The karyotype of Nodipecten nodosus (Bivalvia: Pectinidae)

Earlier karyotypical work on Nodipecten nodosus embryos indicated that this species has a diploid number of 38, with six pairs respectively of metacentric and submetacentric chromosomes and seven pairs of subtelocentric chromosomes, although there were some difficulties in obtaining complete metaphases. The present work provides additional results on specific regions of the chromosomes in N. nodosus and, by meiotic studies, confirms the chromosome number with more reliability. Active nucleolar organizer regions (NOR), detected in mitotic metaphases from embryos, can be characterized in N. nodosus by a high level of heteromorphism of NOR-sites, indicating that these regions are not appropriate as chromosomal markers in this species. The procedure for detecting constitutive heterochromatin of chromosomes allowed us to observe most of the heterochromatic blocks at a pericentromeric position and some at telomeric and interstitial positions. The analysis of meiotic chromosomes from gonad tissue revealed the presence of 19 bivalents during metaphase I, all homomorphic and isopicnotic, confirming the previously described diploid chromosomal number of 38 for N. nodosus. From these results, some evolutionary aspects of the Pectinidae are briefly discussed.     

Morphological and functional specializations of the shell, musculature and pallial glands in the Lithophaginae (Mollusca: Bivalvia)

The structure of the pallial glands in limestone and coral boring species of Lithophaga (Bivalvia: Lithophaginae) is examined and related to evolutionary trends in habitat specialization. Boring glands of all species occur in the middle mantle fold but display a progressive degree of complexity from simple epithelial to ducted sub-epithelial structures coinciding with increasing specialization of habitat from natural limestone to species specific live coral dwellers. The size of this gland is relatively reduced anteriorly and along the entire length of the mantle in live coral borers.     

Tissue Hemoglobins in Bivalvia (Mollusca)

Distribution and properties of tissue hemoglobins in bivalves are reviewed. The mollusk gills are given particular attention; data on their functions, relative weight, and surface area as well as the content, properties, and functional importance of gill hemoglobin are reviewed.     

Mathematical Investigations into the Longevity of the Ocean Quahog Arctica islandica (Mollusca: Bivalvia)

Annual internal growth banding of Arctica islandica has shown that the species has a surprising longevity of up to 149 years as stated by THOMPSON , JONES and DREIBELBIS (1980). The authors have compiled 100 values of valve height against age as reproduced in Figure 1. Visually gained growth curves have been added for single specimens that leave some doubt as to the existence of an inflexion point. In this paper a cross-sectional evaluation of growth behaviour is carried out with nine funtions using nonlinear regressions. Five growth functions yield almost equally good results with a final valve height just below 100 mm. An inflexion point is either present in early youth or lacking altogether, depending on the growth function used.     

Development and morphology of the glochidium larva of Anodonta cygnea (Mollusca: Bivalvia)

The development of the parasitic glochidium larva of Anodonta cygnea L. is outlined, with reference to earlier studies on unionid larvae, and a comparison is made between the mature glochidia of the three genera, Anodonta, Unio , and Margaritifera.
The glochidium of Anodonta cygnea is anatomically and morphologically specialized in connection with its parasitic existence. The specialized structures are described, and their significance discussed.     

Mitogenomics of Vetigastropoda: insights into the evolution of pallial symmetry

The nucleotide sequences of the complete or nearly complete mitochondrial (mt) genomes of seven vetigastropods were determined: Angaria neglecta (Angarioidea), Phasianella solida (Phasianelloidea), Granata lyrata (Seguenzioidea), Tegula lividomaculata and Bolma rugosa (Trochoidea), Diodora graeca (Fissurelloidea) and Lepetodrilus schrolli (Lepetodriloidea). While the mt genomes of the superfamilies Angarioidea, Phasianelloidea, Seguenzioidea and Trochoidea conform generally to the ancestral gene order of Vetigastropoda and Gastropoda, those of the superfamilies Fissurelloidea and Lepetodriloidea have suffered important rearrangements. The gene order of the mtDNA of Chrysomallon squamiferum, a representative of Neomphalina, was also analysed since it has been proposed to be closely related to Vetigastropoda, and showed a distinct arrangement. The reconstructed phylogenies recovered Neomphalina as a distinct gastropod lineage that is the sister group (only with moderate bootstrap support) of a clade including Vetigastropoda and Neritimorpha + Caeno‐gastropoda while the relative position of Heterobranchia and Patellogastropoda in the gastropod tree could not be determined definitively due to their long branches. Within the monophyletic Vetigastropoda, the superfamily Fissurelloidea was recovered as the sister group of two lineages, one including Lepetodriloidea as the sister group of Seguenzioidea + Halitoidea, the other including Phasianelloidea, Angarioidea and Trochoidea without resolved relationships. The long branches of Fissurelloidea were found to introduce significant tree instability in phylogenetic reconstruction. The new phylogeny supports that the loss of the right pallial gill occurred multiple times in vetigastropod evolution as previously suggested and that Phasianelloidea, Angarioidea and Trochoidea radiated from a common asymmetric (single‐gilled) ancestor that lived in the middle Palaeozoic.     

Reconstructing the Anomalodesmata (Mollusca: Bivalvia): morphology and molecules

The extant anomalodesmatan bivalves have always proved rather enigmatic and difficult to interpret, both in terms of their relationships to other bivalve taxa and the interrelationships of the constituent families. These difficulties stem from their diverse and often highly specialized life habits which have resulted in a wide array of disparate morphologies, and also from the fact that many are extremely rare. Classifications based on morphological characters have been dogged by fears that convergent and parallel evolution has masked phylogenetic signals. Molecular surveys of members of 12 of the 15 constituent families, using the 18S rRNA gene, have revealed that anomalodesmatans are robustly monophyletic and lie within the basal heterodonts. The Anomalodesmata should no longer be regarded as a subclass, but as a part of the Heterodonta. Here we present an enhanced analysis of 32 anomalodesmatan species (representatives of 12 families). Our results, subjected to Maximum Parsimony, Maximum Likelihood and Bayesian analyses, challenge our understanding of the internal relationships within the Anomalodesmata. In particular they indicate the need for a re-distribution of the families traditionally placed in the Thracioidea and Pandoroidea into a 'thraciid' lineage (Thraciidae + Cleidothaeridae + Myochamidae) and a 'lyonsiid' lineage (polyphyletic Lyonsiidae + Clavagellidae + Laternulidae + Pandoridae). The endolithic Clavagella and endobenthic Brechites and Penicillus form a robust clade. The hypothesis that the carnivorous septibranchs are monophyletic can, thus far, be neither supported nor rejected. Mapping critical morphological characters onto our molecular results provides evidence of multiple loss of some characters (e.g. prismato-nacreous shell microstructure and shell spicules) and also multiple gain of others (e.g. chondrophores).  © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 148 , 395–420.     

The genus Neilo in New Zealand (Mollusca: Bivalvia)

Abstract

The phylogenetic history, paleontological implications, and ecology of larger New Zealand Recent species of Neilo (sensu stricto) are discussed. N. wairoana delli, N. blacki, and N. (Pseudomalletia) aoteana are described as new. The following taxa are reduced to chronosubspecies: N. jugifera Marwick - of N. australis (Quoy & Gaimard); N. annectens Powell and N. rugata Dell - of N. sublaevis Marwick; N. sinangula Finlay - of N. awamoana Finlay.     

New Zealand Recent Hyriidae (Mollusca: Bivalvia: Unionida)

The New Zealand freshwater mussels are taxonomically revised and compared with key Australian and South American taxa using DNA sequence data. Three living species are recognised: Echyridella menziesii, Echyridella aucklandica and Echyridella onekaka. Cucumerunio websteri websteri and Cucumerunio websteri delli, are treated as junior synonyms of Unio aucklandica, which is transferred from Cucumerunio (and Hyridella) to Echyridella. Lectotypes are designated for Unio waikarensis and Unio hochstetteri, which are illustrated together with primary type specimens of other relevant taxa. The type locality of U. aucklandica is restricted to the catchment of the Kawakawa River.     

Ciliary ultrastructure of protobranchs (Mollusca, Bivalvia)

Abstract. The external epithelial cilia and other surface structures of the nuculoid protobranchs Nuculana pernula and Nucula nitidosa were studied. The gill lamellae and labial palps are partly covered with very long cilia. These have a modified slender distal portion, an ordinary metazoan-type basal body, a basal foot. and a single, long cross-striated rootlet. In cilia on the gills of N. nitidosa , the basal foot is thick and attaches to the next basal body directly behind. Unciliated surface areas on the gills, labial palps, and foot are covered with a dense brushborder of microvilli. We observed no specific homologies between the cilia of the protobranchs studied and the epidermal cilia of the enigmatic Xenoturbella bocki , hence the recent hypothesis of a close connection of the latter to the protobranch bivalves is questioned.     

Development of the larval muscle system in the mussel Mytilus trossulus (Mollusca,Bivalvia)

In the present study we examined muscle development throughout the complete larval cycle of the bivalve mollusc, Mytilus trossulus. An immunofluorescence technique and laser scanning confocal microscopy were used in order to study the organization of the muscle proteins (myosin, paramyosin, twitchin, and actin) and some neurotransmitters. The appearance of the muscle bundles lagged behind their nervous supply: the neuronal elements developed slightly earlier (by 2 h) than the muscle cells. The pioneer muscle cells forming a prototroch muscle ring were observed in a completed trochophore. We documented a well‐organized muscle system that consisted of the muscle ring transforming into three pairs of velar striated retractors in the early veliger. The striations were positive for all muscle proteins tested. Distribution of FMRFamide and serotonin (5‐HT) immunocytochemical staining relative to the muscle ring differed significantly: 5‐HT‐immunioreactive cells were situated in the center of the striated muscle ring, while Phe‐Met‐Arg‐Phe‐NH2 neuropeptide FMRFamid immunoreactive fibers were located in a distal part of this ring. Our data showed clearly that the muscle proteins and the neurotransmitters were co‐expressed in a coordinated fashion in a continuum during the early stages of the mussel development. Our study provides the first strong evidence that mussel larval metamorphosis is accompanied by a massive reorganization of striated muscles, followed by the development of smooth muscles capable of catch‐contraction.     

Food selection by Dreissena polymorpha Pallas (Mollusca: Bivalvia)

SUMMARY. The role of the selection systems in the mantle cavity and the stomach of Dreissena polymorpha was investigated by comparing the composition of particles in the plankton, the stomach and the mid-gut. Both systems selected particles of 15–40 μm for food. The selection mechanism functioned more efficiently in the presence of simple algal mixtures than in the presence of lake plankton, but did not function at all in animals fed after 16 h starvation. The composition of successive pseudo-faecal pellets differed. It is concluded that selection of different kinds of particle occurs in specific places in the mantle cavity. Cryptomonas spp. were highly preferred as food. Indications were found that not only does size-selection occur, but also selection of a chemical nature.     

The fine structure of the pallial eyes of Laternula truncata (Bivalvia: Anomalodesmata: Pandoracea)

The structure of the pallial eyes of Laternula truncata (Lamarck 1818) has been studied using the light and electron microscopes. The eye is complex and can be- considered to be- the most advanced yet described for a bivalve mollusc. The cornea consists of modified flattened epithelial cells with an external border of microvilli. The cornea covers a large, circular, multinucleate lens. The lens comprises (1) centrally located translucent lens cells, (2) laterally located supporting cells from which cell processes interdigitate with processes from the lens cells. The retina is two layered and inverted. The proximal and distal retinae are made up of concentrically arranged laminae derived from the membranes of ciliary basal bodies. The cilia comprise a base and feet, but no root system and have a 9+0 arrangement of filaments.
The pigment cup or tapetum is bounded by a sclerotic coat and is three layered, each layer possessing characteristic pigment granules. From the base of the eye arises a large optic nerve.
The eye possesses an eye appendage, the epithelium of which is invaginated on its internal border to form a groove within which are found some 28 cilia. The cilia, it is thought, make contact with the microvilli of the epithelium when the appendage is touched. Such an action serves to protect the delicate eye from damage.
The structure of the eye is compared with that of other molluscs, particularly members of the Bivalvia.     

Cytogenetics of the razor clam Solen marginatus (Mollusca: Bivalvia: Solenidae)

The razor clam Solen marginatus has a diploid chromosome number of 38. The karyotype consists of one metacentric/submetacentric, three submetacentric/metacentric, five submetacentric, one submetacentric/subtelocentric, one subtelocentric/submetacentric, six subtelocentric and two telocentric chromosome pairs. Staining with chromomycin A3 revealed bright positive bands subcentromerically in the long arms of one medium-sized subtelocentric pair, while DAPI staining showed uniform fluorescence in all chromosomes of the complement. Fluorescence in situ hybridization using an 18S-5.8S-28S rDNA probe locates these loci at the subcentromeric region of one subtelocentric pair and at the subtelomeric region of another subtelocentric pair.     

The pallial organ of Atrina pectinata (Bivalvia: Pinnidae): its structure and function

Hitherto, the unique pallial organ of the Pinnidae has been considered to fulfil a cleansing role, removing sediment and shell fragments from the mantle cavity. This study of the pallial organ of Atrina pectinata describes structure in greater detail than before but was unable to demonstrate a cleansing function. Rather, the organ is thigmotropic and massively secretory. Study of the pallial organ head demonstrates it to have a pH of between 2 and 4 and that tall, apparently empty cells in the head epithelium contain sulphuric acid which can be precipitated as barium sulphate crystals following irrigation with barium chloride.
Acid-secreting cells are recorded from a number of animal groups, notably opisthobranch gastropods, where they are concluded to possess a defensive function. Such a structure has not hitherto been recorded from the Bivalvia.     

Sexual selection and simultaneous hermaphroditism among the Unionidae (Bivalvia: Mollusca)

Simultaneous hermaphroditism is an infrequent mode of reproduction among bivalves of the family Unionidae: only five of the 220 North American species are simultaneous hermaphrodites. However, hermaphroditic individuals of otherwise predominantly dioecious species have been encountered in 30 of I01 species examined. These hermaphroditic individuals as well as simultaneous hermaphrodites can exhibit considerable variability in the ratio of spermatogenic: oogenic tissue within the gonad, and the purposes of this paper are to determine the underlying causes of both this variability and the occurrence of occasional hermaphroditic individuals among dioecious species. Results indicate that the ratio of male: female gonodal tissue of a simultaneous hermaphrodite is bimodally distributed, and several hypotheses to account for this observation are presented. It is proposed that populations occurring in different habitats and under conditions of different individual density are subject to fundamentally different sexual selection pressures acting on ratios of allocation to male and female gametes. Occasional hermaphroditism among otherwise predominantly dioecious species was in this study associated with infection of the gonads by digenean trematodes. A model of sexual determination among unionids presented in this paper proposes that sex is determined by genetically controlled hormone levels: occasional hermaphrodites result from alterations in these hormone levels caused by developmental errors and trematodal infections which mimic the results of such errors. Predictions of this model are consistent with observed levels of variability in male: females gonadal tissue among occasional hermaphrodites.     

Phylogeny and spatio-temporal distribution of European Pectinidae (Mollusca: Bivalvia)

Phylogenetic relationships within Pectinidae (Bivalvia, Pteriomorphia) have been investigated primarily for Pacific and Western Atlantic or commercially valuable taxa. Most molecular phylogenetic studies have revealed monophyly of pectinid bivalves but interrelationships of the different clades are still inconsistent. However, non-commercial European Pectinidae has mostly been neglected in earlier investigations and therefore the evolution and radiation of the European Pectinidae is poorly understood. Since the fossil record of this group is well investigated, the evolutionary age of phylogenetic diversification and radiation events within this group can be dated. Thus, the connection of geological and climatic changes to radiation events within this group can be assessed. We investigated the phylogenetic relationships within European Pectinidae using mitochondrial (12S and 16S) and nuclear (18S, 28S and H3) gene markers and performed relaxed molecular clock approaches to gain information on the evolutionary age and the connection between Cenozoic climatic changes and diversification within this group. The results show concordance of radiation events with the Middle Miocene cooling event and the following climatic period with slowly decreasing temperatures. However, geological changes such as the uplift of the Gomphotherium Landbridge or the closure and re-opening of the Strait of Gibraltar also had great impact on diversification and distribution patterns within European Pectinidae.     

Trends in the sand: Directional evolution in the shell shape of recessing scallops (Bivalvia: Pectinidae)

Directional evolution is one of the most compelling evolutionary patterns observed in macroevolution. Yet, despite its importance, detecting such trends in multivariate data remains a challenge. In this study, we evaluate multivariate evolution of shell shape in 93 bivalved scallop species, combining geometric morphometrics and phylogenetic comparative methods. Phylomorphospace visualization described the history of morphological diversification in the group; revealing that taxa with a recessing life habit were the most distinctive in shell shape, and appeared to display a directional trend. To evaluate this hypothesis empirically, we extended existing methods by characterizing the mean directional evolution in phylomorphospace for recessing scallops. We then compared this pattern to what was expected under several alternative evolutionary scenarios using phylogenetic simulations. The observed pattern did not fall within the distribution obtained under multivariate Brownian motion, enabling us to reject this evolutionary scenario. By contrast, the observed pattern was more similar to, and fell within, the distribution obtained from simulations using Brownian motion combined with a directional trend. Thus, the observed data are consistent with a pattern of directional evolution for this lineage of recessing scallops. We discuss this putative directional evolutionary trend in terms of its potential adaptive role in exploiting novel habitats.