The development of growth lines on articulate brachiopods

Three types of growth lines are recognised on articulate brachiopod shells: (1) very fine diurnal growth lines formed by calcite increments at the shell margin, (2) seasonal growth lines, formed by inward reflection (doubling back) of the mantle edge, seen as concentric steps on the shell surface and marked by re-orientation of growth vectors evidenced by secondary shell fibres, (3) disturbance lines, formed by abrupt regression of the mantle edge, also seen as concentric steps on the shell surface, but indicated by a dislocation in the shell fabric. Lamellose and spinose ornaments of the sort seen in Tegulorhynchia are essentially genetically controlled. Periodic outgrowths from the outer mantle lobe secrete frills of primary shell that project from the shell surface and form short hollow spines where they cross the radial ornament. In longitudinal section spine formation is seen to involve gradual increase in the rate of secretion of primary shell followed by retraction, and often collapse, of the mantle outgrowth, accompanied by regression. Reflection of the mantle edge usually follows spine formation.     

Finite element analysis of a double membrane tube (DMS-tube) and its implication for gastropod shell morphology

Molluscan shells, including those of Gastropoda, are formed by accretionary growth at the mantle edge. The mantle is a thin membrane of skirt-like shape, which extends minutely beyond the aperture, and its edge adds a shell increment to the aperture margin so that each increment copies a configuration of the mantle edge at that time. Thus, regulation of shell morphogeny is almost equivalent to the factors which control the mantle form at the moment of shell growth. Form of the mantle skirt is considered to be kept in a state of balance between the force of its internal stress and forces acting on it such as fluid pressure or muscle contraction. The expansion behavior of the mantle skirt has been numerically analyzed by using an elastic model (DMS-tube), which represents the fundamental structure of the mantle tissue as a double membrane structure with internal springs (DMS). Four characteristic expansion patterns of the DMS-tube have been detected: (1) general outward expansion; (2) developing a ridge-like fold on an initial longitudinal protrusion of the tube edge; (3) drastic shift of the expanded state from a uniformly curved to an elliptical shape in outline, owing to the existence of a fixed boundary condition on the tube wall; and (4) constricted protrusion on the open region of the shell wall surrounding the DMS-tube. These results have the potential for answering the following questions relating to the morphogenesis of gastropod shells. How does the mantle skirt usually make contact with the inner surface of the shell wall so as to ensure continuous accretion of shell materials to the aperture margin? What is the cause of spiral ridges? Why do open coiling or minimally overlapping shells have generally circular apertures, while shells with apertures overlapped by whorls have non-uniformly curved apertural lips? What is the cause of long closed spines and why do they always appear on spiral ridges?     

Zona Localization of Shell Matrix Proteins in Mantle of <Emphasis Type="Italic">Haliotis tuberculata</Emphasis> (Mollusca,Gastropoda)

Organic matrix from molluscan shells has the potential to regulate calcium carbonate deposition and crystallization. Control of crystal growth thus seems to depend on control of matrix protein secretion or activation processes in the mantle cells, about which little is known. Biomineralization is a highly orchestrated biological process. The aim of this work was to provide information about the source of shell matrix macromolecule production, within the external epithelium of the mantle. An in vivo approach was chosen to describe the histologic changes in the outer epithelium and in blood sinus distribution, associated with mantle cells implicated in shell matrix production. Our results characterized a topographic and time-dependent zonation of matrix proteins involved in shell biomineralization in the mantle of Haliotis.     

Functional morphology of the mantle of Nautilus pompilius (Mollusca, Cephalopoda)

This study presents histological and cytological findings on the structural differentiation of the mantle of Nautilus pompilius in order to characterize the cells that are responsible for shell formation. The lateral and front mantle edges split distally into three folds: an outer, middle, and inner fold. Within the upper part of the mantle the mantle edge is divided into two folds only; the inner fold disappears where the hood is attached to the mantle. At the base of the outer fold of the lateral and front mantle edge an endo-epithelial gland, the mantle edge gland, is localized. The gland cells are distinguished by a distinct rough endoplasmic reticulum and by numerous secretory vesicles. Furthermore, they show a strong accumulation of calcium compounds, indicating that the formation of the shell takes place in this region of the mantle. Numerous synaptic contacts between the gland cells and the axons of the nerve fibers reveal that the secretion in the area of the mantle edge gland is under nervous control. The whole mantle tissue is covered with a columnar epithelium having a microvillar border. The analyses of the outer epithelium show ultrastructural characteristics of a transport active epithelium, indicating that this region of the mantle is involved in the sclerotization of the shell. Ultrastructural findings concerning the epithelium between the outer and middle fold suggest that the periostracum is formed in this area of the mantle, as it is in other conchiferan molluscs.     

Bivalves with 'concrete overcoats': Granicorium and Samarangia

Two veneroidean bivalves Granicorium indutum from Australia and Samarangia quadrangularis from the tropical Indo-Pacific region, cement a thick, hard layer of sand over most of their shells. In Granicorium this layer forms low commarginal ribs while in Samarangia it forms more prominent radial features. Sand grains are cemented to the shell and to each other with growths of a crystalline aragonitic cement similar in morphology to inorganic marine cements. Both species secrete mucus layers at the growing shell margin which initially hold the sediment grains together and form a substrate for the nucleation and growth of calcium carbonate crystals. The ribs of Samarangia are formed by the accretion of successive sheets of spherulitic growths. In G. indutum , the middle and outermost of two inner mantle folds are large, glandular and capable of considerable extension beyond the shell margin. Mucus secreted by the folds contains abundant bacteria and small calcium carbonate crystals. It is proposed that initial nucleation of the calcium carbonate cement takes place within this biofilm possibly mediated by the bacteria. The function of the sand layers is unknown but predation resistance and protection of the shells from endobionts are the most likely possibilities.     

Taphofacies analysis of recent shelly cheniers (beach ridges), northeastern baja california,Mexico

Summary This report presents the results of taphofacies analyses of shelly cheniers (mollusk-dominated lag-concentrations) from the tidal flats of northeastern Baja California, Mexico. The three generations of moderm (formed during last 70 years), submodem (younger than 1,500 BP), and subfossil (5,000–2,000 BP) cheniers can be distinguished by their position relative to the shoreline, their topography, and the radiocarbon-age of their shells. The generations differ in the duration and complexity of their taphonomic history. Sixty-one samples from nine localities were collected to test the utility of the taphofacies approach for studying chenier-type shell deposits. The three chenier generations, although all dominated by the bivalve molluskMulinia coloradoensis, differ significantly in their taxonomic composition due to taphonomic and/or biologic factors. The taphofacies analysis included 4,334 specimens ofM. coloradoensis described by nine taphonomic variables. Univariate analysis of those variables indicated that the shells that accumulated in the cheniers are little-affected by biological processes (bioerosion, encrustation), and moderately affected by physical processes (fragmetation, cracking, peeling, edge preservation). Only the luster features of shells (external luster, internal luster, and internal features) vary substantially and consistently with chenier age —a result of subaerial weathering. Multivariate taphofacies analysis discriminates the three generations of cheniers even when the poorly preservable luster variables are excluded from the analysis. This suggests that taphofacies discrimination is possible for fossil cheniers. The shells collected from the chenier surface have substantially poorer preservation than shells from the subsurface, indicating that taphonomic degradation in the chenier plain environment is a surface phenomenon. Chenier plain shelly assemblages are taphonomically distinct from assemblages formed in other marine environments: they have a very low frequency of macroscopically recognizable bioerosion and encrustation. The existence of preservable taphonomic differences between the cheniers that differ in their age (i.e., duration of preburial history), suggests that fossil lag concentrations may be useful in detecting incompleteness gradients along stratigraphic boundaries. A ‘taphonomic clock’—a correlation between a ‘time-sincedeath’ and shell preservation—was found only for luster features, taphonomic attributes that are unlikely to be preserved in the fossil record.     

Morphological studies on the mantle of the fresh-water mussel Amblema (UNIONIDAE): Scanning electron microscopy

The scanning electron microscope has been used to describe the surface morphology of the mantle in mantle-shell preparations from the fresh-water mussel Amblema. In some regions (adductor muscle insertions), the mantle is firmly attached to the shell. In other areas (along the main course of the mantle), transient adhesions between the outer mantle epithelial cells and the nacre appear to temporally further compartmentalize the extrapallial fluid possibly as a prerequisite for the initial crystallization phenomenon. At the mantle edge, as well as at the isthmus, the periostracum was seen to extrude from the periostracal groove. At the siphonal edge, peculiar fingerlike processes were identified; these may represent primitive photoreceptors. The epithelial cells of the outer mantle epithelium are all microvillated whereas those of the inner mantle epithelium are both microvillated and ciliated. Specific differences in surface morphology are described for various regions of the outer mantle epithelium. These may be related to precise regionalized functional differences of this tissue. Several functions of the mantle, in addition to shell formation, and based on its various morphologies, are also discussed.     

Carbonic anhydrase activity and biomineralization process in embryos, larvae and adult blue mussels Mytilus edulis L.

To decide whether a physiological role can be attributed to enzymatic activity with respect to crystal formation and biomineralization of the first larval shell, carbonic anhydrase (CA) activity was measured in embryos and larvae of the blue mussels Mytilus edulis L. Also, CA activity was determined in the mantle edge and gonads of adult mussels with different shell length and condition index. The intention was to find a possible correlation between CA activity and adult shell calcification, i.e. gonadal maturation. The comparison of CA activity in different developmental stages of mussels and the results of an X-ray diffraction study of biomineralization processes in embryonic and larval shells indicate that CA activity is maximal at the end of several developmental stages. Consequently, the increase in CA activity precedes some physiological changes, i.e. the somatoblast 2d formation and the occurrence of the first calcite and quartz crystals in embryos, shell field formation in the gastrula stage, shell gland and periostracum production in trochophores, and rapid aragonite deposition in larval prodissoconch I and prodissoconch II shells. Furthermore, it was found that in adult mussels CA activity was quite variable and that in the mantle edge it was frequently inversely related to the activity in the gonad. Received: 28 November 1998 / Received in revised form: 30 August 1999 / Accepted: 31 August 1999     

Probabilistic Gompertz model of irreversible growth

Characterizing organism growth within populations requires the application of well-studied individual size-at-age models, such as the deterministic Gompertz model, to populations of individuals whose characteristics, corresponding to model parameters, may be highly variable. A natural approach is to assign probability distributions to one or more model parameters. In some contexts, size-at-age data may be absent due to difficulties in ageing individuals, but size-increment data may instead be available (e.g., from tag-recapture experiments). A preliminary transformation to a size-increment model is then required. Gompertz models developed along the above lines have recently been applied to strongly heterogeneous abalone tag-recapture data. Although useful in modelling the early growth stages, these models yield size-increment distributions that allow negative growth, which is inappropriate in the case of mollusc shells and other accumulated biological structures (e.g., vertebrae) where growth is irreversible. Here we develop probabilistic Gompertz models where this difficulty is resolved by conditioning parameter distributions on size, allowing application to irreversible growth data. In the case of abalone growth, introduction of a growth-limiting biological length scale is then shown to yield realistic length-increment distributions.     

Stacking Increments: A New Model and Morphospace for the Analysis of Bivalve Shell Growth

A model employing stacking increments is introduced for the analysis of bivalve shell growth and form. The model is based on the components of shell growth that are potentially independent: the rate of mantle cell proliferation, the rate of precipitation of shell material, and the rate of translation of the pallial line, where the mantle is attached to the shell. This model is defined in terms of the following parameters: (1) the ratio of accretion of shell material at the shell margin to growth of the mantle by cell division, (2) the ratio of shell accretion at the pallial line to mantle growth, and (3) the ratio of the amount of pallial muscle translation, away from the umbo toward the shell margin, to mantle growth. In this model, the shape of a radial section through the shell is simulated by stacking of internal microgrowth increments. The mode of stacking of the increments is determined by the balance among the parameters defining growth. A theoretical morphospace defined on the basis of this model is largely consistent with the range of forms of naturally occurring bivalve shells. Analysis of the distribution of actual shell forms in relation to this morphospace suggests that the absolute rate of shell precipitation and the gradient in precipitation rate away from the shell margin along a radial cross-section are physiologically as well as geometrically constrained.     

Constraints in the ligament ontogeny and evolution of pteriomorphian Bivalvia

A study of ligaments of larval, postlarval and adult shells of fossil and recent pteriomorphian bivalves leads to the following observations and hypotheses: (1) Ligament growth passively follows the general growth pattern of the mantle margin. No independent genetic information fixes the anterior, ventral, or posterior growth direction of the ligament. Further growth constraints relate to physical availability of space on the ligament area and to heterochronic processes. (2) The disjunct ligament and the repetition of fibrous or lamellar sublayers are phenotypic aspects of the same derived ligament Bauplan 1. All Pteriomorphia possess the ability to produce repetitive ligaments. This ability and space reductions of the ligament area in independent phylogenetic lineages are responsible for the iterative evolution of ligament grades. (3) Spondylidae and Plicatulidae are duplivincular, and the Ostreoidea are plesiomorphically multivincular. (4) Larval anterior-helical growth of the soft tissue produces opisthogyrate shells and possibly caused the evolution of the alivincular-multivincular grade. Duplivincular-alivincular and multivincular-alivincular grades can be distinguished if larval shell characters are known. (5) The taxonomic distribution of ligament grades as amended in this paper is largely consistent with modern phylogeny hypotheses based on genetic or morphologic or combined character sets. However, the resolution of early phylogenetic nodes requires more data on larval shells of Lower Palaeozoic taxa.     

Changes in gape frequency, siphon activity and thermal response in the freshwater bivalves Anodonta cygnea and Margaritifera falcata

Physiologically-driven rhythms in bivalve molluscs are predictedto vary as a function of metabolic rate and temperature, incontrast to genetically predisposed biological clocks. Theserhythms can be evaluated using long-term video monitoring techniquesunder controlled conditions in laboratory aquaria. The bivalvesAnodonta cygnea and Margaritifera falcata were used to evaluatethe effect of temperature on rhythms in gape and the formationof siphons at the mantle edge. Frequency and duration of shellclosure vary with temperature in both species, but with differentresponses. Mean duration of intervals of valve closure decreasesas temperature rises in both species, and is consistent withphysiological limitation by increased biological oxygen demand.For A. cygnea, cumulative gape duration peaks at 25°C, withless time spent closed than at any other temperature, but increasingtemperatures correspond to an increase in gape frequency witha strong increase observed at 31°C. In contrast, frequencyof adduction and valve closure peak at 25°C in M. falcata,and continuous gaping is observed above 29.5°C. This physiologicalstress is consistent with evidence from sclerochronologically-calibratedstable isotope studies of shells, where growth breaks in manymarine taxa coincide with maximum temperatures above 31°Cas derived for ¹⁸O_carbonate. The results of this study suggestthat these growth breaks may be due to physiological limitationsin oxygen uptake and metabolic activity, rather than being adirect consequence of elevated temperature alone. (Received 17 March 2008; accepted 3 October 2008)     

Standardization in Biological Staining. The Influence of Dye Manufacturing

The purpose of biological staining is to obtain specimens of biological material that can be assessed in the microscope. These specimens are influenced by all processes from removal from the intact organism to mounting on the microscopic slide. To achieve comparable results with various techniques for biological staining, standardization of all procedures and reagents is mandatory. In this paper, I focus particularly on dyes and consider the possibilities for obtaining standardized dyes. In general practice, most biological staining takes place with available commercial dyes. These dyes may or may not have been subjected to quality assessment either internally by the producer or vendor or externally by independent investigators or organizations such as the Biological Stain Commission. Concerted attempts at standardization in Europe are discussed. The latest results of this work, the European standard EN 12376, is presented. This standard is concerned with information supplied by the manufacturer with in vitro diagnostic reagents for biological staining. The standard has been prepared by a Working Group on Staining in Biology under Technical Committee 140, In Vitro Medical Devices, of the European committee for standardization, CEN.     

Standardization in biological staining. The influence of dye manufacturing.

The purpose of biological staining is to obtain specimens of biological material that can be assessed in the microscope. These specimens are influenced by all processes from removal from the intact organism to mounting on the microscopic slide. To achieve comparable results with various techniques for biological staining, standardization of all procedures and reagents is mandatory. In this paper, I focus particularly on dyes and consider the possibilities for obtaining standardized dyes. In general practice, most biological staining takes place with available commercial dyes. These dyes may or may not have been subjected to quality assessment either internally by the producer or vendor or externally by independent investigators or organizations such as the Biological Stain Commission. Concerted attempts at standardization in Europe are discussed. The latest results of this work, the European standard EN 12376, is presented. This standard is concerned with information supplied by the manufacturer with in vitro diagnostic reagents for biological staining. The standard has been prepared by a Working Group on Staining in Biology under Technical Committee 140, In Vitro Medical Devices, of the European committee for standardization, CEN.     

Involvement of Hox genes in shell morphogenesis in the encapsulated development of a top shell gastropod (Gibbula varia L.)

Regulatory gene expression during the patterning of molluscan shells has only recently drawn the attention of scientists. We show that several Hox genes are expressed in association with the shell gland and the mantle in the marine vetigastropod Gibbula varia (L.). The expression of Gva-Hox1, Gva-Post2, and Gva-Post1 is initially detected in the trochophore larval stage in the area of the shell field during formation of embryonic shell. Later, during development, these genes are expressed in the mantle demonstrating their continuous role in larval shell formation and differentiation of mantle edge that secretes the adult shell. Gva-Hox4 is expressed only late during the development of the veliger-like larva and may also be involved in the adult shell morphogenesis. Additionally, this gene also seems to be associated with secretion of another extracellular structure, the operculum. Our data provide further support for association of Hox genes with shell formation which suggest that the molecular mechanisms underlying shell synthesis may consist of numerous conserved pattern-formation genes. In cephalopods, the only other molluscan class in which Hox gene expression has been studied, no involvement of Hox genes in shell formation has been reported. Thus, our results suggest that Hox genes are coopted to various functions in molluscs.     

Growth rate and age effects on Mya arenaria shell chemistry: Implications for biogeochemical studies

The chemical composition of bivalve shells can reflect that of their environment, making them useful indicators of climate, pollution, and ecosystem changes. However, biological factors can also influence chemical properties of biogenic carbonate. Understanding how these factors affect chemical incorporation is essential for studies that use elemental chemistry of carbonates as indicators of environmental parameters. This study examined the effects of bivalve shell growth rate and age on the incorporation of elements into juvenile softshell clams, Mya arenaria. Although previous studies have explored the effects of these two biological factors, reports have differed depending on species and environmental conditions. In addition, none of the previous studies have examined growth rate and age in the same species and within the same study. We reared clams in controlled laboratory conditions and used solution-based inductively coupled plasma mass spectrometry (ICP-MS) analysis to explore whether growth rate affects elemental incorporation into shell. Growth rate was negatively correlated with Mg, Mn, and Ba shell concentration, possibly due to increased discrimination ability with size. The relationship between growth rate and Pb and Sr was unresolved. To determine age effects on incorporation, we used laser ablation ICP-MS to measure changes in chemical composition across shells of individual clams. Age affected incorporation of Mn, Sr, and Ba within the juvenile shell, primarily due to significantly different elemental composition of early shell material compared to shell accreted later in life. Variability in shell composition increased closer to the umbo (hinge), which may be the result of methodology or may indicate an increased ability with age to discriminate against ions that are not calcium or carbonate. The effects of age and growth rate on elemental incorporation have the potential to bias data interpretation and should be considered in any biogeochemical study that uses bivalves as environmental indicators.     

Models for the morphogenesis of the molluscan shell

We give a review of current theories of morphogenesis of both the general coiling and the ornamentation of molluscan shells. These two aspects of shell growth are closely connected, as ornamentation is primarily due to local perturbations of the general apertural growth field controlling coiling. Also, a new, generalized, free-form apertural growth map model is presented in this paper, illustrating some aspects of the regulation of logarithmic spiral growth. This model is used to simulate the formation of megastriae in ammonoids. We emphasize the importance of damaged specimens and how they regenerated, as illustrated with examples from ammonoids. The phenomenon of ornamental compensation can be explained by a mechanism involving a pre-pattern in the mantle. However, simple reaction-diffusion models for ornamental pattern formation should be regarded only as useful abstractions.     

The Paleozoic evolution of the gastropod larval shell: larval armor and tight coiling as a result of predation‐driven heterochronic character displacement

Early and middle Paleozoic gastropod protoconchs generally differ strongly from their corresponding adult morphologies, that is, most known protoconchs are smooth and openly coiled, whereas the majority of adult shells are ornamented and tightly coiled. In contrast, larval and adult shells of late Paleozoic gastropods with planktotrophic larval development (Caenogastropoda, Neritimorpha) commonly resemble each other in shape and principle ornamentation. This is surprising because habitat and mode of life of planktonic larvae and benthic adults differ strongly from each other. Generally, late Paleozoic to Recent protoconchs are tightly coiled. This modern type of larval shell resembles the adult shell morphology and was obviously predisplaced onto the larval stage during the middle Paleozoic. The oldest known planktonic‐armored (strongly ornamented) larval shells are known from the late Paleozoic. However, smooth larval shells are also common among the studied late Paleozoic gastropods. The appearance of larval armor at the beginning of the late Paleozoic could reflect an increase of predation pressure in the plankton. Although there are counter examples in which larval and adult shell morphology differ strongly from each other, there is statistical evidence for a heterochronic predisplacement of adult characters onto the larval stage. Larval and adult shells are built in the same way, by accretionary secretion at the mantle edge. It is likely that the same underlying gene expression is responsible for that. If so, similarities of larval and adult shell may be explained by gene sharing, whereas differences may be due to different (planktic vs. benthic life) epigenetic patterns.     

Comparing ontogenetic trajectories using growth process data

Ontogenetic trajectories are commonly quantified by characterizing changes in the sizes and shapes of organisms over the course of development. This formulation of ontogenetic transformations can be misleading in that it ignores critical aspects of the biological processes responsible for constructing morphology. Hypothetical examples are used to illustrate some of the shortcomings of methods that rely exclusively on size and shape data for ontogenetic analyses. By characterizing growth as a vector field, and representing growth vectors as complex numbers, one can simultaneously analyze size, shape, and growth processes. The utility of such an approach is demonstrated in a study of shape and growth process variation in turtle shells.