The effects of taxonomic standardization on sampling-standardized estimates of historical diversity

Occurrence-based databases such as the Palaeobiology database (PBDB) provide means of accommodating the heterogeneities of the fossil record when evaluating historical diversity patterns. Although palaeontologists have given ample attention to the effects of taxonomic practice on diversity patterns derived from synoptic databases (those using first and last appearances of taxa), workers have not examined the effects of taxonomic error on occurrence-based diversity studies. Here, we contrast diversity patterns and diversity dynamics between raw data and taxonomically vetted data in the PBDB to evaluate the effects of taxonomic errors. We examine three groups: Palaeozoic gastropods, Jurassic bivalves and Cenozoic bivalves. We contrast genus-level diversity patterns based on: (i) all occurrences assigned to a genus (i.e. both species records and records identifying only the genus), (ii) only occurrences for which a species is identified, and (iii) only occurrences for which a species is identified, but after vetting the genus to which the species is assigned.Extensive generic reassignments elevate origination and extinction rates within Palaeozoic gastropods and origination rates within Cenozoic bivalves. However, vetting increases generic richness markedly only for Cenozoic bivalves, and even then the increase is less than 10%. Moreover, the patterns of standing generic richness are highly similar under all three data treatments. Unless our results are unusual, taxonomic standardization can elevate diversity dynamics in some cases, but it will not greatly change inferred richness over time.     

Gastropods and bivalves of commercial interest from the continental shelf of Jalisco and Colima,México

The distribution and abundance with respect to depth and type of substratum of 20 species of gastropods and four species of bivalves of economic importance were examined in the continental shelf of Jalisco and Colima, México. These species were taken with net trawls at depths from 24 to 83 m in August, 1988. Most individuals and species of gastropods were collected in stations with sandy silt substratum. Bivalves were collected in sandy silt and medium sand substrata. The six most abundant species represented 81.2% of all gastropods and bivalves collected. These species are: Cantharus pallidus, Fusinus dupetittouarsi, Ficus ventricosa, Hexaplex brassica, Harpa conoidalis and Arca pacifica.     

Phylogenetic analysis of molluscan mitochondrial LSU rDNA sequences and secondary structures

Mollusks are an extraordinarily diverse group of animals with an estimated 200,000 species, second only to the phylum Arthropoda. We conducted a comparative analysis of complete mitochondrial ribosomal large subunit sequences (LSU) of a chiton, two bivalves, six gastropods, and a cephalopod. In addition, we determined secondary structure models for each of them. Comparative analyses of nucleotide variation revealed substantial length variation among the taxa, with stylommatophoran gastropods possessing the shortest lengths. Phylogenetic analyses of the nucleotide sequence data supported the monophyly of Albinaria, Euhadra herklotsi + Cepaea nemoralis, Stylommatophora, Cerithioidea, and when only transversions are included, the Bivalvia. The phylogenetic limits of the mitochondrial LSU rRNA gene within mollusks appear to be up to 400 million years, although this estimate will have to be tested further with additional taxa. Our most novel finding was the discovery of phylogenetic signal in the secondary structure of rRNA of mollusks. The absence of entire stem/loop structures in Domains II, III, and V can be viewed as three shared derived characters uniting the stylommatophoran gastropods. The absence of the aforementioned stem/loop structure explains much of the observed length variation of the mitochondrial LSU rRNA found within mollusks. The distribution of these unique secondary structure characters within mollusks should be examined.     

Larval ecology, geographic range, and species survivorship in Cretaceous mollusks: organismic versus species-level explanations

The observation that geographic range size in Cretaceous mollusks is correlated with species survivorship and is heritable at the species level has figured repeatedly in discussions of species selection over the past two decades. However, some authors have suggested that the relationship between mode of larval development and geographic range supports the reduction of this example to selection on organismic properties. Our reanalysis of Jablonski's work on heritability at the species level finds that geographic range is significantly heritable (using a randomization test) in both bivalves and gastropods, even within a single larval mode. Further, generalized linear models show that geographic range size is more important than larval mode in predicting extinction probability in both gastropods and bivalves. These results reaffirm the role and heritability of geographic range as a species-level property that can promote species selection; the model-based approach applied here may help to operationalize "screening off " and related approaches to evaluating hierarchical explanations in evolution.     

Gastropod first intermediate hosts for two species of Monorchiidae Odhner, 1911 (Trematoda): I can’t believe it’s not bivalves!

The trematode superfamily Monorchioidea comprises three families of teleost parasites: the Monorchiidae Odhner, 1911, Lissorchiidae Magath, 1917, and Deropristidae Cable & Hunninen, 1942. All presently known lissorchiid and deropristid life cycles have gastropods as first intermediate hosts, whereas those of monorchiids involve bivalves. Here, we report an unexpected intermediate host for monorchiids; two species of Hurleytrematoides Yamaguti, 1954 use gastropods as first intermediate hosts. Sporocysts and cercariae were found infecting two species of the family Vermetidae, highly specialised sessile gastropods that form calcareous tubes, from two locations off the coast of Queensland, Australia. These intramolluscan infections broadly corresponded morphologically to those of known monorchiids in that the cercariae have a spinous tegument, oral and ventral suckers, a simple tail and distinct eye-spots. Given the simplified morphology of intramolluscan infections, genetic data provided a definitive identification. ITS2 rDNA and cox1 mtDNA sequence data from the gastropod infections were identical to two species of Hurleytrematoides, parasites of butterflyfishes (Chaetodontidae); Hurleytrematoides loi McNamara & Cribb, 2011 from Moreton Bay (south-eastern Queensland) and Heron Island (southern Great Barrier Reef) and Hurleytrematoides morandi McNamara & Cribb, 2011 from Heron Island. Notably, species of Hurleytrematoides are positioned relatively basal in the phylogeny of the Monorchiidae and are a sister lineage to that of species known to infect bivalves. Thus, the most parsimonious evolutionary hypothesis to explain infection of gastropods by these monorchiids is that basal monorchiids (in our analyses, species of Cableia Sogandares-Bernal, 1959, Helicometroides Yamaguti, 1934 and Hurleytrematoides) will all prove to infect gastropods, suggesting a single host switching event into bivalves for more derived monorchiids (17 other genera in our phylogenetic analyses). A less parsimonious hypothesis is that the infection of vermetids will prove to be restricted to species of Hurleytrematoides, as an isolated secondary recolonisation of gastropods from a bivalve-infecting lineage. Regardless of how their use arose, vermetids represent a dramatic host jump relative to the rest of the Monorchiidae, one potentially enabled by their specialised feeding biology.     

Death assemblages of molluscs on St Martin's Flats, Isles of Scilly: a surrogate for regional biodiversity?

We test the hypothesis that the biodiversity of the death assemblage of shelled molluscs (gastropods and bivalves) on a single sandflat is representative of the biodiversity of the regional living fauna from the complete spectrum of habitat types, and can therefore be used as a surrogate for the purposes of rapid biodiversity assessment. Two biodiversity indices considered appropriate for such an assessment are average taxonomic distinctness (⁺) and the variation in taxonomic distinctness (⁺), which are, respectively, measures of the degree to which the species are taxonomically related to each other (i.e. the average taxonomic spread), and the degree to which taxa are over or under represented (i.e. the evenness of the distribution across the taxonomic tree). In each case, measured values were compared with the regional living species pool using a randomisation test. Collections and identifications of shells were made by both a non-expert in mollusc taxonomy and by taxonomic experts, the latter using microscopic examination of sediment samples as well as simple surface searches for macro-species. In all studies, ⁺ for the gastropods in the death assemblage was fully representative of the regional living fauna, but for bivalves it was significantly lower. This is because most of the bivalves were characteristic of the sandflat habitat, which introduced a bias, whereas post-mortem transport of gastropods, characterising exotic habitats, is more random. For gastropods, in all studies but one, ⁺ was significantly higher than expected from a random sample of the regional species pool, and for bivalves this was the case for all studies, suggesting the over-representation of some taxa and the under-representation of others. For the total shelled molluscan fauna (gastropods + bivalves) all ⁺ values were below, and all ⁺ values above, expectation. Results obtained by experts and a non-expert in mollusc taxonomy did not differ. In general, death assemblages at a single location are only likely to be representative of a wider region if there is randomisation as a result of post-mortem transport processes.     

Mid-domain models as predictors of species diversity patterns: bathymetric diversity gradients in the deep sea

Geometric constraints represent a class of null models that describe how species diversity may vary between hard boundaries that limit geographic distributions. Recent studies have suggested that a number of large scale biogeographic patterns of diversity (e.g. latitude, altitude, depth) may reflect boundary constraints. However, few studies have rigorously tested the degree to which mid-domain null predictions match empirical patterns or how sensitive the null models are to various assumptions. We explore how variation in the assumptions of these models alter null depth ranges and consequently bathymetric variation in diversity, and test the extent to which bathymetric patterns of species diversity in deep sea gastropods, bivalves, and polychaetes match null predictions based on geometric constraints.
Range–size distributions and geographic patterns of diversity produced by these null models are sensitive to the relative position of the hard boundaries, the specific algorithms used to generate range sizes, and whether species are continuously or patchily distributed between range end points. How well empirical patterns support null expectations is highly dependent on these assumptions. Bathymetric patterns of species diversity for gastropods, bivalves and polychaetes differ substantially from null expectations suggesting that geometric constraints do not account for diversity–depth patterns in the deep sea benthos.     

Early Shell Formation During Molluscan Embryogenesis, with New Studies on the Surf Clam, Spisula solidissima

Shell formation in molluscs begins early in embryogenesis duringsome stage of archenteron formation. Ultrastructural informationon early formation of external shells is available from onlya few bivalves and gastropods. Secretion of the very first shellmaterial by shell field epithelial cells is preceded by an invaginationof the dorsal ectoderm in the region of the shell field. A centuryago, this invagination was termed the "shell gland." As a secretoryfunction for this invagination has not yet been demonstratedand as the term "shell gland" has taken on various meaningsin the literature, the invagination will be referred to as theshell field invagination. The opening into the shell field invaginationseems to be circular in gastropods and elongate in bivalves.Accordingly initial organic shell material seems to form a ringin gastropods and a saddle in bivalves. As in adult molluscs,shells of pre-metamorphic molluscs are composed of both organicand inorganiccomponents. Ultrastructural data from bivalvesand gastropods indicate that the initial organic shell materialis secreted just outside the shell field invagination (acrossthe pore). Initial inorganic shell materials have not been localizednor their pathway traced into or through any pre-metamorphicmolluscs. New SEM and TEM data show that the invagination inthe bivalve Spisula solidissima is composed of a wide outerregion and very narrow inner region with the first shell materialforming at the junction between the two. This is unlike ultrastructuraldata available for other species. Many sections give the falseimpressions that: 1) the shell field invagination is closedto the outside and, 2) that the first organic shell materiallines the innermost region of the invagination. It is not clearwhether the cells of the outer invagination in this speciesare shell field cells. It is suggested that they are not.     

Morphological Variability and Levels of Nuclear Ploidy in Neurons of the Central Nervous System of Bivalves Relative to the Problem of Somatic Polyploidy

Ploidy of neurons was assessed by the method of nuclear DNA cytophotometry in ganglia of the central nervous system (CNS) in 28 species of Bivalvia belonging to 5 existing subclasses and having various bio-ecological characteristics. A significant variation in the nucleus size and chromatin structure was revealed on crush preparations and slides; however, almost all nuclei were diploid. Some species had nuclei with tetraploid DNA value (from 0.2 up to 1%) and with condensed chromosomes, the probable mitotic fraction of cells. It was concluded that variation in the sizes of neurons and of their nuclei in the given instance was of a functional character, and polyploidy was untypical for the nervous system of bivalves. Apparently, the polyploidy, as an evolutionary strategy of neuron growth, had no anatomical prerequisites in bivalves, which is similar to CNS oligomerization in higher gastropods, as they are devoid of any functional adaptive sense.     

Diversity and abundance of mollusks in the sublittoral epifaunal community of Punta Patilla, Venezuela

The diversity of a sublittoral epifaunal mollusk community of Punta Patilla, Sucre State, Venezuela, was studied from September 1990 to September 1991. We identified 25 species (14 bivalves and 11 gastropods) of mollusks that inhabit gravel, soft sand and bottoms covered by Thalassia testudinum. Total diversity indices were H' = 3.42. J' = 0.74 and 1-D = 0.85. Monthly diversity reached its maximum in March 1991 (3.12 bits/ ind.), June 1991 (2.88 bits/ind.) and September 1991 (2.95 bits/ind.); minimum diversity was recorded in August 1991 (1.20 bits/ind.). A log series model showed a diversity index alpha = 4.56 for species abundance data and alpha = 3.11 for biomass data. The more abundant species were Chione cancellata, Anigona listeri, Chione granulata and Area zebra among the bivalves, and Chicoreus brevifrons, Turritella variegata and Phllonotus pomum among the gastropods (which present maximum biomass). The average total biomass (56.80 g/m2) is low when compared to reports from other tropical zones.     

Molluscan biological and chemical diversity: secondary metabolites and medicinal resources produced by marine molluscs

The phylum Mollusca represents an enormous diversity of species with eight distinct classes. This review provides a taxonomic breakdown of the published research on marine molluscan natural products and the medicinal products currently derived from molluscs, in order to identify priority targets and strategies for future research. Some marine gastropods and bivalves have been of great interest to natural products chemists, yielding a diversity of chemical classes and several drug leads currently in clinical trials. Molluscs also feature prominently in a broad range of traditional natural medicines, although the active ingredients in the taxa involved are typically unknown. Overall secondary metabolites have only been investigated from a tiny proportion (<1%) of molluscan species. At the class level, the number of species subject to chemical studies mirrors species richness and our relative knowledge of the biology of different taxa. The majority of molluscan natural products research is focused within one of the major groups of gastropods, the opisthobranchs (a subgroup of Heterobranchia), which are primarily comprised of soft‐bodied marine molluscs. Conversely, most molluscan medicines are derived from shelled gastropods and bivalves. The complete disregard for several minor classes of molluscs is unjustified based on their evolutionary history and unique life styles, which may have led to novel pathways for secondary metabolism. The Polyplacophora, in particular, have been identified as worthy of future investigation given their use in traditional South African medicines and their abundance in littoral ecosystems. As bioactive compounds are not always constitutively expressed in molluscs, future research should be targeted towards biosynthetic organs and inducible defence reactions for specific medicinal applications. Given the lack of an acquired immune system, the use of bioactive secondary metabolites is likely to be ubiquitous throughout the Mollusca and broadening the search field may uncover interesting novel chemistry.     

Geographic variations in freshwater molluscs

The phenomenon of geographic variation is known in practically all taxa of living beings. However, the reality of this phenomenon in freshwater molluscs (snails and bivalves) has many times been questioned in the past. It was accepted that these animals do not demonstrate spatially-oriented variation, where specific "local race" is arisen in each specific habitat. Till the beginning of 1970s, there was no statistical evidence that geographic clines in freshwater molluscs really exist. However, a few species of freshwater molluscs has been studied in this respect so far, therefore it is almost impossible to draw any general patterns of geographical variation in this group of animals. Most species of freshwater molluscs studied to the date exhibit statistically significant decrease of their body size in the south-north direction. Perhaps, it may be explained by decrease of the duration of the growth season in high latitudes. Some species of freshwater snails demonstrate clinal changes in shell proportions. This allows to reject subspecies separation within these species since diagnostic characters of such "subspecies" may blur when geographic variation is taken into consideration. The data on geographic variation in anatomical traits in freshwater molluscs is much more scarce. At least one species of pond snails (Lymnaea terebra) demonstrates clinal variation in proportions of the copulative apparatus in the south-north direction. Further studies of geographic variation in freshwater molluscs should reveal whether it is truly adaptive, i.e. whether geographical clines have underlying genetic basis. Otherwise, the clines may arise as a result of direct modifying effect of a habitat.     

Distribution of marine birnavirus (MABV) in marine organisms from Okinawa,Japan, and a unique sequence variation of the VP2/NS region

Distribution of marine type of Aquabirnavirus (MABV) was examined in shellfish and fish from Okinawa and Ishigaki Islands, Japan, where water temperature is higher than 25°C through the year. Genome detection and virus isolation were performed for shellfish and fish samples, and the results revealed the prevalent distribution of MABV in diverse species in the area, although isolation was not frequently. Detection rate of MABV genome in bivalves was higher than gastropods, which was similar result to former report in mainland of Japan. Furthermore, the unique five-nucleotide deletion was found with a high rate of occurrence in the MABV genome from shellfish and fish. This study showed distribution status of MABV in organisms in subtropical waters by wide monitoring, and discovered new genome variation in VP2/NS region of this virus.     

First results of a study of the feeding habits of <Emphasis Type="Italic">Synodontis</Emphasis><Emphasis Type="Italic">comoensis</Emphasis> (Siluriformes: Mochokidae) in a West African river (Comoé River,Comoé National Park,Côte d’Ivoire)

The feeding habits of Synodontis comoensis (Siluriformes, Mochokidae) have been studied for the first time. Specimens caught in the Comoé River (Comoé National Park, Côte d’Ivoire) had fed on different types of food including insects (Diptera, Ephemeroptera, Hymenoptera, and Isoptera), molluscs (gastropods and bivalves), oligochaetes, and macrophytes. The food items most frequently found in the stomach contents were chironomid larvae, oligochaetes, vegetable detritus, fruit, and gastropods. Correlation studies based on the index of preponderance of food items revealed noticeable variation in S. comoensis diet composition with fish size and season. Preferred food items of young fish were oligochaetes in the rainy season and vegetable detritus in the dry season. Food items preferred by larger fish were fruit and gastropods in the rainy season and chironomid larvae in the dry season.     

Molecular evidence to suggest the origin of a colonization: Drosophila subobscura in America

The recent colonization of America by Drosophila subobscura represents a great opportunity for evolutionary biology studies. Knowledge of the populations from which the colonization started would provide an understanding of how genetic composition changed during adaptation to the new environment. Thus, a 793 nucleotide fragment of the Odh (Octanol dehydrogenase) gene was sequenced in 66 chromosomal lines from Barcelona (western Mediterranean) and in 66 from Mt. Parnes (Greece, eastern Mediterranean). No sequence of Odh fragment in Barcelona or Mt. Parnes was identical to any of those previously detected in America. However, an Odh sequence from Barcelona differed in only one nucleotide from another found in American populations. In both cases, the chromosomal lines presented the same inversion: O(7), and the Odh gene was located within this inversion. This evidence suggests a possible western Mediterranean origin for the colonization. Finally, the molecular and inversion data indicate that the colonization was not characterized by multiple reintroductions.     

A novel role for dpp in the shaping of bivalve shells revealed in a conserved molluscan developmental program

During the molluscan evolution leading to the bivalves, the single dorsal shell was doubled. To elucidate the molecular developmental basis underlying this prominent morphological transition, we described the cell cleavage and expression patterns of three genes, brachyury, engrailed, and dpp in the Japanese spiny oyster Saccostrea kegaki, and examined the function of dpp in this species. The cleavage pattern of the S. kegaki embryo was nearly the same as the previously described pattern of other bivalve species, suggesting that the pattern itself is highly important for the establishment or the maintenance of the bivalve body plan. The expression pattern of a brachyury homolog in S. kegaki (SkBra) was similar to the pattern in gastopods even at the single cell level despite the deep divergence of gastropods and bivalves. Engrailed and dpp were previously found to be expressed around the shell anlagen in gastropods. Like that of gastropods, an engrailed homolog in S. kegaki (SkEn) was found to be expressed around the shell anlagen. However, the dpp homolog in S. kegaki (SkDpp) was expressed only in the cells along the dorsal midline. ZfBMP4 treatment experiments revealed the importance of dpp in establishing the characteristic shape of the bivalve shell anlagen.     

Diet and niche overlap of southern populations of brill Scophthalmus rhombus and turbot Scophthalmus maximus

The diets of adult brill Scophthalmus rhombus and turbot Scophthalmus maximus from the Portuguese coast relied mostly on fishes. There was a higher diversity of food items compared to their northern counterparts, and several of the identified prey are the first records of these species, including a brown alga, echinoderms, nematodes, oligochaetes, gastropods, bivalves and various fish species. The diet of the two species was significantly different and niche overlap was low.     

The vertical distribution and abundance of gastropods and bivalves from rocky beaches of Cuastecomate Bay, Jalisco, México

The vertical distribution and abundance of conspicuous gastropod and bivalve species were studied at five rocky beaches in Cuastecomate Bay, Jalisco. Sampling was done from September, 1993 through March, 1994 with 0.75 m2 quadrants placed along replicate transect lines (10 m long) in the supralittoral and mesolittoral (upper, middle and lower intertidal) zones. A total of 6,643 mollusks were collected. Gastropods dominated the samples (6,272 individuals, 44 species); the bivalves were less abundant and diverse (371 individuals, five species). Seventeen species comprised 89.8% of all individuals collected. The gastropods Nodilittorina aspera and Nerita scabricosta were the most abundant with 637.8 and 71.43 individuals/m2, respectively. The most abundant bivalves were Brachidontes adamsianus and Chloromytilus palliopunctatus with 60.7 and 61.3 individuals/m2 respectively. The abundance of gastropods decreased from the supralittoral to the lower tidal zones while the number of species increased in the same direction. The number of species of bivalves also increased from the supralittoral to the lower intertidal zone; the abundance of individuals was higher at the middle intertidal zone. Affinities between groups of species among sampling stations were identified by computing Pearson's correlation coefficient using abundance values (ind./m2) and Jaccard's dissimilarity index using species presence or absence in the lower intertidal zone. Affinity among stations was not dependent upon their vicinity but on the high dominance of few species, the occurrence of many secondary species and beach characteristics.     

The Small-Subunit rRNA Gene Sequences of Venerids and the Phylogeny of Bivalvia

The complete nucleotide sequence of the 18S subunit of ribosomal DNA (rDNA) was determined for the venerid clams Callista chione (Pitarinae) and Venus verrucosa (Venerinae). Comparison of the new sequences with the published sequences of 1 annelid, 2 gastropods, 2 polyplacophorans, and 19 bivalves showed that when the annelids are used as outgroup the gastropods diverge from the bivalves, which form a cluster including the polyplacophorans. When the gastropods alone were compared with the bivalves, the latter split in two groups corresponding to the two subclasses of Heterodonta and Pteriomorpha. The former include two taxa that diverged early, Galeomma and Tridacna, while the Veneridae and Mactridae form two sister groups. In contrast to previous reports and in line with morphological data, the Ostreidae are included in the Pteriomorphia and form a monophyletic group. Received: 16 May 1998 / Accepted: 11 August 1998