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?     

A light- and electron-microscopic study of enzymes in the embryonic shell-forming tissue of the freshwater snail, Biomphalaria glabrata

Abstract. The mode of formation of the molluscan exoskeleton is still poorly understood, but studies on adult snails indicate that enzymes involved in vertebrate bone formation also participate in mollusc shell formation. The enzymes peroxidase, alkaline phosphatase, and acid phosphatase are expressed in a constant pattern and help to identify the different zones of the adult shell-forming tissue. The present study evaluates whether the expression of these enzymes is also a tool for the identification of the developing zones of the embryonic shell-forming tissue. Thus, we analyzed the temporal and spatial activity of the above-mentioned enzymes and of tartrate-resistant acid phosphatase in the shell forming tissues in Biomphalaria glabrata. Embryos of different age groups and adults were studied; alkaline phosphatase activity was seen in very young embryos in the shell field invagination prior to the secretion of any shell material, while peroxidase activity was present from the start of the periostracum production. Acid phosphatase, found in considerable amounts in yolk granules and albumen cells, appeared in the embryonic shell-forming tissue in relatively few Golgi stacks. Tartrate-resistant phosphatase was not present in embryos, but was found in adults in the same zone of the mantle edge as acid phosphatase. Using the enzymes as cell markers, the differentiation of the embryonic shell-forming tissue to the different zones of the adult mantle edge could clearly be followed.     

淡水贝类贝壳多层构造形成研究

对几种淡水贝（包括蚌、螺）进行形态及组织学观察，并通过实验方法重现贝壳三种物质，即：角质、棱柱质、珍珠质的生成过程，结果表明：外套膜外表皮细胞是由相同类型细胞组成，这些相同细胞在不同的作用条件下形成贝壳多层构造。     

The mechanism of shell elevation in Haliotis (Mollusca: Gastropoda) and a consideration of the evolution of the hydrostatic skeleton in Mollusca

Early in molluscan evolution, the development of a conical shell with shell or pedal retractor muscles led to the need of a mechanism for the extension of the foot or the raising of the shell. The forces generated during pedal retraction and extension have been studied in Haliotis midae , an easily obtainable and conveniently large archaeogastropod. In the mantle cavity, cephalopedal venous sinus and ventricle pressure pulses were observed during pedal retraction elicited by the shadow withdrawal reflex, but were never present during extension. However, pressure pulses were recorded in the proximal region of the columellar (or shell) muscle, both during retraction and pedal extension. Sections of this region of the muscle show a three dimensional network of muscle fibres, consisting of retractor fibres passing down to the foot and circumferential and radial fibres. Contraction of the two latter sets of fibres would bring about extension of the retractors, without the use of a discrete hydrostatic skeleton, and appears to be the principal mechanism of pedal extension. Similar muscular structures, here termed the muscular antagonistic system, have been observed in the columellar muscle of other gastropods and in the cephalopod mantle. In contrast, this system has not been observed in the proximal region of the pedal retractors of bivalves or scaphopods, for the pedal haemocoel, which allows muscular antagonism in the manner of a classical hydrostatic skeleton, has developed in association with the burrowing habit. The significance of the muscular antagonistic system in molluscan evolution is discussed.     

Epibiont habitation patterns and their implications for life habits and orientation among trigoniid bivalves

The bivalve superfamily Trigoniacea has persisted from the Late Paleozoic to the Recent. Late Jurassic and terminal Cretaceous mass extinctions decimated this once-dominant group in shallow marine facies; only a single genus with seven species survives today in the Austral Province. Trigoniacea retain a vestigial byssus and primitive but efficient schizodont dentition. They have been widely considered as infaunal bivalves, burrowing with a very large foot to shallow depths, with inhalant and exhalant apertures at or slightly below the sediment-water interface (SWI). Yet the Trigoniacea are poorly adapted for this life habit. The mantle in living species is unfused and non-siphonate, and some fossil Trigoniacea have permanent shell gapes over these apertures, enhancing the probability of sediment fouling of feeding and respiratory structures. Some living Neotrigonia , e.g., N. margaritacea , solve this problem by having a semi-infaunal life habit, with the inhalant and exhalant apertures elevated above the SWI and the zone of active sediment transport. Semi-infaunal species commonly have epibionts cohabiting the exposed posterior-posteroventral portion of the shell. Numerous well-preserved species of South American Mesozoic Trigoniacea have phototropically and geotropically oriented epibionts on co-attached valves, strongly suggesting a semi-infaunal life mode for at least some members of these taxa. These shell symbionts allow orientation of extinct trigoniid shells relative to the SWI during life, as well as analysis of their depth of burial. Careful analyses of the kinds, size classes, orientation, and dispersion of various epibionts on fossil Trigoniacea thus yield important new information on their life habits, and demonstrate that semi-infaunal life modes were far more common than previously supposed.     

Ontogeny of the Molluscan Shell Field: a Review

In the gastropod, scaphopod, lamellibranch, and cephalopod gastrulae a thickened portion of the posttrochal region is referred to as the embryonic shell field. It invaginates and gives rise to the shell gland. In species with an at least temporarily external shell, the shell gland evaginates and again forms a shell field. In lamellibranchs, the shell field grows into two halves connected by the ligament-secreting isthmus. In polyplacophorans plate fields are produced without invagination. Slugs and endocochleate cephalopods overgrow the embryonic shell field to form an internal shell sac. The calcified part of the shell is secreted by the flattened central region. The periostracum has its origin in the permanently thickened peripheral region of the shell field. In many forms, this region is depressed in a periostracal groove. If the shell is external, the central region of flattened cells, the mantle roof, along with the two or three marginal folds of the free mantle edge and, in species with internal shell, the shell sac are parts of the mantle. The shell field descends from the first somatoblasts. Either of 2 d or 2 c alone is able to form the shell field. There are arguments that the formation of the embryonic shell field is not autonomic, but induced by the entoderm during a period of contact. The shell gland and the shell field grow by mitotic cell divisions. Cells secreting organic material are highly prismatic, have a well developed ergastoplasm and large dictyosornes, and contain much peroxidase. The secretion of calcium manifests itself in very flat cells, rich in alkaline phosphatase and glycogen. The shell gland and the rosette of ectocochleate conchifera together are homologous to the proximal part of the shell sac in slugs and endocochleate cephalopods.     

A contribution to the ultrastructural knowledge of the pollen exine in subtribe Inulinae (Inuleae, Asteraceae)

To better understand the relationships within the Asteroideae and Inuleae, the structure of the pollen exine was investigated in seven genera and nine species of the subtribe Inulinae using LM, TEM and SEM. All taxa have a senecioid pattern of exine. The tectal complex consists of three main layers that differ in thickness and morphology: a tectum, a columellar layer, and a layer consisting of the basal region of the columellae. The absence or the vestigial condition of the foramina is considered as a plesiomorphy within the Asteroideae. All taxa have a complex apertural system that consists of an ecto-, a meso-, and an endoaperture. These apertures intersect respectively the tectal complex, the foot layer and the upper part of the endexine, and the inner layer of the endexine. A continuous transition among the different species of Inulinae was found for all the quantitative characters examined. This relative homogeneity of the pollen morphological characters enhances the naturality of the subtribe Inulinae.     

Morphological studies on the calcification process in the fresh-water mussel Amblema

Light microscopy, transmission electron microscopy, scanning electron microscopy, various histochemical procedures for the localization of mineral ions, and analytical electron microscopy have been used to investigate the mechanisms inherent at the mantle edge for shell formation and growth in Amblema plicata perplicata, Conrad. The multilayered periostracum, its component laminae formed from the epithelia lining either the periostracal groove or the outer mantle epithelium (of the periostracal cul de sac), appears to play the major regulatory and organizational role in the formation of the component mineralized layers of the shell. Thus, the inner layer of the periostracum traps and binds calcium and subsequently gives rise to matricial proteinaceous fibrils or lamellar extensions which serve as nucleation templates for the formation and orientation of the crystalline subunits (rhombs) in the forming nacreous layer. Simultaneously, the middle periostracal layer furnishes or provides the total ionic calcium pool and the matricial organization necessary for the production of the spherical subunits which pack the matricial ‘bags’ of the developing prismatic layer. The outer periostracal layer appears to be a supportive structure, possibly responsible for the mechanical deformations which occur in the other laminae of the periostracum. The functional differences in the various layers of the periostracum are related to peculiar morphological variables (foliations, vacuolizations, columns) inherent in the structure and course of this heterogeneous (morphologically and biochemically) unit. From this study, using the dynamic mantle edge as a morphological model system, we have been able to identify at least six interrelated events which culminate in the production of the mature mineralized shell layers (nacre, prisms) at the growing edge of this fresh-water mussel.     

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.     

THE FUNCTIONAL ANATOMY OF THE NERITACEAN LIMPET PHENACOLEPAS OMANENSIS BIGGS AND SOME COMPARISON WITH SEPTARIA

The shell of Phenacolepas and Seplaria is produced by a disproportionateexpansion of the last whorl of the typical neritid shell, withthe right and left columellar muscle an extension of that partarising from the shell and not the columella. The columellais a posterior ridge near the level of the protoconch. In contrastto zeugobranchs the mantle cavity is large, extending beneaththe viscera: the organization and disposition of the organsrelated to it are similar to those of Nerita. Poor development of pedal musculature of Phenacolepas may berelated to the sheltered environment. As in other limpets thereis a marginal pedal flange acting as a seal around the areaof low pressure which arises when the foot is used as a sucker.The internal anatomy reflects neritacean conservatism, but thereare characters, previously undescribed, which justify the separationof the genus in a separate family, the Phenacolepadidae: theelaboration of the mantle edge to resemble that of some zeugobranchs;the retention of an internal operculum; relatively sparse developmentof pedal musculature; the loss of the right auricle; erythrocytesin the blood; a vaginal papilla related to an ovipositor; theabsence of a crystal sac and modifications of the nervous systemto give more direct nervous pathways. (Received 12 September 1982;     

Swimming behavior and morphometry of the file shell Limaria fragilis

Swimming has evolved in only a few orders of Bivalves. In this study, the behavior, morphometry, and mechanics of swimming in the file shell Limaria fragilis were characterized and compared to the better understood scallops. Absolute swimming speed (cm sec^-1) increased with increasing shell height, although relative swimming speed (body lengths sec^-1) did not covary with shell height. The increase in absolute swimming speed was due to an increase in the distance covered during each valve clap as clap distance (cm clap^-1) also increased with shell height while clapping frequency (claps sec^-1) did not covary with animal size. Limaria fragilis displayed a variety of morphological changes related to size. Shell length was negatively allometric with shell height indicating the shell became proportionately slimmer in larger animals. Dry shell mass was negatively allometric with shell height, while both dry adductor muscle mass and dry mantle + tentacle mass were positively allometric. Autotomy of mantle tentacles significantly decreased clap distance by 13% without affecting clapping frequency or swimming speed.     

绣球亚科花粉形态的研究

借助光学显微镜和扫描电镜观察了绣球亚科９属３１种植物的花粉形态．本亚科的花粉多为近球形至近长球形,少数为扁球形．多具三孔沟。在叉叶蓝属观察到了极度缩短的花粉沟．花粉外壁表面纹饰为具颗粒状或棱形突起、孔穴状或明显网纹,表现出一定程度的变异．花粉形态表明黄山梅属在本亚科与其它属具一致性,冠盖藤属和钻地风属亲缘较近．绣球属花粉形态所表现出来的变异范围大致将其它几属的都包括在内了,表明绣球属可能在本亚科演化上处于一个中心位置。花粉形态结合外部宏观形态又表明绣球亚科各属之间没有十分明确的界限,存在着复杂的性状上的重叠．     

Three into two doesn't go: two-dimensional models of bird eggs, snail shells and plant roots

Three published two-dimensional analyses of three-dimensional structures are reinvestigated. (1) In their 1997 paper, Barta & Szekely sought shapes of bird eggs that maximized the size of eggs that packed under a circular brood patch. Inappropriately they measured egg size by cross-sectional area; maximizing volume implies different optima. Also their genetic algoridim mislocated their optima, probably because of too much mutation. (2) In this journal in 1985, Heath considered a section of an idealized snail shell; altering the overlap of adjacent whorls alters the ratio of shell material to volume enclosed. Heath located an overlap that minimized perimeter/area of the cross-section, which is different from minimizing surface-area/volume. I derive the surface area of a logarithmic helicospiral; some formulae used previously are slightly incorrect. Heath's alteration of overlap changed shell volume and aperture area; a more meaningful reanalysis keeps these characters constant. (3) In 1987, Fitter used a two-dimensional model of plant roots to show that topology, growth rate, and nutrient diffusion rate affect exploitation efficiency (area of nutrient depletion zone/volume of root). Recalculation and reanalysis show that only part of the effect of topology depends on overlap of depletion zones. I compare Fitter's model to coplanar root systems with three-dimensional depletion zones and then to fully three-dimensional branching. Finally I discuss further examples in which two-dimensional models could mislead.     

Columellar muscle of neogastropods: muscle attachment and the function of columellar folds

Malacologists often assume that ornamentation on snail shells is functional, and therefore adaptive. I conducted the first comprehensive test of the widely accepted hypothesis that columellar folds, a type of internal ornamentation, enhance the performance of the columellar muscle, which attaches the snail to its shell. Careful dissections of live, non-relaxed specimens reveal that the physical attachment between the columellar muscle and the columella is not restricted to a small, circular patch located deep within the shell. Instead, the attachment is long and narrow, extending approximately a full whorl along the length of the columella. I developed a novel technique for preparing three-dimensional reconstructions from photographs documenting the dissections. These reconstructions were then used to measure four parameters that describe the muscle: (1) the surface area of the physical attachment between the muscle and columella, (2) the total contact area between the muscle and the columella, (3) the depth of attachment, and (4) the length of attachment. None of these parameters differed significantly between species with and without folds. In light of the biomechanics of muscular hydrostats, values of the first parameter indicate that columellar folds probably do not guide the columellar muscle as the animal moves in and out of its shell. Values of the other parameters indicate that columellar folds neither increase an animal's ability to maneuver its shell nor facilitate deeper withdrawal. These results, and the fact that folds have evolved convergently several times, might indicate that folds are an easily evolvable solution to many functional problems, none of which are currently understood.     

江西弯螺属一新种记述(肺螺亚纲,柄眼目,扭轴蜗牛科)

在整理江西地区陆生贝类标本时,经比对鉴定发现1新种,龙潭弯螺Sinoennea longtanensis sp.nov.,隶属肺螺亚纲、柄眼目、扭轴蜗牛科、弯螺属。对新种形态特征、栖息环境作了记述,并与其近似种进行讨论。     

Swimming behavior and morphometry of the file shell Limaria fragilis

Swimming has evolved in only a few orders of Bivalves. In this study, the behavior, morphometry, and mechanics of swimming in the file shell Limaria fragilis were characterized and compared to the better understood scallops. Absolute swimming speed (cm?sec^?1) increased with increasing shell height, although relative swimming speed (body lengths?sec^?1) did not covary with shell height. The increase in absolute swimming speed was due to an increase in the distance covered during each valve clap as clap distance (cm?clap^?1) also increased with shell height while clapping frequency (claps?sec^?1) did not covary with animal size. Limaria fragilis displayed a variety of morphological changes related to size. Shell length was negatively allometric with shell height indicating the shell became proportionately slimmer in larger animals. Dry shell mass was negatively allometric with shell height, while both dry adductor muscle mass and dry mantle + tentacle mass were positively allometric. Autotomy of mantle tentacles significantly decreased clap distance by 13% without affecting clapping frequency or swimming speed.     

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.     

Shell repair process in the green ormer Haliotis tuberculata: a histological and microstructural study

In the present paper, juvenile and adult shells of the green ormer Haliotis tuberculata ('Oreille de Saint-Pierre') were perforated in a zone close to the shell edge and the shell repair process was followed at two levels: (1) by observing the histology of the calcifying mantle in the repair zone and (2) by analyzing with SEM the microstructure of the shell repair zone. Histological data clearly show the presence of calcium carbonate granules into the connective tissues, but not in the epithelial cells. This suggests that calcium carbonate granules are synthesized by sub-epithelial cells and actively transported through the epithelium to the repair zone, via a process which may be similar to that described by Mount et al. [Mount, A.S., Wheeler, A.P., Paradkar, R.P., Snider, D., 2004. Hemocyte-mediated shell mineralization in the eastern oyster. Science 304, 297-300]. Furthermore, SEM observations show that the repair zone exhibits different stratified microstructures (spherulitic, thin prismatic, blocklike, sub-nacreous, nacreous, foliated-like), some of which are not continuous (i.e. lenticular) along the repair zone. This suggests a complex secreting regime of the calcifying mantle and an elaborate geometry of the epithelium involved in shell repair.     

Changes in the specific cytochemical properties of rat decidual cells during differentiation

The antimesometrial part of rat's decidua of 8-9 day of gestation was divided into three parts: epithelial, transition and basal zones. Cells of either zone have their own morphological and cytochemical features. Cells of the epithelial zone are characterized by synthesis of tissue specific antigens and fat (fatty acids). Cells of the transition zone synthesize glycogen. Acid mucopolysaccharides are located in the basal zone only. Decidual cells do not synthesize cholesterol. Con A receptors are localized on the surface of cells of the basal and transition zones and disappear from the surface of epithelial zone cells. It is concluded that differentiation of large decidual cells of the antimesometrial part of rat's decidua are accompanied by a significant change in cytochemical features of cell precursors the synthesis of acid mucopolysaccharides and glycogen stops, while tissue specific antigens and fat (fatty acids) start their syntheses, Con A receptor disappear.     

An unusual trochiform gastropod from the upper Ordovician of Sweden

An unusual trochiform gastropod, Semizona bella gen. et sp. nov., is described from the Boda Limestone carbonate mounds (upper Ordovician, Ashgill) of central Sweden. A second species, S. glindmeyeri (Rohr, 1996), is recognised from the Ordovician (Whiterockian) of Nevada. The shell shape and the strongly prosocline tangential aperture of Semizona suggest that balancing of the shell on the head-foot mass was accomplished by tilting of the axis of coiling of the shell to about 65 degrees with 10–30 degrees of regulatory detorsion. The rounded aperture allowed straight contraction of the retractor muscles, suggesting clamping behaviour often associated with a sedate, grazing snail. This agrees with the environmental setting, which suggests a hard substrate with rich microbial growth. Besides clamping, the subsutural nodes and thick shell were probably effective against predation; repaired injuries indicate failed predatory attacks. Semizona shows morphological similarities with some pleurotomariin vetigastropods, and with the family Pseudophoridae Miller, 1889.