Skeletal mineralogy of scaphopods : an unusual uniformity

As Earth’s high-CO2 atmosphere drives changes in ocean chemistry, marine carbonate geochemists are delving into the composition of marine skeletons, with a view to understanding possible effects on dissolution and calcification. Detailed studies of skeletal carbonate mineralogy have revealed more complexity and variability than originally assumed. Some coralline algae, for example, far from being entirely high-magnesium calcite as had long been thought, have been shown to contain low-Mg calcite and even aragonite (Smith et al., 2012). Similarly, invertebrates such as bryozoans (Smith, Key & Gordon, 2006; Taylor et al., 2009) and serpulid worms (Smith, Riedi & Winter, 2013) have proved to be mineralogically complex, with variability related to phylogeny, environment and/or skeletal element. Smaller skeletal elements, such as radulae or statoblasts of varying compositions, can add to the biomineral complexity. Molluscs in particular are known to exert a great deal of control over many various minerals they precipitate; some cephalopod species may secrete more than 10 different minerals (Smith, 2012). Scaphopods appear to be a group awaiting mineralogical study. Of the 10 or so shells actually tested (Clarke & Wheeler, 1917, 1922; Bøggild, 1930; Alzuria, 1984), all are from the North Atlantic or Mediterranean and all belong to the family Dentaliidae (Table 1). They encompass seven species (there is almost no replication) and all 10 shell specimens have been shown to be aragonitic, low in magnesium and strontium. The radular teeth in (at least some) scaphopods may be mineralized, perhaps with an amorphous calcium phosphate with, sometimes, iron and copper in the teeth (Vovelle & Grasset, 1983; but see Glover, Taylor & Whittaker, 2003). There may be gravity receptors (statocysts), larval stages or protoconchs which could be mineralized, but nothing is known about the mineralogy of these life stages. It is quite an extrapolation to assert that all 500 or so species of scaphopods from around the world are always aragonitic (e.g. de Paula & Silveira, 2009) on the basis of such a small set of measurements (10 specimens, seven species, all in a single family). We set out to determine whether there is more mineralogical variability among scaphopods by studying material from a wide range of families and geographic areas. It is, perhaps, not surprising that very little is known about the skeletal carbonate mineralogy of the Scaphopoda. They are relatively rare and often buried in soft sediments from intertidal waters (Glover et al., 2003) down to abyssal depths (4500 m) (Mulcrone, 2005). The shell is a long, narrow, slightly curved cone (hence ‘tusk shells’), open at both ends. Scaphopod shells are usually 2 to 4 cm long, but can range from about 0.5 to 15 cm; some fossils are reportedly up to 30 cm long (Mulcrone, 2005). The scaphopod fossil record stretches back to at least the Carboniferous and possibly the Ordovician (about 460 Ma). They were probably the last molluscan class to evolve, although their relationships to other classes remain unclear (Schrödl & Stöger, 2014). There are two recognized orders in the class Scaphopoda. The Dentaliida are characterized by having the widest part of the shell at the aperture, with the outer surface often longitudinally sculptured and the middle radular tooth broader than it is high (Habe, 1964; Reynolds & Okusu, 1999). There are eight families in the order and about 290 species, the vast majority in the family Dentaliidae (Steiner & Kabat, 2001, 2004). In contrast, in the Gadilida the widest part of the shell is somewhat behind the aperture. They are usually completely smooth on the outside and the middle radular tooth is higher than it is wide (Habe, 1964; Reynolds & Okusu 1999). This order contains four families with about 560 species (Steiner & Kabat, 2001, 2004). Very little is known about biomineralization in scaphopods. In their comprehensive review of biomineralization, Lowenstam & Wiener (1989: 94) stated only this: “The shell is composed of aragonite (Bøggild, 1930) and is differentiated into three layers: a thick middle layer (crossed-lamellar ultrastructure) and two thin surface layers (homogeneous or finely prismatic) (Bøggild, 1930). The mantle is completely fused into a cylindrical cone. Almost nothing appears to be known about the shell formation processes of Scaphopoda. ” A detailed search of the literature has found little more. A few studies have investigated scaphopod shell structure (Couvreur, 1979; Alzuria, 1984), protoconchs (Engeser, Riedel & Bandel, 1993) and the composition of radulae (Vovelle & Grasset, 1983; Glover et al., 2003). Most studies cite only Bøggild (1930), who published results on four species, for the mineralogy of this class. In order to address this shortfall in data, we acquired scaphopod shells from around the world from collections at: Natural History Museum (NHM), National Institute of Water and Atmosphere (New Zealand), National Museum of Scotland (NMS), Auckland Museum (AM) and Museum of New Zealand Te Papa Tongarewa (MNZ), focusing in particular on families whose mineralogy has heretofore been unknown. In addition, we have collected scaphopods from around New Zealand, with the animal intact. Here we report on the shell