Barbatia mytiloides and the evolution of shell torsion in arcid pelecypods

Barbatia mytiloidrs (Brocchi) (Arcidae, Arcinae) from the Neogene of northwestern Italy shows a variable but significant amount of dextral shell torsion about the hinge axis. This phenomenon represents a morphological and functional convergence with the sinistrally torted ark Trisidos . Polyphyletic evolution of shell torsion in mud-dwelling, semi-infaunal descendants of epibyssate Barbatia is probably due to ventilation problems that forced these forms to lie with the posterior commissure horizontal. Depending on which valve happened to lie lowermost in the non-torted reclining or semi-infaunal ancestors, shell torsion became established in dextral or sinistral direction.     

Functional morphology and autecology of Pseudoptera (Bakevelliid bivalves,upper cretaceous of Portugal)

Pseudoptera sp. nov. is sinistrally twisted about the hinge axis, i.e., in the opposite direction to that of Hoernesia and the other twisted bakevelliids. Field observations confirm the semi-infaunal pleurothetic life habit predicted from the shell morphology. The twisted Bakevelliidae are unlikely to have evolved from soft-bottom forms. A transition from an epibyssate habit on solid substrates to an endobyssate, semi-infaunal life habit probably triggered the evolution of shell torsion in this family, as in the Arcidae and Mytilidae.     

Torsion in Patella caerulea (Mollusca, Patellogastropoda): ontogenetic process, timing, and mechanisms

Abstract. Torsion is a process in gastropod ontogenesis where the visceral body portion rotates 180° relative to the head/foot region. We investigated this process in the limpet Patella caerulea by using light microscopy of living larvae, as well as scanning electron microscopy (SEM) of larvae fixed during the torsion process. The completion of the 180° twist takes considerably less time in larvae of Patella caerulea than previously described for other basal gastropod species. At a rearing temperature of 20–22°C, individuals complete ontogenetic torsion in ?2 h. Furthermore, the whole process is monophasic, i.e., carried out at a constant speed, without any evidence of distinct ‘fast” or ‘slow” phases. Both larval shell muscles—the main and the accessory larval retractor—are already fully contractile before the onset of torsion. During the torsion process both retractors perform cramp‐like contractions at ～30 s intervals, which are followed by hydraulic movements of the foot. However, retraction into the embryonic shell occurs only after torsion is completed. The formation of the larval operculum is entirely in‐dependent from ontogenetic torsion and starts before the onset of rotation, as does the mineralization of the embryonic shell. The reported variability regarding the timing (mono‐ versus biphasic; duration) of torsion in basal gastropod species precludes any attempt to interpret these data phylogenetically. The present findings indicate that the torsion process in Patella caerulea, and probably generally in basal gastropods, is primarily caused by contraction of the larval shell muscles in combination with hydraulic activities. In contrast, the adult shell musculature, which is independently formed after torsion is completed, does not contribute to ontogenetic torsion in any way. Thus, fossil data relying on muscle scars of adult shell muscles alone appear inappropriate to prove torted or untorted conditions in early Paleozoic univalved molluses. Therefore, we argue that paleontological studies dealing with gastropod phylogeny require data other than those based on fossilized attachment sites of adult shell muscles.     

Beiträge zur Torsion und Frühevolution der Gastropoden

Contributions to torsion and early evolution of the gastropods The phylogenetical model, which is presented here, is directed by trying to explain torsion of gastropods out of biomechanical conditions. It is based on morphological and functional reflexions upon the evolution of an ancestral mollusc out of an annelid-like segmented coelomate and further evolution of this to a monoplacophoran. Starting point of gastropod evolution is a high-arched monoplacophoran with serial pairs of dorsoventral muscles. There are always alternating one pair of vertically and one pair of obliquely running, crossed dorsoventral muscles. Evolution of higher arches causes a reduction of many dorsoventral muscles and a magnification of the pallial cavity. This gives place for a few pairs of massive gills instead of many serrated little monoplacophoran gills. A second consequence is a waist (“Taille”) between cephalopodium and visceral hump. It functions as an axis for at first little, with further muscle reductions greater torsional motions of the shell and the visceral hump. At last there exists only one pair of oblique and crossed dorsoventral muscles. This situation forces a fixation of the torted position, because it causes less energy consumption. There is also a lot of other advantages for the “torted gastropod”. The construction of a bilateral ancestor is the base for the evolution of recent asymmetric snails as a bundle of different phylogenetic lines.     

Morphological perspective on the classification and evolution of Recent Pterioidea (Mollusca: Bivalvia)

The evolutionary relationships of the Recent Pterioidea are inferred from a phylogenetic analysis of representatives of all pterioidean genera based on original observations of anatomy and shell morphology, and an extensive survey of bivalve literature. The well-resolved cladogram supports monophyly for the superfamily, but renders all but one family (the monotypic Pulvinitidae) polyphyletic. In addition, these results reveal a considerable level of convergence and parallelisms through the Pterioidea. The branching order of pterioid genera in the morphological analysis is largely corroborated by the sequence of their appearance in the fossil record. The palaeontological evidence provides important information on dating lineage splitting events and transitional taxa. The proposed phylogeny integrates the cladistic analysis of the Recent Pterioidea with the fossil record and suggests that the crown-group pterioideans probably originated in the Triassic from the Bakevelliidae, an extinct paraphyletic stem group from which the Ostreoidea are also ultimately derived.  © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 148 , 253–312.     

Spermatic cord torsion, reactive oxygen and nitrogen species and ischemia-reperfusion injury

Mammalian testes are highly sensitive to oxidative free radical damage. Acute scrotum is a clinical syndrome mainly caused by torsion of the spermatic cord that constitutes a surgical emergence affecting newborns, children and adolescents. This syndrome often leads to infertility of the ipsilateral (torted) and contralateral (not torted) testis, an outcome that makes surgical intervention mandatory. There is a controversy involving the effects of ischemia and reperfusion on ipsilateral and contralateral testes after unilateral torsion and detorsion of the spermatic cord. Conflicting reports have led to two distinct and opposite recommendations regarding surgical intervention: detortion and preservation of the ipsilateral testis, or ipsilateral orchiectomy to preserve contralateral fertility. Early detortion surgery in humans preserves fertility, but after prolonged torsion periods followed by preservation of the ipsilateral fertility of both testis is jeopardized. Lowered contralateral blood flow after unilateral testicular torsion is associated with reactive oxygen species (ROS) overgeneration and therefore with the corresponding tissue damage. Reperfusion time appears to be determinant of contralateral testes damage due to the consequent oxidative insult that accompanies the rise in ROS following ischemia-reperfusion. Nevertheless, more investigations on the molecular mechanisms and the antioxidant status in testis are necessary to ascertain the contribution of ROS to the tissue damage produced by spermatic cord torsion in experimental animals and humans.     

Modelling various sculptures in the Cretaceous bivalve Inoceramus hobetsensis

The geometry of the external shell sculpture in the Late Cretaceous inoceramid bivalve Inoceramus hobetsensis Nagao & Matsumoto, 1939 was studied both empirically and theoretically. A large sample, collected from the Upper Cretaceous of Hokkaido, Japan, shows remarkably high intraspecific variation in the shell sculptural pattern. Quasi-commarginal ribs, slightly oblique to the external growth increments, occur in some specimens. These sculptures are commonly irregular in strength and spacing, and their features are successfully modelled by computer simulations when the commarginal ribs are superposed with nearly concentric divaricate rib. Computer models indicate that the divergent sculpture element, often found in other inoceramids, was present throughout the evolution of I. hobetsensis and was developing in the evolutionary lineage from I. hobetsensis nonsulcatus to I. hobetsensis hobetsensis, although it was only weakly expressed. The results also suggest that some apparently distinct sculptural patterns of I. hobetsensis are the result of minor changes in the morphogenetic program.     

Ontogenetic torsion in two basal gastropods occurs without shell attachments for larval retractor muscles

Results of this study on two species of vetigastropods contradict the long-standing hypothesis, originally proposed by Garstang (1929), that the larval retractor muscles power the morphogenetic movement of ontogenetic torsion in all basal gastropods. In the trochid Calliostoma ligatum and the keyhole limpet Diodora aspera, the main and accessory larval retractor muscles failed to establish attachments onto the protoconch (larval shell) when the antibiotics streptomycin sulfate and penicillin G were added to cultures soon after fertilization. Defects in protoconch mineralization were also observed. Despite these abnormalities, developing larvae of these species accomplished complete or almost complete ontogenetic torsion, a process in which the head and foot rotate by 180 degrees relative to the protoconch and visceral mass. Analysis by using phalloidin-fluorophore conjugate and transmission electron microscopy showed that myofilaments differentiated within myocytes of the larval retractor muscles and adherens-like junctions formed between muscle and mantle epithelial cells in both normal and abnormal larvae. However, in abnormal larvae, apical microvilli of mantle cells that were connected to the base of the larval retractor muscles failed to associate with an extracellular matrix that normally anchors the microvilli to the mineralized protoconch. If morphogenesis among extant, basal gastropods preserves the original developmental alteration that created gastropod torsion, as proposed by Garstang (1929), then the alteration involved something other than the larval retractor muscles. Alternatively, the developmental process of torsion has evolved subsequent to its origin in at least some basal gastropod clades so that the original alteration is no longer preserved in these clades.     

Some morphological adaptations in freshwater bivalves

Savazzi, E. & Yao, P. 1992 04 15: Some morphological adaptations in freshwater bivalves. Lethaia , Vol. 25, pp. 195–209. Oslo. ISSN 0024–1164.
Several freshwater bivalves possess peculiar shell morphologies. An extension of the postero-dorsal shell margins above the hinge line evolved convergently in several unionids. This extension supplements the opening momentum of the ligament, but must be broken off periodically in order to allow further shell growth. Arconaia and Cuneopsis have evolved twisted commissure planes, comparable to those found in unrelated marine bivalves. In marine forms, byssus is believed to have played a fundamental role in the evolution of shell torsion. However, the twisted Unionidae do not possess a byssus in the adult stage, thus forcing us to re-evaluate our ideas on the adaptive value and evolution of shell torsion. Solenaia oleivora is apparently incapable of reburrowing and of retracting its foot within the shell. The foot may be functional as an anchor, and is perhaps involved in chemosynthesis by storing sulphur extracted as sulphide from the surrounding sediment. Other adaptations of freshwater bivalves include selective thickening of portions of the shell that enhance its stability, permanent anterior and posterior gapes, and oyster-like morphologies and shell structures. * Functional morphology, constructional morphology, burrowing, shell torsion, Mollusca, Bivalvia, Unionacea, Recent, Quaternary, People's Republic of China .     

Localization of serotonin and fmrfamide in the bivalve molluscMytilis edulis at early stages of its development

Location and time of appearance of serotonin and FM RFamide in ontogenesis of the bivalve mollusc Mytilus adulis L. has been studied. Serotonin first appears at a transfer from the last trochophore stage to the early veliger stage, 30–32 h after fertilization and is detected in a large cell located immediately at the base of the apical plume. FMRFamide appears 14–15 h after fertilization, during the transfer from the conchostoma stage to the early trochophore stage before the turning out of the shell gland and is a marker of the future shell site. It is located as a transverse stretch on the back of the larva in the area of the future shell. After the turning out of the shell gland the stretch is markedly shortened, while its processes approach the larval surface and are located under the shell. A suggestion is made about a morphogenetic function of FMRFamide.     

Ecological aspects of a silicified bivalve fauna from the Silurian of Gotland

The silicified Wenlockian (Silurian) bivalve fauna from MÖllbos, Gotland, is part of a life assemblage. The vast number of shells show unusual phenomena, e.g. shell repair, pearl and tumour formation, etc. A number of shells contain epibionts and bored, round holes. Presumptive predators of the bivalve community are discussed. Size-frequency distribution of the two most abundant species possibly reflects age classes. The fauna, comprising eleven species, is dominated by deposit-feeders (90 %). They exhibit niche diversification, including at least three different feeding levels within the sediment.     

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 Arcoidea (Mollusca: Bivalvia): a review of the current phenetic-based systematics

The diversity and adaptive radiations of modern Arcoidea, here considered to contain the families Arcidae, Noetiidae, Cucullaeidae, and Glycymerididae, are reviewed. Most fall into either epibyssate or endobyssate life habits with only the Glycymerididae living as free burrowers. The phenetic characters of the families within the Arcoida are reviewed and the families are shown to be supported by very few synapomorphic characters. Homoplasy is shown to be widespread and is illustrated in a series of discussions on the ligament, epibyssate–endobyssate radiations, and possible parallelism within genera, and in a review of arcoid anatomical characters. Previously published molecular data are reviewed and these support the inclusion of the Glycymerididae in the Arcoidea. They also indicate, however, that polyphyly is probably widespread at the subfamily level.  © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 148 , 237–251.     

Stages in the ontogeny and a model of the evolution of bivalves (Mollusca)

Comparison of the ontogenies of bivalves of different habitats and systematic position provide two main conclusions. The first is that bivalves stick to a certain basic program of ontogeny which can be divided into six phases. The first three phases are parallel to those of some other invertebrates, the 4th phase is parallel to that of conchiferan molluscs, with, the 5th phase, bivalve characters are acquired, and in the 6th phase, genus-specific adult organization is reached. The second is that within this basic developmental programm specific differences occur. The ontogeny ofTeredora is similar to that of many marine bivalves and is characterized by a strong plasticity. The adult, in contrast, is highly specialized for living within wood. InAnodonta, the opposite is true and embryonic development even includes a parasitic stage, while the adult is of a more general organization. Due to brood-protection and embryonic nursing, the whole organogenesis ofSphaerium becomes directed towards adult organization. Ontogenetic development of a general body plan of the Bivalvia provides a model for the evolution of the first bivalves (protobranchs excluded) from univalves by a one-step alteration connected with shell mineralization and loss of the buccal mass affecting the embryo near end of embryogenesis.     

Localization of serotonin and FMRFamide in the bivalve molluscMytilis edulis at early stages of its development

Location and time of appearance of serotonin and FMRFamide in ontogenesis of the bivalve mollusc Mytilus adulis L.has been studied. Serotonin first appears at a transfer from the last trochophore stage to the early veliger stage, 30–32 h after fertilization and is detected in a large cell located immediately at the base of the apical plume. FMRFamide appears 14—15 h after fertilization, during the transfer from the conchostoma stage to the early trochophore stage before the turning out of the shell gland and is a marker of the future shell site. It is located as a transverse stretch on the back of the larva in the area of the future shell. After the turning out of the shell gland the stretch is markedly shortened, while its processes approach the larval surface and are located under the shell. A suggestion is made about a morphogenetic function of FMRF-amide.     

Functional morphology and palaeontological significance of the conchiolin layers in corbulid pelecypods

The Corbulidae, which today are slow, cumbersome, very shallow burrowers, developed special morphological features by which they obtained an outstanding capability to withstand the physical and biological stresses characteristic of their preferred habitat. These features are: an inequivalve, globose shape, thick shells, and conchiolin layers (at least one) embedded within their valves in a unique way. These features enable the corbulids to close their valves tightly during the unfavourable environmental conditions (e.g. low salinity, low oxygen content) which may prevail in the marginal marine regions inhabited by several corbulid species. The conchiolin layers act as a barrier preventing all chemically boring organisms from penetrating into the bivalve shell, or shell dissolution by sea water undersaturated with respect to calcium carbonate. The layered conchiolin weakens the shell mechanically, however, especially during fossilization, when the conchiolin is decomposed. The valve splits apart into two shells so completely different in appearance that they may be attributed to different taxa. The conchiolin layers are therefore of great ecological and palaeontological significance. The nature of these conchiolin layers in Corbula (Varicorbula) gibba (Olivi) is described and illustrated and their functional significance discussed in relation to other living and fossil corbulid species.     

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.     

Cavity defects in the procapsid of bacteriophage P22 and the mechanism of capsid maturation

Bacteriophage P22 belongs to a family of double-stranded DNA viruses that share common morphogenetic features like DNA packaging into a procapsid precursor and maturation. Maturation involves cooperative expansion of the procapsid shell with concomitant lattice stabilization. The expansion is thought to be mediated by movement of two coat protein domains around a hinge. The metastable conformation of subunit within the procapsid lattice is considered to constitute a late folding intermediate. In order to understand the mechanism of expansion it is necessary to characterize the interactions stabilizing procapsid and mature capsid lattices, respectively. We employ pressure dissociation to compare subunit packing within the procapsid and expanded lattice. Procapsid shells contain larger cavities than the expanded shells, presumably due to polypeptide packing defects. These defects contribute to the metastable nature of the procapsid lattice and are cured during expansion. Improved packing contributes to the increased stability of the expanded shell. Comparison of two temperature-sensitive folding (tsf) mutants of coat protein (T294I and W48Q) with wild-type coat revealed that both mutations markedly destabilized the procapsid shell and yet had little effect on relative stability of the monomeric subunit. Thus, the regions affected by these packing defects constitute subunit interfaces of the procapsid shell. The larger activation volume of pressure dissociation observed for both T294I and W48Q indicates that the decreased stability of these particles is due to increase of cavity defects. These defects in the procapsid lattice are cured upon expansion suggesting that the intersubunit contacts affected by tsf mutations are absent or rearranged in the mature shell. The energetics of the in vitro expansion reaction also suggests that entropic stabilization contributes to the large free energy barrier for expansion.     

First elucidation of a didymozoid life cycle: Saccularina magnacetabula n. gen. n. sp. infecting an arcid bivalve

The first first-intermediate host for a species of Didymozoidae (Trematoda: Hemiuroidea), a bivalve of the family Arcidae, is identified using multi-loci molecular data. First intermediate, (likely) third intermediate, and adult stages of a new didymozoid taxon (Saccularina magnacetabula n. gen. n. sp.) from Moreton Bay, Queensland, Australia were collected from the Sydney cockle Anadara trapezia (Deshayes) (Arcoidea: Arcidae), Sillago sp. (Sillaginidae) and Elops hawaiensis Regan (Elopiformes: Elopidae), respectively, and genetically matched. Infections in A. trapezia were present as sporocysts and cystophorous cercariae, and infected tissue at the base of the gills. Morphologically, S. magnacetabula is distinctive relative to all other didymozoids in the combination of hermaphroditism, mate-pairing, filiform body shape, the presence of a ventral sucker, a single testis, and a saccular excretory vesicle at the posterior extremity. Molecular sequence data were generated for S. magnacetabula and 42 other putative didymozoid species to explore relationships within the Didymozoidae and Hemiuroidea. In molecular phylogenetic analyses of the 28S rDNA region, the new genus forms a clade with an undescribed taxon from the redthroat emperor, Lethrinus miniatus (Bloch & Schneider) (Perciformes: Lethrinidae), from the Great Barrier Reef, and another uncharacterised taxon from E. hawaiensis. This clade is sister to a moderately well-supported clade comprising all other didymozoid species for which sequences are available, including representatives of five of the six presently recognised subfamilies. The infection of a bivalve by a didymozoid is discussed in the context of the overwhelming use of gastropod molluscs as first intermediate hosts by the Hemiuroidea.