Calcite and aragonite in the egg shell of Chelonia mydas L.

The structure of the calcified layer of egg shells from farm-reared and wild turtles has been examined using scanning electron microscopy and infrared analysis. Farm-reared egg shells contained discrete morphologically distinct regions of blocks of calcite and spherulites of aragonite. In contrast, the egg shells from feral populations consisted only of the spherulites. Differences in the impurity contents of the calcite and aragonite structures have been observed. SEM revealed a thin cuticular membrane.     

CONVERGENT SHELL MORPHOLOGY IN INTERTIDAL GASTROPODS

The functional morphology of shell infrastructure in 2 speciesof intertidal trochid was compared with that in 2 species ofnerite. The shell of Monodonta constrictais typical of the majorityof trochids. The shell is composed of 4 layers: a distal layer(calcite), anouter prismatic layer (aragonite), a nacreous layer(aragonite), and an oblique prismatic layer (aragonite). Monodontalabio lacks a distal layer and an oblique prismatic layer. Theoblique prismatic layer is replaced by an inner prismatic layerwhich forms an apertural ridge as a result of deposition andresorption. The shells of Nerita versicolor and N. tessellataconsistof 3 layers: an outer prismatic layer (calcite), a crossedlamellar layer (aragonite), and a complex crossed lamellar layer(aragonite). The complex crossed lamellar layer is covered withinclined platelets which superficially resemble the surfaceof the ique prismatic layer of trochids. In addition, the complexcrossed lamellar layer forms an apertural ridge which is similarin appearance to that of Monodonta labio. The outer surfaceof the mantle of Nerita versicolor and N. tessellata is throwninto a series of large folds which lie in contact with the inclinedplatelets of the omplex crossed lamellar layer. The interactionof the mantle folds with the inclined platelets is thought toserve as a rachet mechanism to aid in extension of themantle;a similar function has previously been proposed for trochids.The apertural ridges of Monodonta labio and Nerita are thoughtto prevent excessive desiccation when these gastropodsare exposedat low tide. ¹Contribution No. 56 of the Tallahassee, Sopchoppy & GulfCoast Marine Biological Association (Received 6 July 1979;     

Shell disintegration and taphonomic loss in rudist biostromes

Radiolitid biostromes in the Upper Cretaceous of Austria and Italy record a marked taphonomic loss controlled mainly by the composition of the biocoenosis, by the density of rudist colonization, by the style of radiolitid shell disintegration and by early diagenetic processes. Radiolitid shells consisted of a calcitic ostracum and an originally aragonitic hypostracum. The attached valve of most radiolitids was built of (1) an outermost ostracal layer of delicate calcite lamellae, (2) a thick layer of ‘boxwork ostracum’ built of radial funnel plates and cell walls, (3) a thin, inner ‘ostracal layer 3’ of thick-walled boxwork, and (4) the hypostracum that formed the innermost shell layer. The attached valve disintegrated by spalling of radial funnel plates of layer 2, and by selective removal of the boxwork ostracum. In the free valve, the ostracum consisted of two layers: (a) an inner, lid-shaped layer of dense calcite, and (b) an outer layer composed of calcite lamellae. The free valve disintegrated by spalling into ostracal and hypostracal portions, by spalling of the ostracum into layers a and b, and by disintegration of layer b into packages of calcite lamellae and individual lamellae. The specific style of disintegration of the radiolitids was aided or induced by discontinuities in shell structure. Lamellar fragments from the ostracum of the upper valve and from the radial funnel plates of the lower valve locally are abundant in free-valve-funnel-plate floatstones that comprise the matrix of or occur in lenses within radiolitid biostromes. In biostromes with an open parautochthonous fabric, selective removal of the boxwork ostracum of the attached valve occurred by mechanical spalling and, most probably, by early diagenetic dissolution. Complete removal of the boxwork ostracum yielded thin, relict shells composed of the ‘ostracal layer 3’ and the hypostracum. During early diagenesis, the hypostracum was replaced by blocky calcite spar, or was dissolved and became filled by internal sediments. The combination of both selective removal of boxwork ostracum and early diagenetic dissolution of aragonite locally resulted in the formation of ghost biostromes that entirely or largely consist of faint relics of radiolitids. The syndepositional formation of radiolitid shell relics and the presence of radiolitid ghost biostromes produced by bios-tratinomic and early diagenetic processes show that rudist biostromes can undergo marked taphonomic loss during fossilization. The presence of ghost biostromes with a burrowed, open parautochthonous rudist fabric indicates that the final preservation of a rudist biostrome was directly influenced by the characteristics of the biocoenosis, including unpreserved burrowing taxa. Rudist biostromes may be of markedly different taphonomy as a result of the taxonomic composition of the entire assemblage and the density of colonization by the rudists.     

Aragonitic dendritic prismatic shell microstructure in Thracia (Bivalvia,Anomalodesmata)

The shells of most anomalodesmatan bivalves are composed of an outer aragonitic layer of either granular or columnar prismatic microstructure, and an inner layer of nacre. The Thraciidae is one of the few anomalodesmatan families whose members lack nacreous layers. In particular, shells of members of the genus Thracia are exceptional in their possession of a very distinctive but previously unreported microstructure, which we term herein “dendritic prisms.” Dendritic prisms consist of slender fibers of aragonite which radiate perpendicular to, and which stack along, the axis of the prism. Here we used scanning and transmission electron microscopical investigation of the periostracum, mantle, and shells of three species of Thracia to reconstruct the mode of shell calcification and to unravel the crystallography of the dendritic units. The periostracum is composed of an outer dark layer and an inner translucent layer. During the free periostracum phase the dark layer grows at the expense of the translucent layer, but at the position of the shell edge, the translucent layer mineralizes with the units typical of the dendritic prismatic layer. Within each unit, the c‐axis is oriented along the prismatic axis, whereas the a‐axis of aragonite runs parallel to the long axis of the fibers. The six‐rayed alignment of the latter implies that prisms are formed by {110} polycyclically twinned crystals. We conclude that, despite its distinctive appearance, the dendritic prismatic layer of the shell of Thracia spp. is homologous to the outer granular prismatic or prismatic layer of other anomalodesmatans, while the nacreous layer present in most anomalodesmatans has been suppressed.     

Ultrastructural features and elemental distribution in eggshell during pre and post hatching periods in the green turtle, Chelonia mydas at Ras Al-Hadd, Oman

Eggshells were randomly collected from turtle nests immediately after oviposition and at the end of incubation to examine the ultrastructural features using scanning JSM-5600LV microscopy. Three layers were recognized; an outer calcareous, a middle multistrata and an inner membrane. The calcareous layer had loose nodular units varying in shape and size without interlocking attachments. In freshly laid eggs, each nodular unit had spicules arranged in folded stacks. The spicules became unfolded during incubation, to form radiating configurations. Elemental composition and mapping of the layers were analyzed using energy dispersive spectroscopy (EDS). The elements were unevenly distributed throughout the eggshell and Ca²⁺ decreased significantly after hatching. X-ray diffraction was used to identify the crystals of the eggshells. It revealed that nodular units of the calcareous were made up of CaCO₃, as aragonite (91%), calcite (6%) and vaterite (3%). The middle layer consisted of organic amorphous material with aragonite (89%) and calcite (11%). The shell membrane consisted of reticular fibers with crystals predominantly of NaCl halite. Thermogravimetry analysis of the calcareous layer indicated a complete evaporation of bonded H₂O at 480 °C and CO₂ at 830 °C. Using the differential thermal analysis (DTA), aragonite was transformed to stable calcite at 425 °C.     

Shell microstructure and mineralogy of the Littorinidae: ecological and evolutionary significance

An examination of the shell microstructure and mineralogy of species from 30 of the 32 genera and subgenera of the gastropod family Littorinidae shows that most species have a shell consisting of layers of aragonitic crossed-lamellar structure, with minor variations in some taxa. However, Pellilitorina, Risellopsis and most species of Littorina have partly or entirely calcitic shells. In Pellilitorina the shell is made entirely of calcitic crossed-foliated structure, while in the other two genera there is only an outer calcitic layer of irregular-prismatic structure. A cladistic analysis shows that the calcitic layers have been independently evolved in at least three clades. The calcite is found only in the outermost layers of the shell and in species inhabiting cooler waters of both northern and southern hemispheres. Calcium carbonate is more soluble in cold than warm water and, of the two polymorphs, calcite is about 35% less soluble than aragonite. We suggest that calcitic shell layers are an adaptation of high latitude littorinids to resist shell dissolution.     

In vitro regulation of CaCO(3) crystal polymorphism by the highly acidic molluscan shell protein Aspein

Biominerals, especially molluscan shells, generally contain unusually acidic proteins. These proteins are believed to function in crystal nucleation and inhibition. We previously identified an unusually acidic protein Aspein from the pearl oyster Pinctada fucata. Here we show that Aspein can control the CaCO(3) polymorph (calcite/aragonite) in vitro. While aragonite is preferentially formed in Mg(2+) -rich solutions imitating the extrapallial fluids of marine molluscs, Aspein exclusively induced calcite precipitation. Our results suggest that Aspein is involved in the specific calcite formation in the prismatic layer. Experiments using truncated Aspein demonstrated that the aspartic acid rich domain is crucial for the calcite precipitation.     

Shell mineralogical trends in epifaunal Mesozoic bivalves and their relationship to seawater chemistry and atmospheric carbon dioxide concentration

The Late Triassic-Early Jurassic change from aragonite- to calcite-facilitating conditions in the oceans, which was caused by a decrease of the Mg²⁺/Ca²⁺ ratio of seawater in combination with an increase of the partial pressure of carbon dioxide, also affected the shell mineralogy of epifaunal bivalves. In the “calcite sea” of the Jurassic and Cretaceous, the most diverse and abundant families of epifaunal bivalves had largely calcitic shells. Some of them, such as the Inoceramidae, acquired this shell mineralogy earlier in Earth's history but did not significantly diversify until the onset of “calcite sea” conditions. Others, however, replaced aragonite by calcite in their shell at the beginning of the Jurassic, as shown for the Ostreidae, Gryphaeidae, Pectinidae, Plicatulidae, and Buchiidae. In these families, replacement of aragonite by calcite took place in the middle and inner layer of the shell and was not associated with changes in morphology and life habit. It is therefore proposed that lower metabolic costs rather than higher resistance against dissolution or advantageous physical properties triggered the calcite expansion in their shells. This explanation fits well the observation that clades of thin-shelled bivalves were less affected by the change of seawater chemistry. Thick-shelled clades, by contrast, may suffer a severe decline in diversity until they adapt their shell mineralogy, as demonstrated by the Hippuritoida: The diversity of the Megalodontoidea, which failed to adapt their shell mineralogy to “calcite sea” conditions, dramatically decreased at the end of the Triassic, whereas their descendents became dominant carbonate producers during the Late Mesozoic after they acquired a calcitic outer shell layer in the Late Jurassic. These examples indicate that changes in the seawater chemistry and in the partial pressure of carbon dioxide are factors that influence the diversity of carbonate-secreting animals, and, as in the case of the decline of the Megalodontoidea, may contribute to mass extinctions.     

Lipids from the nacreous and prismatic layers of two Pteriomorpha mollusc shells

Mollusc shells are acellular biominerals, in which macromolecular structures are intimately associated with mineral phases. Most studies are devoted to proteins, despite sugars have been described. Lipids were extracted from the calcite prismatic and aragonite nacreous layer of two mollusc shells. Fourier Transform Infrared Spectrometry shows that lipids are present in both samples, but they are not similar. Thin layer chromatography confirms that lipids are different in the two studied layers, so that it may be suggested they are species-dependant. Although not yet deciphered, their role in biomineralization and fossilisation processes is probably important.     

Aragonite shells are more ancient than calcite ones in bivalves: new evidence based on omics

Two calcium carbonate crystal polymorphs, aragonite and calcite, are the main inorganic components of mollusk shells. Some fossil evidences suggest that aragonite shell is more ancient than calcite shell for the Bivalvia. But, the molecular biology evidence for the above deduction is absent. In this study, we searched for homologs of bivalve aragonite-related and calcite-related shell proteins in the oyster genome, and found that no homologs of calcite-related shell protein but some homologs of aragonite-related shell proteins in the oyster genome. We explained the results as the new evidence to support that aragonite shells are more ancient than calcite shells in bivalves combined the published biogeological and seawater chemistry data.     

Eggshell formation during prolonged gravidity of the tuatara Sphenodon punctatus

Scanning electron microscopy (SEM) was used to examine the process of shell formation in tuatara. Tuatara carry eggs in the oviducts for ∼ 7–8 mo before nesting, a period of gravidity more than three times as long as in any other oviparous reptile. Our aim was to determine whether shell formation occurred rapidly after ovulation, or whether it occurred gradually throughout gravidity. Eggs were obtained from females in early gravidity (May, ∼ 1 mo after ovulation), midgravidity (August and September, 4–5 mo after ovulation), and late gravidity, immediately prior to nesting (December, 8 mo after ovulation). The shell membrane (fibrous layer) was well formed by May, but calcification of the outer surface had only just begun. Vertical columns of calcium carbonate were embedded in the shell membrane and appeared to erupt through the outer surface between early and midgravidity. Changes in the appearance of the outer calcareous layer were evident as gravidity progressed. In all shells, calcium carbonate was present as calcite. The appearance of the inner boundary (innermost layer of eggshell) was variable; some shells had a smooth and amorphous inner boundary as previously reported for tuatara and other reptiles, whereas other shells had an inner boundary composed of small spherical granules on the inner surface of which small calcareous spicules were scattered. A previously published model of the process of shell formation in tuatara eggshells is refined in light of our observations. We interpret the ability of female tuatara to shell their eggs gradually during winter as further evidence of their unusual physiological tolerance of cold conditions. © 1996 Wiley-Liss, Inc.     

Mineralogical composition and biomass studies of the microbial mats sediments from the Ebro Delta, Spain.

The mineral composition of the microbial mats at La Banya spit was studied. The spit is formed by a narrow sand bar and a peninsula and is located south of the main body of the Ebro Delta (Tarragona, Spain). Although quartz was the predominant mineral component in all sampling sites, clay, feldspars, calcite, aragonite, halite, dolomite and gypsum were also found. An increase in both the fine material (clay) and the halite content was observed in the sites influenced by nearby salterns. The amount of each mineral did not differ significantly along a 55 cm deep profile, except for halite and aragonite, which reached a maximum in the surface and decreased with depth. Dolomite, which ranged from 0.5 to 5% (w/w), is a possible indicator of sulfate-reducing bacteria activity in the past. Organic carbon and total nitrogen were quantified for biomass assessment. Total nitrogen ranged from 0.1 to 0.56% in the uppermost layer, where the microbial mat is active, but was undetectable at deeper layers. Organic carbon ranged from 1 to 5.5% in the active microbial mat layers and decreased to 0.3% at deeper layers. During the summer, both organic carbon and total nitrogen contents (biomass) of the microbial mat samples from some sites increase, whereas other sites show constant concentrations throughout the year, and others have a fluctuant biomass content.     

Multiscale structure of calcite fibres of the shell of the brachiopod Terebratulina retusa

The shells of rhynchonelliform brachiopods have an outer (primary) layer of acicular calcite and an inner (secondary) layer of calcite fibres which are parallel to the shell exterior. Atomic force microscopy (AFM) reveals that these fibres are composed of large triangular nanogranules of about 600-650 nm along their long axis. The nanogranules are composites of organic and inorganic components. As the shell grows, the fibres elongate with the calcite c-axis perpendicular to the fibre axis as demonstrated by electron backscatter diffraction (EBSD). Thus, despite being a composite structure comprising granules that are themselves composites, each fibre is effectively a single crystal. The combination of AFM and EBSD reveals the details of the structure and crystallography of these fibres. This knowledge serves to identify those aspects of biological control that must be understood to enable comprehension of the biological control exerted on the construction of these exquisite biomineral structures.     

Hydrogeochemistry of Lake Gallocanta (Aragón,NE Spain)

Lake Gallocanta has undergone drastic changes during the last thirteen years. Water level changed from a high level (Z_max = 2 m) to total dryness in 1985. From 1986 to 1988 slow refilling occurred. The water volume fluctuations have been studied in relation to climatic variations recorded for that period. Variations in the major dissolved ions were related to water volume fluctuations from data at two different stages, one corresponding to the drying phase and another to the refilling phase. Mineralogical composition of the salts precipitated at different stages was examined by X-ray diffraction. Interstitial water and mineralogical composition of recent sediments were also studied along a transect through the lake.The water column decrease from 1977 to 1985 is related to decreasing annual rainfall (500-250 mm respectively). The refilling in 1986–1988 is due to high annual rainfall (537 mm). In addition to these fluctuations, seasonal changes of the water level between 20 and 50 cm occurred every year.Gallocanta is a Na-Mg-Cl-(SO₄) type lake. During the drying period a typical salt enrichment occurs with linear relationships between TDS, Cl, Na and K. Alkalinity is linearly correlated with Ca at relatively low salinities. As salinity increases a linear relationship between Ca and SO₄ is observed. Minerals formed from the brine are halite, bischofite, epsomite, hexahydrite, mirabilite, gypsum, aragonite, calcite and dolomite. The molar ratio Mg/Ca of the interstitial water changes from 1.5 along the shorelines, where calcite and aragonite precipitate, to 40 in the center of the lake. Sediment cores from the central part of the lake show aragonite in the top layers, magnesian calcite and low proportions of quartz and illite, while at 20 cm depth a high proportion of gypsum is present. In contrast, cores from the shore of the lake are mainly composed of low magnesium calcite in the top layers and low magnesium calcite together high magnesium calcite and dolomite between 30 and 70 cm depth. Gypsum deposits only occur in significant proportions at 80–100 cm depth.The refilling process showed relationships between volume and salt concentration following the Langbein model. The salt mass in solution decreased about 50% from the drying to the refilling phase. However, Mg content decreased about 70 % for the same period, suggesting a contribution of this element to the dolomite formation.     

Shell structure of two polar pelagic molluscs, Arctic Limacina helicina and Antarctic Limacina helicina antarctica forma antarctica

The purpose of this study was to investigate shell growth performance in two thin-shelled pelagic gastropods from cold seawater habitats. The shells of Arctic Limacina helicina and Antarctic Limacina helicina antarctica forma antarctica are very thin, approximately 2–9 μm for shells of 0.5–6 mm in diameter. Many axial ribbed growth lines were observed on the surface of the shell of both Limacina species. Distinct axial ribs were observed on the outermost whorl, while weak or no rib-like structures were observed on the inner whorls in the larger shell of L. helicina antarctica forma antarctica. For L. helicina, no ribs were observed on small individuals with three whorls, while larger individuals had distinct ribs on the outer whorls. Shell microstructure was examined in both species. There is an inner crossed-lamellar and extremely thin outer prismatic layer in small individuals of both species, and a distinct thick inner prismatic layer was observed beneath the crossed-lamellar layer in large Antarctic individuals. Various orientations of the crossed-lamellar structure were observed in one individual. Shell structure appeared to be different between the Antarctic and Arctic species and among shells of different size.     

Calcite and aragonite distributions in the skeletons of bimineralic bryozoans as revealed by Raman spectroscopy

Abstract. Bryozoans are among a diverse range of invertebrates capable of secreting calcium carbonate skeletons. Relatively little is known about biomineralization in bryozoans, despite the importance of understanding biomineralization processes for nanotechnology and the threats imposed by ocean acidification on organisms having calcareous skeletons. Ten species of cheilostome bryozoans that are reported to have bimineralic skeletons of calcite and aragonite are studied here using Raman spectroscopy. This technique allowed identification of the two mineral phases at submicron spatial resolution, allowing the distributions of calcite and aragonite within bryozoan skeletons to be determined with unprecedented precision. Confirming previous findings based on the use of chemical stains, most of the bimineralic species analyzed exhibited a calcitic skeletal framework, composed of basal, vertical, and inner frontal walls, having aragonite deposited subsequently onto the outer surfaces of the frontal walls. In one species ( Odontionella cyclops ), aragonite formed the superstructure above the autozooids, and in two others, traces of aragonite were detected on the undersides of the frontal shields. Using Raman spectroscopy, it was possible for the first time to determine the mineralogy of small-scale structures, including orificial rims, condyles and hinge teeth, avicularian pivotal bars and rostra, and ascopore rims and sieve plates. Even when surrounded by aragonitic frontal shields, these structures were found typically to be calcitic, the two exceptions being the aragonitic avicularia of Stylopoma inchoans and O. cyclops . Unexpectedly, the first-formed part of the basal wall at the distalmost growing edge of Pentapora foliacea was found to consist mainly of aragonite. This may point to a precursory phase of biomineralization comparable with the unusual mineralogies identified previously in the earliest-formed skeletons of members of some other invertebrate phyla.     

In situ chemical speciation of sulfur in calcitic biominerals and the simple prism concept

The microstructure and composition of two mollusc shells were investigated using a combination of light microscopy, SEM, EPMA, and XANES. The shells of Pinna and Pinctada are composed of calcite prisms separated by organic walls. The prismatic units of Pinna are monocrystalline, and those of Pinctada are polycrystalline with internal organic radial membranes. High-spatial-resolution XANES maps for the different S species across adjacent prisms show that sulfate is the principal component in both the intraprismatic organic matrices and the outer membranes. Additionally, these maps confirm that the inner structures of the prismatic units are different for both genera. In many ways, the prisms of Pinna and Pinctada are different and invalidate the "simple prism" concept.     

Structure and composition of the boundary zone between aragonitic crossed lamellar and calcitic prism layers in the shell of Concholepas concholepas (Mollusca,Gastropoda)

Mollusc shells are composed of two or three layers. The main layers are well‐studied, but the structural and chemical changes at their boundaries are usually neglected. A microstructural, mineralogical, and biochemical study of the boundary between the inner crossed lamellar and outer prismatic layers of the shell of Concholepas concholepas showed that this boundary is not an abrupt transition. Localized structural and chemical analyses showed that patches of the inner aragonitic crossed lamellar layer persist within the outer calcitic prismatic layer. Moreover, a thin aragonitic layer with a fibrous structure is visible between the two main layers. A three‐step biomineralization process is proposed that involves changes in the chemical and biochemical composition of the last growth increments of the calcite prisms. The changes in the secretory process in the mantle cells responsible for the shell layer succession are irregular and discontinuous.     

Multiscale Mechanics and Optimization of Gastropod Shells

A vast majority of mollusks grow a hard shell for protection.The structure of these shells comprises several levels of hierarchy that increase their strength and their resistance to natural threats.This article focuses on nacreous shells,which are composed of two distinct layers.The outer layer is made of calcite,which is a hard but brittle material,and the inner layer is made of nacre,a tough and ductile material.The inner and outer layers are therefore made of materials with distinct structures and properties.In this article,we demonstrate that this system is optimum to defeat attacks from predators.A two-scale modeling and optimization approach was used.At the macroscale,a two-layer finite element model of a seashell was developed to capture shell geometry.At the microscale,a representative volume element of the microstructure of nacre was used to model the elastic modulus of nacre as well as a multiaxial failure criterion,both expressed as function of microstructural parameters.Experiments were also performed on actual shells of red abalone to validate the results obtained from simulations and gain insight into the way the shell fails under sharp perforation.Both optimization and experimental results revealed that the shell displays optimum performance when two modes of failure coincide within the structure.Finally,guidelines for designing two-layer shells were proposed to improve the performance of engineered protective systems undergoing similar structural and loading conditions.