Mechanical constraints on aperture form in gastropods

Gastropod apertures reflect expanded states of their mantle edge under variable boundary conditions. The apertures are divided into two groups: apertures without distinct overlap zones (e.g., whorl overlap) and those with overlap zones. Each group follows a unique morphological rule. Apertures without overlap zones are generally circular in outline. Apertures with overlap zones are either elongated perpendicularly to the overlap zone or inflated abapically. Moreover, the latter abapically inflated apertures are generally accompanied by a straight section anterior to the overlap zone along the columellar axis (columellar part). Numerical analysis of an elastic double membrane tube whose main frame simulates the gastropod mantle indicates that these morphological rules are the products of mantle edge expansion under the condition that the head-foot mass presses against the mantle edge in both the overlap zone and the columellar part. The mantle edge in these two zones is thus in a completely or partly fixed boundary condition at the moment of shell growth. The rest of the mantle edge is free to expand either symmetrically or asymmetrically. It is hypothesized that the head-foot mass is a driving force for regulating the pattern of shell coiling and apertural shape.     

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.     

How the 'forceps' of Lithophaga aristata (Bivalvia: Mytiloidea) are formed

The calcareous substratum borer, Lithopaga aristata Dillwyn, 1817, secretes posterior incrustations that take the form of interlocking 'forceps'. These are secreted, initially, as asymmetrical ridges by similarly asymmetrical dorsal and ventral glands in the left and right middle folds of the posterior mantle lobes. In the adult, the secretions are more uniformly spread over the posterior shell margins, concealing the juvenile ridges.
Opening and closing of the valves smooths the outer surfaces of the 'forceps' against the burrow wall, which also comes to be lined with calcium carbonate that is reciprocally smoothed, so that they occlude the borehole more effectively. Extension and retraction of the siphons probably smooths the inner surfaces of the 'forceps' which are sharpened by abrasion, one against the other, during valve opening and closing. The pair of inequilateral spikes so produced, project from the burrow aperture, occluding it, but probably, more importantly, distancing aperturally attacking predators from the true posterior shell margin. Interlocking of the 'forceps' and their sharpness may further deter would-be predators.     

对黑蜗牛生物学特性的初步研究

利用对比观察法,研究黑蜗牛的形态结构、生殖特点、死亡原因,发现黑蜗牛的螺旋贝壳短,只有壳顶和两个螺层,体螺层不能完全容纳软体,螺旋贝壳中碳酸钙含量约为60．3％,易软化。体背上生长着外套膜和外壳膜,外套膜在螺旋贝壳内包裹着内脏囊,外壳膜覆盖在螺旋贝壳的表面,能向不同方向伸展。壳口处的外壳膜上有排泄孔,排泄孔与外套膜上的呼吸孔连通,具有呼吸、排泄粪便和排泄尿液的多种功能。证明黑蜗牛是一种新的软体动物——最原始的蜗牛,也是陆生软体动物贝壳退化成内壳的过渡物种。隶属于琥珀蜗牛科（Zonitidae）,夏威夷琥珀蜗牛属（Hawaiia）,沂水琥珀蜗牛种（Hawaiia yishuiusculeLi）。     

Mechanics of sculpture formation in Magadiceramus? rangatira rangatira (Inoceramidae,Bivalvia) from the Upper Cretaceous of New Zealand

The surface sculpture of the inoceramid bivalve Magadiceramus? rangatira rangatira consists of commarginal ribs and curious, transverse wrinkles. The wrinkles typically are at a high angle or orthogonal to the shell margin (‘antimarginal’) and thus differ from purely radial structures. They show features of distribution and morphology that reveal them to be products of margin‐parallel compression of the shell‐secreting mantle and its adjacent, flexible, uncalcified periostracum. The interaction of wrinkles with commarginal ribs indicates that the ribs also formed as folds of the mantle margin. During growth, commarginal folding caused withdrawal of the entire mantle margin towards the umbo, with a consequent reduction in perimeter length. Measurement of specimens indicates that fabrication of the commarginal ribs resulted in the magnitude of commarginal shortening that is required for the formation of transverse wrinkles. We infer that early in ontogeny, at the first development of these sculptures, the wrinkles resulted entirely from mantle contraction and resultant commarginal shortening. With subsequent growth, total wrinkling included a component of ‘pre‐wrinkling’ inherited from the preceding growth stage; the contribution of pre‐wrinkling to total wrinkling increased with shell size. The proposed mechanical model is two‐phase. First, the transversely corrugated (pre‐wrinkled) mantle and periostracum advanced and secreted a slightly concave growth increment. Secondly, the mantle subsequently contracted to create a commarginal rib and increase the number and amplitude of transverse wrinkles. This model is consistent with a homogeneous mantle lacking any differentiated and specialized rib‐constructing segments.     

Nassarius kraussianus shell beads from Blombos Cave: evidence for symbolic behaviour in the Middle Stone Age

Since 1991, excavations at Blombos Cave have yielded a well-preserved sample of faunal and cultural material in Middle Stone Age (MSA) levels. The uppermost MSA phase, M1, is dated to c. 75 ka by optically stimulated luminescence (OSL) and thermoluminescence, and the middle M2 phase to a provisional c. 78 ka. Artefacts unusual in a MSA context from these phases include bifacial points, bone tools, engraved ochre and engraved bone. In this paper, we describe forty-one marine tick shell beads recovered from these MSA phases and tick shell beads from Later Stone Age (LSA) levels at Blombos Cave and the Die Kelders site. Thirty-nine shell beads come from the upper M1 phase and two from M2. Morphometric, taphonomic and microscopic analysis of modern assemblages of living and dead tick shell demonstrate that the presence of perforated Nassarius kraussianus shells in the Blombos MSA levels cannot be due to natural processes or accidental transport by humans. The types of perforation seen on the MSA shells are absent on modern accumulations of dead shells and not attributable to post-depositional damage. Their location, size, and microscopic features are similar to those obtained experimentally by piercing the shell wall, through the aperture, with a sharp bone point. Use-wear, recorded on the perforation edge, the outer lip, and the parietal wall of the aperture indicates the shells having being strung and worn. MSA shell beads differ significantly in size, perforation type, wear pattern and shade compared to LSA beads and this eliminates the possibility of mixing across respective levels. Thirty-one beads were found in four groups of five to twelve beads, each group being recovered in a single square or in two adjacent sub-squares during a single excavation day. Within a group, shells display a similar shade, use-wear pattern and perforation size suggesting their provenance from the same beadwork item, lost or disposed during a single event. The likely symbolic significance of these finds suggests levels of cognitively modern behaviour not previously associated with MSA people.     

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.     

Caedichnus,a New Ichnogenus Representing Predatory Attack on the Gastropod Shell Aperture

Predatory traces, in which the tracemaker has damaged the prey animal's skeleton to kill and consume it, have a deep fossil history and have received much scientific attention. Several types of predatory traces have been assigned to ichnotaxa, but one of the most studied predatory traces, the wedge-shaped excision produced as a result of attacks mainly by crustaceans on the apertures of gastropod shells, has yet to be described as an ichnotaxon. We propose the ichnogenus Caedichnus to describe the shell damage produced by aperture peeling behavior. Caedichnus is produced by predators that are unable to crush their prey's shells outright. Depending on the predator's peeling ability and the prey's withdrawal depth within the shell, the trace can extend through several whorls of the shell. Aperture peel attacks may fail, allowing such damage to be repaired by surviving gastropods. Thus, the types of attacks that produce Caedichnus may exert selective pressure on prey to evolve better-defended shells (in the case of gastropods) or to inhabit better-defended shells (in the case of hermit crabs). The identification of these trace fossils will enhance our understanding of how predation influences the morphological, and even behavioral, evolution of prey organisms.     

The Micro/Nanostructure Characteristics and the Mechanical Properties of Hemifusus tuba Conch Shell

<正> The mollusk shell mobilizes calcium from environment for skeletal mineralization.This occurs through synthesizing solidsin solution in the presence of organic molecules of specific interior regions of the conch shell.The ultrastructure and microhardnessof the Hemifusus tuba conch shell living in the Huang/Bo sea area are investigated in the paper.It is shown that thecomposition and microstructure of the mollusk shell vary in different positions.The prodissoconch shell consists only of aragonitewith the crossed-lamellar microstructure.While the spiral shell and the body shell of the Hemifusus tuba conch shell arecomposed of one calcite layer and several aragonite layers.The calcite layer consists of cylindrical grains,but the aragonitelayers are crossed-lamellar ultrastructure at three size scales.The minimum structure size (the third-order lamella) is at about20 nm - 80 nm.The margin of shell aperture is only composed of calcite with cylindrical grains.This natural optimization of theshell microstructure is intimately due to the growth of the Organic matrix.At different positions the microhardness of molluscshell is different due to different crystal structures and crystal arrangements.The growth process of shells allows a constantrenewal of the material,thus enabling their functional adaptation to external environments.     

Crab shell-crushing predation and gastropod architectural defense

The shell-breaking behavior of the crabs Ozius verreauxii Saussure 1853 and Eriphia squamata, Stimpson 1859 from the Bay of Panama is described. The master claws of both these crabs are well designed for breaking shells. Small shells, relative to the size of a crab predator, are crushed by progressively breaking off larger segments of a shell's apex, while larger shells are peeled by inserting a large dactyl molar into the aperture of a shell and progressively chipping away the lip of the shell.

Heavy gastropod shells are shown to be less vulnerable to crab predators than lighter shells, and narrow shell apertures and axial shell sculpture are demonstrated to be architectural features that deter crab predation. The incidence of architectural features which deter crab predation appears to be higher for smaller gastropod species than for larger gastropods which are too large for most crab predators. Large fish predators prey upon both gastropods and shell-crushing crabs. To avoid fish predators, both these prey groups seek refuge under rocks when covered by the tide. Fish predation thus appears to enforce a close sympatry between smaller gastropods and their crab predators.     

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。     

Paleobiological implications of shell repair in recent marine gastropods from the northern Gulf of California

Shell repair frequencies in eleven species of Recent gastropods from the northern Gulf of California vary with habitat, shell morphology and intensity of durophagous predation. Squat shells with large apertures tend to have high repair frequencies (0.25–0.50). Shell thickness at the aperture and shell size are not correlated with frequency of repair. Significant intraspecific variation in repair frequency exists between habitats. Samples from rocky habitats have statistically higher repair frequencies than samples of the same species from sandy habitats. However, habitat‐related variation between species is not apparent.

Trends in co‐evolution of gastropods and their durophagous predators are based on the indirect evidence of shell repair frequencies through time. Variation in repair frequency due to environmental and morphological factors may obscure predator‐related temporal trends in repair frequency.     

A possible mechanism for the formation of annual growth lines in bivalve shells

We report a unique shell margin that differed from the usual shell structure of Pinctada fucata. We observed empty organic envelopes in the prismatic layer and the formation of the nacreous layer in the shell margin. All the characteristics of the growing margin indicated that the shell was growing rapidly. To explain this anomaly, we propose the concept of “jumping development”. During jumping development, the center of growth in the bivalve shell jumps forward over a short time interval when the position of the mantle changes. Jumping development explains the unusual structure of the anomalous shell and the development of annual growth lines in typical shells. Annual growth lines are the result of a discontinuity in the shell microstructure induced by jumping development.     

Morphogenetic regulation in the shells of bivalves and brachiopods: evidence from the geometry of the spiral

Ackerly. S. C. 1992 07 15: Morphogenetic regulation in the shells of bivalves and brachiopods: evidence from the geometry of the spiral.
Analyses of the spiral geometry in shells of the mollusc Pecten maximus and the brachiopod Terebratulina retusa indicate a relative reduction in morphological variability within the population during growth. The spiral, as measured by a model of exponential radial expansion, tends to converge on a particular adult form, irrespective of irregularities during early growth phases. The recurrence of this pattern of variability in populations from two separate phyla (molluscs and brachiopods) suggests a common mechanism controlling shell form. Brachiopoda. Mollusca, coiling, spiral. morphogenetic regulation, growth .     

Trade-offs between force and fit: extreme morphologies associated with feeding behavior in carabid beetles

We explored how functional trade-offs in resource handling strategies are associated with the divergent morphology of predators. The malacophagous carabid Damaster blaptoides shows two extreme morphologies in the forebody; there is an elongate small-headed type and a stout large-headed type. A feeding experiment showed that the small-headed type obtained a high feeding performance on snails with a thick shell and a large aperture by penetrating the shell with its head. In contrast, the large-headed type showed a high feeding performance on snails that had a thin shell and a small aperture, and they ate these prey by crushing the shell. The large-headed, strong-jawed beetles are efficient at shell crushing but are ineffective at shell entry; the large mandibles and musculature that allow for shell crushing make the beetle's head too wide to penetrate shell apertures. On the other hand, small-headed, weak-jawed beetles crush poorly but can reach into shells for direct predation on snail bodies. These findings are hypothesized to be functional trade-offs between force and fit due to morphological constraints. This trade-off would be a primary mechanism affecting both resource handling ability in animals and phenotypic diversity in predators and prey.     

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.     

Structure and formation of the adventitious tube of the Japanese watering-pot shell Stirpulina ramosa (Bivalvia, Anomalodesmata, Clavagellidae) and a comparison with that of the Penicillidae

Abstract. Stirpulina ramosa is the only extant endobenthic representative of the Clavagellidae and is restricted to the waters of Japan. A single intact adventitious tube of this species has been obtained and its structure is described. The right valve is 16 mm long and located within the adventitious tube. It has an opisthodetic ligament located on resilifers. There are anterior and posterior adductor muscle scars, a thick pallial line, and pallial and pedal gape (right valve only) sinuses. The left shell valve is but 9 mm long and is united into the fabric of the adventitious tube via the intermediary of a shelly saddle. Internally, only the anterior adductor muscle scar and a small element of the pallial line scar are identifiable on the left valve. The posterior adductor and the rest of the pallial line scar (including a pallial sinus) are, remarkably, located on the adventitious tube beyond the shell valve margin. The adventitious tube of S. ramosa is formed in a manner wholly dissimilar from that of Brechites vaginiferus (Penicillidae). In B. vaginiferus, the tube is secreted as a single entity from the general outer mantle surface, including the siphons, covering the body. As a consequence, both shell valves are incorporated into the structure of the tube and the watering pot is bilaterally symmetrical. In S. ramosa, the tube and watering pot are secreted from the mantle margin and surface surrounding and extending from the left shell valve, so that only the left valve is incorporated into its structure. A dorsally derived mantle element is progressively extended over to the right side of the body, meeting a ventrally derived counterpart that passes beneath it, forming a pleat in the calcareous structure of the right side of the tube that they secrete. This pleat extends into the complex of watering‐pot tubules and forms the pedal gape. The watering pot is thus Ω shaped. The ventrally derived mantle element forms a sinusoidal crest on the right‐hand base of the watering pot, creating a pedal gape sinus scar on the right valve. The Clavagellidae radiated widely in the Mesozoic, leaving behind a rich fossil record for Stirpulina. Only S. ramosa, however, has survived until the present. In contrast, the Cenozoic Penicillidae has a poor fossil record, but there is a rich variety of extant endobenthic watering‐pot shells. It has been argued hitherto that the two families represent a remarkable example of convergent evolution. In view of the success of the Penicillidae and thus the endobenthic, tube‐dwelling lifestyle, however, it is hard to understand why Stirpulina has largely died out—even S. ramosa being known by but one or two specimens. A study of the anatomy of S. ramosa might one day answer this question.     

Chamber growth in ammonites inferred from colour markings and naturally etched surfaces of Cretaceous vascoceratids from Nigeria

Cretaceous Vascoceras and Jurassic Lytoceras show colour markings and etched surfaces representing original organic membranes between the septa. The main difference between the formation of ammonite and Nautilus chambers involved the continuous secretion of a gelatinous cameral liquid to support the ammonite mantle when it moved forward. The gel containing cyclically secreted membranes. here named pseudosepta, resembled the intra-cameral structures of the cuttlebone in Sepia. Pseudoscpta are attached to the shell wall in pseudosutures (Pseudoloben) which are particularly visible in the saddles of the septal suture and tend to mimic them. Their shape suggests reconstruction of posterior mantle shape during translocation. Drag-bands (Schleppstreifen) are spiral markings formed by the overlapping pseudosepta along the axial traces of the foliole folds. The chamber of ammonites was formed by a locally muscular mantle in a tripartite cycle: (1) the mantle initially remained attached to the saddles of the completed septal suture while muscular tissue within the umbilical lobes was contracted and rapidly reattached to the side of the lateral saddles; (2) the whole mantle subsequently crept forward by secreting a gelatinous matrix which contained telescoped membranes, with an adhesive function on pseudolobc flanks; (3) the mantle almost ceased to move within the sites of future lobules, but expanded and crept on before forming the mural and 'gutter' ridges of the septum. □ Ammonites, chamber growth, vascoceratids, LYTOCERAS, Nigeria.     

The functional significance of aperture form in gastropods

Aperture form of marine prosobranch gastropods has evolved under the influence of a number of different selective forces, including: generation of shell form; protection from predation; accommodation of the foot during clamping behavior: and accommodation of water currents in and out of the mantle cavity. Aperture form correlates positively with foot shape in most gastropods and foot shape, in turn, correlates moderately well with substrate preference. Almost all gastropods that have non-round apertures elongate the aperture parallel to the foot so that water currenth tend to flow anteriorly to posteriorly. Fresh-water pulmonates have responded to somewhat different stresses. They exhibit clamping behavior and thus show correspondence between foot shape and aperture shape. They show less apertural strengthening as crab (or crayfish) predation is less of a factor and presumably because calcium carbonate is less available. They also lack anterior-posterior apertural elongation due to the absence of water currents through their mantle cavity. Due to the absence of mantle cavity water currents and clamping behavior, terrestrial gastropods do not show the apertural modifications associated with these two factors. In addition. few adaptations of apertural form are present to resist predation. Instead, many of the apertural modifications of terrestrial pulmonates seem to be concer-ned with the problems of water loss during estivation.     

Assessing cockle shells (Anadara granosa) for reconstruction subdaily environmental parameters: Implication for paleoclimate studies

This study aims to investigate the potential of cockle shells as an environmental recorder, examining the environmental factors controlling the shell growth of the intertidal Anadara granosa from west coast of Malaysia. Subdaily environmental factors were recorded from December 2011 to November 2012. A total of 600 individuals were collected on a monthly basis and the shells sectioned from umbo to ventral margin, polished, etched and photographed under a light microscope to observe microgrowth bands and increments. Comparison of correlation matrix between mean increment width and each environmental factor indicated that shell growth had the highest positive correlation with seawater temperature (+0.72) and weak positive correlation with salinity (+0.53). Multiple regression analysis was used to assess independent associations between shell mean increment width and environmental parameters. Study model showed that 60.8% of the variation in shell growth could be explained by temperature, salinity, rainfall and tidal change. Individually, temperature and salinity made the greatest unique contribution to explain shell growth, respectively (p < 0.01). Laboratory results showed shell growth was in a linear trend to optimum temperature and salinity. These findings provide a basis for the interpretation of the temporal changes in shell microgrowth patterns in terms of environmental conditions of cockle shells.