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IN the initial stages of the biomineralization of abalone shells, a primer layer of oriented calcite crystals grows on a nucleating protein sheet1,2. The deposition of this primer is followed by an abrupt transition to c-axis-oriented crystals of aragonite, another crystalline form of calcium carbonate. The formation of each of the two crystal types is accompanied by the synthesis of specific polyanionic proteins1–3, suggesting that cooperative interactions between these proteins and the inorganic ions during crystal nucleation and growth control the phase of the deposited mineral and that differential expression of the proteins allows the organism to induce phase changes. It is known that soluble shell proteins can control crystal morphology4–10, but it has been suspected that the switch in phase—from calcite to aragonite—might require the deposition of a new nucleating protein sheet. Here we describe in vitro studies of the crystallization of calcium carbonate in the presence of soluble polyanionic proteins extracted from abalone shell. We find that these proteins alone are sufficient to control the crystal phase, allowing us to switch abruptly and sequentially between aragonite and calcite without the need for deposition of an intervening protein sheet. These results show that soluble organic components can exert greater control over hierarchical biomineral growth than hitherto suspected, offering the prospect of similar phase control in materials chemistry.

Mollusc shells consist of the nacreous mother-of-pearl layer and the prismatic layer. Both layers are microlaminate composites of CaCO3 crystals (aragonite in the nacre and calcite in the prismatic layer) and biopolymers. The main biopolymers are structural proteins, insoluble in water and methanoic acid, which determine the framework of each shell layer1,4 and bind soluble polyanionic proteins2,4 which determine the type of CaCO3 crystal that grows5,6. Here we report the sequences and structures of the framework proteins for the nacreous and prismatic layers of the pearl oyster, Pinctada fucata.

Odontogriphus omalus was originally described as a problematic non-biomineralized lophophorate organism. Here we re-interpret Odontogriphus based on 189 new specimens including numerous exceptionally well preserved individuals from the Burgess Shale collections of the Royal Ontario Museum. This additional material provides compelling evidence that the feeding apparatus in Odontogriphus is a radula of molluscan architecture comprising two primary bipartite tooth rows attached to a radular membrane and showing replacement by posterior addition. Further characters supporting molluscan affinity include a broad foot bordered by numerous ctenidia located in a mantle groove and a stiffened cuticular dorsum. Odontogriphus has a radula similar to Wiwaxia corrugata but lacks a scleritome. We interpret these animals to be members of an early stem-group mollusc lineage that probably originated in the Neoproterozoic Ediacaran Period, providing support for the retention of a biomat-based grazing community from the late Precambrian Period until at least the Middle Cambrian.

The main function of postural nervous mechanisms in different species, from mollusc to man, is to counteract the force of gravity and stabilize body orientation in space. Here we investigate the basic principles of postural control in a simple animal model, the marine mollusc Clione limacina. When swimming, C. limacina maintains its vertical orientation because of the activity of the postural neuronal network. Driven by gravity-sensing organs (statocysts), the network causes postural corrections by producing tail flexions. To understand how this function occurs, we studied network activity by using a new method. We used an in vitro preparation that consisted of the central nervous system isolated with the statocysts. Output signals from the network (electrical activity of tail motor neurons) controlled an electrical motor which rotated the preparation in space. We analysed the activity of individual neurons involved in postural stabilization under opened or closed feedback loop. When we closed this artificial feedback loop, the network stabilized the vertical orientation of the preparation. This stabilization is based on the tendency of the network to minimize the difference between the activities of the two antagonistic groups of neurons, which are driven by orientation-dependent sensory inputs.

The fossil Kimberella quadrata was originally described from late Precambrian rocks of southern Australia1. Reconstructed as a jellyfish2, it was later assigned to the cubozoans (‘box jellies’), and has been cited as a clear instance of an extant animal lineage present before the Cambrian3,4,5,6,7. Until recently, Kimberella was known only from Australia, with the exception of some questionable north Indian specimens8. We now have over thirty-five specimens of this fossil from the Winter Coast of the White Sea in northern Russia. Our study of the new material does not support a cnidarian affinity. We reconstruct Kimberella as a bilaterally symmetrical, benthic animal with a non-mineralized, univalved shell, resembling a mollusc in many respects. This is important evidence for the existence of large triploblastic metazoans in the Precambrian and indicates that the origin of the higher groups of protostomes lies well back in the Precambrian.

Palaeozoic invertebrate fossils may pose severe problems in assigning them to Recent taxa. Sutton et al.1 describe the beautifully preserved and illustrated Silurian fossil Acaenoplax hayae as a “plated aplacophoran” mollusc, interpreting its polychaete-like characters as convergent features. In our opinion, it is more parsimonious to place this organism in the Polychaeta, as the molluscan similarities are limited to serial valve-like structures, suggesting polyplacophoran affinities. It is unlikely that Acaenoplax represents a primitive organization that is neither molluscan nor polychaete — instead, it appears to represent a highly derived, specialized line of invertebrate evolution.

Studies of the origin and radiation of the molluscs have yet to resolve many issues regarding their nearest relatives, phylogeny and ancestral characters1,2,3,4,5,6,7,8,9,10,11. The Polyplacophora (chitons) and the Aplacophora are widely interpreted as the most primitive extant molluscs2,3,9,10, but Lower Palaeozoic fossils of the former lack soft parts, and the latter were hitherto unrecognized as fossils. The Herefordshire Lagerstatte12 is a Silurian (about 425 Myr bp) deposit that preserves a marine biota in remarkable three-dimensional detail. The external surface of even non-biomineralized cuticle was preserved by entombment in volcanic ash, subsequent incorporation into concretions, and infilling of the fossils with sparry calcite13. Here we describe, from this deposit, a complete vermiform mollusc, which we interpret as a plated aplacophoran. Serial grinding at intervals of tens of micrometres, combined with computer-based reconstruction methods, renders the fossils in the round14.

DATING of deposits and materials less than 350 years old is hindered by the very poor time resolution of the radiocarbon method over this period1,2. Fluctuations in atmospheric 14C levels result in the existence of several possible calendric ages for any given radiocarbon age for terrestrial samples. In the marine record, these fluctuations are dampened. However, only a small change in marine radiocarbon ages occurs over this period (from AD 1700 to 1950, radiocarbon ages become only 100 yr younger2). Consequently, age resolution is poor. Furthermore, there is an uncertainty in the amount of the correction for the reservoir effect, the apparent radiocarbon age of modern marine carbon (typically ~400yr), which reflects the average residence time of carbon in the oceans. Amino-acid racemization/epimerization analysis has been used primarily for dating samples older than the limit of radiocarbon dating (40,000–50,000 yr BP)3,4. Here I show that aspartic acid (Asp) in mollusc shells has racemized particularly rapidly over the past few centuries. Analyses done on land snails from deposits in the Negev Desert, eolianites in Madeira, cave deposits in Jamaica and from museum collections indicate racemization rates of 2–5% per century. Asp racemization thus provides a means of dating recent faunal assemblages of molluscs and other biogenic carbonates, as well as recent terrestrial, fluvial and marine sedimentary sequences that contain molluscs.

THE interneuronal messenger nitric oxide1,2 (NO) may play a cen-tral role in the processing of olfactory information3. Several circuit elements in the mammalian olfactory bulb contain NO synthase4 or its functional equivalent, NADPH diaphorase5–7. The effects of NO on cellular excitability or circuit dynamics in the olfactory bulb are unknown, although NO effects on other rhythmic cells8 and circuits9 have been described. I have studied the role of NO in central olfactory processing using the procerebral (PC) lobe, the major central site of odour processing in terrestrial molluscs10,11. As in the mammalian olfactory bulb during odour stimulation, the basic dynamics of electrical activity in the molluscan PC lobe is an oscillation12–14. Here I report an obligatory role for NO in the oscillatory dynamics of the PC lobe of Limax maximus. Nitric oxide mediation of the olfactory oscillation may relate to the highly developed odour sensitivity and odour-learning ability of Limax15,16.

Determination of the dynamical structure of neural circuits--the general principles of how neural activity varies with time and manipulates information--is a prerequisite to understanding their computational function. Rhythmically active or oscillating neural circuits are particularly interesting dynamical structures, as rhythms and oscillations are a prominent feature of mammalian central nervous system electrophysiology. Coherent oscillations by networks of interneurons are observed in the vertebrate olfactory system and have recently been described in mammalian visual cortex. These interneuronal networks display oscillations in local field potential (LFP) and probability of producing action potentials that are highly correlated between subcircuits sharing the same stimulus features. Much less is known about the existence and importance of network oscillations in the higher centres of invertebrates. Here we report that a network of olfactory interneurons in the cerebral ganglion of the terrestrial mollusc Limax maximus also displays coherent oscillations in LFP which are modified by odour input. This dynamical structure could be central to the odour recognition and odour learning ability of Limax.

Bivalve molluscs form dense populations that exert profound effects on the particle loads and phytoplankton composition of coastal waters1. It has long been known that bivalves can select among different particle types, including selecting against those of poor nutritional value2,3,4,5, but because of difficulties in observing particle transport processes in the pallial cavity in vivo, the mechanism of selection was not known. We now use a combination of video endoscopy6 and flow cytometry7 to show that oysters can select living particles from non-living detritus on the gills. Our methods could aid the study of suspension feeding in many animal groups.

Steiner and Salvini-Plawen claim that Acaenoplax cannot be a mollusc because it would sit uneasily within the caudofoveates, solenogastres or polyplacophorans; however, the fossil record frequently reveals character combinations that have not survived to Recent times1. Although Acaenoplax displays a mosaic of molluscan characters that does not accord with any single modern group, this does not necessarily eliminate it from the phylum as a whole.

An animal's shell is subjected to stress throughout its life, and residual stress can be released and measured as strain. The shape and texture of the shell are advantageous features of design.

Carbon in marine calcareous tests is not necessarily derived only from dissolved inorganic carbon (DIG) in ambient sea water; metabolic carbon can also be incorporated into carbonate tests, as shown by experiments using 14C-labelled food during the incubation of sea urchin embryos and coral1,2. 14C-rich organic matter (relative to seawater DIC) from terrestrial vegetation as well as marine organic matter (reflecting seawater DIC) is used as food by marine organisms in near-shore environments. This use provides the basis for a natural experiment on the systematics of metabolic carbon incorporation into carbonate tests. Here we have combined 14C/12C and 13C/12C ratio measurements on both the calcareous and the organic parts of marine organisms and on DIC, plankton and other carbon-bearing materials collected in and around New Haven Harbor (Connecticut, USA) in Long Island Sound, where the various sources of carbon can be identified, and we deduce that a large percentage of the carbon in calcareous tests is metabolic carbon. Thus, it is at best difficult to use the δ13C values of ancient biogenic carbonate from molluscs to predict the ancient δ13C values of seawater DIC