the so-called “nacre” of monoplacophorans Philips CM20) observations were carried out on both intact is important in two main respects. First, the pattern of and in completely decalcified samples (2% ethylenediami- organization of nacre is a main theme in biomineralization netetraacetic acid for several days). Samples were coated (Addadi et al. 2006; Cartwright and Checa 2007). It is well with carbon (Hitachi UHS evaporator). known that nacre is arranged terrace like in bivalves and tower like in gastropods (e.g., Erben 1972; Wise 1970; Electron diffraction Mutvei 1978; Nakahara 1991). The knowledge of how nacre is organized in other groups, such as cephalopods and The electron backscatter diffraction (EBSD) technique monoplacophorans, is particularly relevant for biomineral- analyzes the diffraction pattern produced when backscat- ization models. Second, the relationship of Tryblidiida to tered electrons are diffracted by a crystalline material. This other molluscan classes is presently unknown and the pattern, once indexed, provides information about the unraveling of how their “nacre” is fabricated might provide orientation of the crystal lattice and the space group of the insights into this central issue. crystal structure. Data from different positions can either be We have analyzed the internal shell layer of three species integrated into an orientation map or processed to represent of monoplacophorans included in the family Neopilinidae. pole figures in stereographic projection. Samples were Naturwissenschaften (2009) 96:111–122 113 analyzed unpolished and only the organic matter was Smart Apex (CSIRC, Universidad de Granada). The infor- removed from the samples prior to analysis. mation obtained has been represented as pole figures with Two equipments were used: (1) EBSD detector TSL the specially designed software XRD2DScan. OIM 3.5 from EDAX Inc. coupled with a SEM Philips XL- 30 (CITIUS, Universidad de Sevilla) and (2) Inca Crystal (Oxford Instruments) detector coupled to a Gemini-1530 Results (Carl Zeiss) FESEM (CSIRC, Universidad de Granada). Morphology and arrangement of aragonite crystals X-ray diffraction R. euglypta Towards the shell edges, crystals are very For comparative purposes, additional samples of other