Reproductive biology of Donax striatus (Linnaeus, 1758) (Mollusca: Bivalvia) on Gado Bravo Beach, municipality of Tibau do Norte, state of Rio Grande do Norte, Brazil.

Aspects of the reproductive biology of Donax striatus were studied from individuals collected from Gado Bravo Beach in the municipality of Tibau do Norte, state of Rio Grande do Norte, Brazil. Donax striatus is a dioic species without external (on the shell) or internal (gonads) macroscopic dimorphism. Thus, a microscopic examination of the reproductive cells is necessary. For the characterization of the gonadal development stages and determination of the size at first sexual maturity (L50), 30 specimens were selected monthly between February 2021 and January 2022 and submitted to histological processing. The condition index (CI) of each individual was estimated and monthly variations were statistically assessed. The size at first maturity (L50) was estimated to be 14.2 mm in shell length. To foster conservation of the species, catches of individuals larger than 14.2 mm is recommended. The lowest condition indices were found in the dry season, with a greater occurrence of organisms in the elimination stage and exhibiting gonad tissue reorganization. Higher indices were found in the rainy season, with the presence of mature individuals. The continuous nature of gametogenesis in Donax stritatus reflects the influence of rainfall in the region. Males and females have peak gamete elimination with pauses during the year, but with the presence of maturing and eliminating individuals throughout the year. As shellfish gathering targeting Donax striatus is excessive on Gado Bravo Beach in the state of Rio Grande do Norte, it is hoped that the results of the present study can contribute to the establishment of management measures for the activity and conservation strategies for the species.

Thyasirid species composition (Bivalvia: Thyasiridae) and genetic connectivity of Parathyasira equalis (A. E. Verrill & K. J. Bush, 1898) in deep basins of sub-Arctic fjords.

BACKGROUND: Thyasirid bivalves are often recorded as a dominant component of macrobenthic infaunal communities in depositional environments such as fjord basins. Fjord basins comprise patchy soft-bottom habitats bounded by steep walls and sills; however, little is known how this semi-isolated nature of fjords affects benthic populations. Accordingly, data on the composition and population connectivity of thyasirids can provide valuable information on the ecology of these ecosystems. RESULTS: The species composition of thyasirid bivalves has been studied in the basins of three sub-Arctic fjords (Nordland, Northern Norway). Overall, six thyasirid species were recorded: Parathyasira equalis, Parathyasira dunbari, Mendicula ferruginosa, Genaxinus eumyarius, Thyasira sarsii, and Thyasira obsoleta. The species composition remained stable within the basins during the sampling period (2013-2020) and suggested the importance of local reproduction over advection of individuals for population dynamics. Only one species, Parathyasira equalis, was common in all fjords. We have further investigated the population genetics of this species by combining two types of genetic markers: a 579 bp fragment of the cytochrome c oxidase subunit I (COI) gene and 4043 single-nucleotide polymorphisms (SNPs) generated by genotyping-by-sequencing. The latter provided a more in-depth resolution on the population genetics of this species and revealed a weak but significant differentiation of populations within fjords, further indicating limited connectivity between basins. CONCLUSION: Based on our findings, we conclude that limited dispersal between the basin communities results in weakly connected populations and might be an important structuring factor for macrobenthic communities.

Doubling the known diversity of a remote island fauna: marine bivalves of the Juan Fernández and Desventuradas oceanic archipelagos (Southeastern Pacific Ocean).

Juan Fernández and Desventuradas are two oceanic archipelagos located in the southeastern Pacific Ocean far off the Chilean coast that received protected status as marine parks in 2016. Remoteness and access difficulty contributed to historically poor biodiversity sampling and limited associated research. This is particularly noticeable for bivalves, with most prior regional publications focused on single taxa or un-illustrated checklists. This study investigates marine bivalves collected between the intertidal and 415 m depth during (1) the 1997 IOC97 expedition aboard the M/V Carlos Porter, with special focus on scuba-collected micro-mollusks of both archipelagos, (2) two expeditions by the R/V Anton Bruun (Cruise 12/1965 and Cruise 17/1966), and (3) Cruise 21 of USNS Eltanin under the United States Antarctic Program, which sampled at Juan Fernández in 1965. Also, relevant historical material of the British H.M.S. Challenger Expedition (1873-1876), the Swedish Pacific Expedition (1916-1917), and by German zoologist Ludwig H. Plate (1893-1895) is critically revised. A total of 48 species are recognized and illustrated, including 19 new species (described herein) and six other potentially new species. The presence of two species mentioned in the literature for the region (Aulacomya atra and Saccella cuneata) could not be confirmed. The genera Verticipronus and Halonympha are reported for the first time from the Eastern Pacific, as are Anadara and Condylocardia from Chilean waters. Lectotypes are designated for Arca (Barbatia) platei and Mytilus algosus. These findings double the number of extant bivalve species known from the Juan Fernández and Desventuradas archipelagos, highlighting the lack of attention these islands groups have received in the past. A high percentage of species endemic to one or both archipelagos are recognized herein, accounting for almost 78% of the total. The newly recognized level of bivalve endemism supports the consideration of Juan Fernández and Desventuradas as two different biogeographic units (Provinces or Ecoregions) of the Eastern Pacific Ocean.

Structure, Evolution, and Mitochondrial Genome Analysis of Mussel Species (Bivalvia, Mytilidae).

Based on the nucleotide sequences of the mitochondrial genome (mitogenome) of specimens taken from two mussel species (Arcuatula senhousia and Mytilus coruscus), an investigation was performed by means of the complex approaches of the genomics, molecular phylogenetics, and evolutionary genetics. The mitogenome structure of studied mussels, like in many other invertebrates, appears to be much more variable than in vertebrates and includes changing gene order, duplications, and deletions, which were most frequent for tRNA genes; the mussel species' mitogenomes also have variable sizes. The results demonstrate some of the very important properties of protein polypeptides, such as hydrophobicity and its determination by the purine and pyrimidine nucleotide ratio. This fact might indirectly indicate the necessity of purifying natural selection for the support of polypeptide functionality. However, in accordance with the widely accepted and logical concept of natural cutoff selection for organisms living in nature, which explains its action against deleterious nucleotide substitutions in the nonsynonymous codons (mutations) and its holding of the active (effective) macromolecules of the polypeptides in a population, we were unable to get unambiguous evidence in favor of this concept in the current paper. Here, the phylogeny and systematics of mussel species from one of the largest taxons of bivalve mollusks are studied, the family known as Mytilidae. The phylogeny for Mytilidae (order Mytilida), which currently has no consensus in terms of systematics, is reconstructed using a data matrix of 26-27 mitogenomes. Initially, a set of 100 sequences from GenBank were downloaded and checked for their gender: whether they were female (F) or male (M) in origin. Our analysis of the new data confirms the known drastic differences between the F/M mitogenome lines in mussels. Phylogenetic reconstructions of the F-lines were performed using the combined set of genetic markers, reconstructing only protein-coding genes (PCGs), only rRNA + tRNA genes, and all genes. Additionally, the analysis includes the usage of nucleotide sequences composed of other data matrices, such as 20-68 mitogenome sequences. The time of divergence from MRCA, estimated via BEAST2, for Mytilidae is close to 293 Mya, suggesting that they originate in the Silurian Period. From all these data, a consensus for the phylogeny of the subfamily of Mytilinae and its systematics is suggested. In particular, the long-debated argument on mussel systematics was resolved as to whether Mytilidae, and the subfamily of Mytilinae, are monophyletic. The topology signal, which was strongly resolved in this paper and in the literature, has refuted the theory regarding the monophyly of Mytilinae.

Testing ultraconserved elements (UCEs) for phylogenetic inference across bivalves (Mollusca: Bivalvia).

Bivalves constitute an important resource for fisheries and as cultural objects. Bivalve phylogenetics has had a long tradition using both morphological and molecular characters, and genomic resources are available for a good number of commercially important species. However, relationships among bivalve families have been unstable and major conflicting results exist between mitogenomics and results based on Sanger-based amplicon sequencing or phylotranscriptomics. Here we design and test an ultraconserved elements probe set for the class Bivalvia with the aim to use hundreds of loci without the need to sequence full genomes or transcriptomes, which are expensive and complex to analyze, and to open bivalve phylogenetics to museum specimens. Our probe set successfully captured 1,513 UCEs for a total of 263,800 bp with an average length of 174.59 ± 3.44 per UCE (ranging from 28 to 842 bp). Phylogenetic testing of this UCE probe set across Bivalvia and within the family Donacidae using different data matrices and methods for phylogenetic inference shows promising results at multiple taxonomic levels. In addition, our probe set was able to capture large numbers of UCEs for museum specimens collected before 1900 and from DNAs properly stored, of which many museums and laboratories are well stocked. Overall, this constitutes a novel and useful resource for bivalve phylogenetics.

New records of non-indigenous species from the eastern Mediterranean Sea (Crustacea, Mollusca), with a revision of genus Isognomon (Mollusca: Bivalvia).

We report new data on non-indigenous invertebrates from the Mediterranean Sea (four ostracods and 20 molluscs), including five new records for the basin: the ostracods Neomonoceratina iniqua, Neomonoceratina aff. mediterranea, Neomonoceratina cf. entomon, Loxoconcha cf. gisellae (Arthropoda: Crustacea)-the first records of non-indigenous ostracods in the Mediterranean-and the bivalve Striarca aff. symmetrica (Mollusca). Additionally, we report for the first time Electroma vexillum from Israel, and Euthymella colzumensis, Joculator problematicus, Hemiliostraca clandestina, Pyrgulina nana, Pyrgulina microtuber, Turbonilla cangeyrani, Musculus aff. viridulus and Isognomon bicolor from Cyprus. We also report the second record of Fossarus sp. and of Cerithiopsis sp. cf. pulvis in the Mediterranean Sea, the first live collected specimens of Oscilla galilae from Cyprus and the northernmost record of Gari pallida in Israel (and the Mediterranean). Moreover, we report the earliest records of Rugalucina angela, Ervilia scaliola and Alveinus miliaceus in the Mediterranean Sea, backdating their first occurrence in the basin by 3, 5 and 7 years, respectively. We provide new data on the presence of Spondylus nicobaricus and Nudiscintilla aff. glabra in Israel. Finally, yet importantly, we use both morphological and molecular approaches to revise the systematics of the non-indigenous genus Isognomon in the Mediterranean Sea, showing that two species currently co-occur in the basin: the Caribbean I. bicolor, distributed in the central and eastern Mediterranean, and the Indo-Pacific I. aff. legumen, at present reported only from the eastern Mediterranean and whose identity requires a more in-depth taxonomic study. Our work shows the need of taxonomic expertise and investigation, the necessity to avoid the unfounded sense of confidence given by names in closed nomenclature when the NIS belong to taxa that have not enjoyed ample taxonomic work, and the necessity to continue collecting samples-rather than relying on visual censuses and bio-blitzes-to enable accurate detection of non-indigenous species.

[Structure and Evolution of DNA Transposons of the L31 Superfamily in Bivalves].

The mobile genetic elements IS630/Tc 1/mariner (ITm) are widespread DNA transposons that make a significant contribution to the evolution of eukaryotic genomes. With the start of large-scale application of next-generation sequencing (NGS) technologies and the emergence of many new whole genome sequences of organisms in nucleotide sequence collections, the ITm elements have been identified in most taxa of the eukaryotic tree of life. Although ITm diversity has been studied in detail, new elements are still found, thus expanding the respective DNA transposon group and calling for review of its classification. Bivalve L31 elements were for the first time analyzed in detail to describe their structures, diversity, distribution, and phylogenetic position among the ITm elements. The L31 transposons were found to form an independent superfamily of an ancient origin within the ITm group. Rather high diversity was observed within the L31 clade; i.e., five phylogenetic clusters were identified. In mollusks, the L31 transposons have been detected only in the subclass Autobranchia and predominate in diversity and number in the infraclass Pteriomorphia. A protein encoded by open reading frame 2 (ORF2) was shown to be an integral structural component of almost all full-length L31 elements. The results provide for a better understanding of the evolution of particular ITm transposons. Further study of the L31 transposons in other taxa (cnidarians) and functional investigation of the ORF2 protein product will help to better understand the evolution of DNa transposons, the mechanisms of their horizontal transfer, and their contribution to eukaryotic biodiversity.

The Chinese pond mussel Sinanodonta woodiana demographically outperforms European native mussels.

Unionid mussels are essential for the integrity of freshwater ecosystems but show rapid worldwide declines. The large-sized, thermophilic Chinese pond mussel Sinanodonta woodiana s.l., however, is a successful global invader, spread with commercially traded fish encysted with mussel larvae; its negative impacts on native mussels are expected. Here, we exploit a natural experiment provided by a simultaneous introduction of S. woodiana and four species of native unionids for water filtration to a pond in north-eastern Poland. Sinanodonta woodiana established a self-sustaining population and persisted for 19 years in suboptimal thermal conditions (mean annual temperature, 7.4 °C; mean temperature of the coldest month, - 3.7 °C, 73-day mean yearly ice-formation), extending the known limits of its cold tolerance. Over four study years, its frequency increased, and it showed higher potential for population growth than the native mussels, indicating possible future dominance shifts. Outbreaks of such sleeper populations are likely to be triggered by increasing temperatures. Additionally, our study documents the broad tolerance of S. woodiana concerning bottom sediments. It also points to the importance of intentional introductions of adult individuals and the bridgehead effect facilitating its further spread. We argue that S. woodiana should be urgently included in invasive species monitoring and management programmes.

Global biogeography of chemosynthetic symbionts reveals both localized and globally distributed symbiont groups.

In the ocean, most hosts acquire their symbionts from the environment. Due to the immense spatial scales involved, our understanding of the biogeography of hosts and symbionts in marine systems is patchy, although this knowledge is essential for understanding fundamental aspects of symbiosis such as host-symbiont specificity and evolution. Lucinidae is the most species-rich and widely distributed family of marine bivalves hosting autotrophic bacterial endosymbionts. Previous molecular surveys identified location-specific symbiont types that "promiscuously" form associations with multiple divergent cooccurring host species. This flexibility of host-microbe pairings is thought to underpin their global success, as it allows hosts to form associations with locally adapted symbionts. We used metagenomics to investigate the biodiversity, functional variability, and genetic exchange among the endosymbionts of 12 lucinid host species from across the globe. We report a cosmopolitan symbiont species, Candidatus Thiodiazotropha taylori, associated with multiple lucinid host species. Ca. T. taylori has achieved more success at dispersal and establishing symbioses with lucinids than any other symbiont described thus far. This discovery challenges our understanding of symbiont dispersal and location-specific colonization and suggests both symbiont and host flexibility underpin the ecological and evolutionary success of the lucinid symbiosis.

Molecular Characterization of Complement Component 3 (C3) in the Pearl Oyster Pinctada fucata Improves Our Understanding of the Primitive Complement System in Bivalve.

As the central component in the complement system, complement component 3 (C3) plays essential roles in both the innate and adaptive immune responses. Here, a C3 gene (designated as pf-C3) was obtained from the pearl oyster Pinctada fucata by RT-PCR and rapid amplification of cDNA ends (RACE). The pf-C3 cDNA consists of 5,634 bp with an open reading frame (ORF) of 5,193 bp encoding a protein of 1,730 amino acids with a 19 residue signal peptide. The deduced pf-C3 protein possessed the characteristic structural features present in its homologs and contained the A2M_N_2, ANATO, A2M, A2M_comp, A2M_recep, and C345C domains, as well as the C3 convertase cleavage site, thioester motif, and conserved Cys, His, and Glu residues. Phylogenetic analysis revealed that pf-C3 is closely related to the C3s from other mollusks. Pf-C3 mRNA was expressed in all examined tissues including gill, digestive gland, adductor muscle, mantle and foot, while the highest expression was found in the digestive gland. Following the challenge with Vibrio alginolyticus, pf-C3 expression was significantly induced in hemocytes. Luciferase reporter assays indicated that pf-C3a could activate the NF-κB signal pathway in HEK293T cells. Further knockdown of pf-C3 by specific siRNA could significantly reduce the phagocytosis of V. alginolyticus by hemocytes in vitro. These results would help increase understanding of the function of C3 in the invertebrate immune system and therefore provide new insights into the roles of the primitive complement system in invertebrates.

Systematic redescription of Solen/ (Ensisolen/) tehuelchus/ and Ensis/ macha/ (Bivalvia: Solenoidea) from Argentina, southwestern Atlantic Ocean.

In the present study, bivalves belonging to the superfamily Solenoidea that inhabit the Argentinean Sea are redescribed. The specimens studied in this work were collected by scuba diving in the Gulf of San José, Argentina. Additional specimens from different malacological collections were revised to update the geographical distribution of valid species. According to the literature, there exist two species for the "Argentinean" and "Magellan" biogeographical provinces. However, ten nominal taxa, with uncertain taxonomic status, have been mentioned for the study area. Field work, local collection survey and type material examination allow confirming two valid species: Solen (Ensisolen) tehuelchus Hanley (Solenidae) and Ensis macha (Molina) (Pharidae). A morphological comparison between S. tehuelchus and S. gaudichaudi Chenu and S. obliqua Spengler is provided as well type locality, repository and synonymy list. The type material of all related species are illustrated whenever possible. Establishing the valid names of these species is the first step towards more integrative studies including genetic and zoogeographical analyses.

Phylogenetic Relationships and Adaptation in Deep-Sea Mussels: Insights from Mitochondrial Genomes.

Mitochondrial genomes (mitogenomes) are an excellent source of information for phylogenetic and evolutionary studies, but their application in marine invertebrates is limited. In the present study, we utilized mitogenomes to elucidate the phylogeny and environmental adaptation in deep-sea mussels (Mytilidae: Bathymodiolinae). We sequenced and assembled seven bathymodioline mitogenomes. A phylogenetic analysis integrating the seven newly assembled and six previously reported bathymodioline mitogenomes revealed that these bathymodiolines are divided into three well-supported clades represented by five Gigantidas species, six Bathymodiolus species, and two "Bathymodiolus" species, respectively. A Common interval Rearrangement Explorer (CREx) analysis revealed a gene order rearrangement in bathymodiolines that is distinct from that in other shallow-water mytilids. The CREx analysis also suggested that reversal, transposition, and tandem duplications with subsequent random gene loss (TDRL) may have been responsible for the evolution of mitochondrial gene orders in bathymodiolines. Moreover, a comparison of the mitogenomes of shallow-water and deep-sea mussels revealed that the latter lineage has experienced relaxed purifying selection, but 16 residues of the atp6, nad4, nad2, cob, nad5, and cox2 genes have underwent positive selection. Overall, this study provides new insights into the phylogenetic relationships and mitogenomic adaptations of deep-sea mussels.

A High-Quality Reference Genome for a Parasitic Bivalve with Doubly Uniparental Inheritance (Bivalvia: Unionida).

From a genomics perspective, bivalves (Mollusca: Bivalvia) have been poorly explored with the exception for those of high economic value. The bivalve order Unionida, or freshwater mussels, has been of interest in recent genomic studies due to their unique mitochondrial biology and peculiar life cycle. However, genomic studies have been hindered by the lack of a high-quality reference genome. Here, I present a genome assembly of Potamilus streckersoni using Pacific Bioscience single-molecule real-time long reads and 10X Genomics-linked read sequencing. Further, I use RNA sequencing from multiple tissue types and life stages to annotate the reference genome. The final assembly was far superior to any previously published freshwater mussel genome and was represented by 2,368 scaffolds (2,472 contigs) and 1,776,755,624 bp, with a scaffold N50 of 2,051,244 bp. A high proportion of the assembly was comprised of repetitive elements (51.03%), aligning with genomic characteristics of other bivalves. The functional annotation returned 52,407 gene models (41,065 protein, 11,342 tRNAs), which was concordant with the estimated number of genes in other freshwater mussel species. This genetic resource, along with future studies developing high-quality genome assemblies and annotations, will be integral toward unraveling the genomic bases of ecologically and evolutionarily important traits in this hyper-diverse group.

Natural hybridization between pen shell species: Pinna rudis and the critically endangered Pinna nobilis may explain parasite resistance in P. nobilis.

Recently, Pinna nobilis pen shells population in Mediterranean Sea has plummeted due to a Mass Mortality Event caused by an haplosporidian parasite. In consequence, this bivalve species has been included in the IUCN Red List as "Critically Endangered". In the current scenario, several works are in progress to protect P. nobilis from extinction, being identification of hybrids (P. nobilis x P. rudis) among survivors extremely important for the conservation of the species.Morphological characteristics and molecular analyses were used to identify putative hybrids. A total of 10 individuals of each species (P. nobilis and P. rudis) and 3 doubtful individuals were considered in this study. The putative hybrids showed shell morphology and mantle coloration intermingled exhibiting both P. nobilis and P. rudis traits. Moreover, the analyses of 1150 bp of the 28S gene showed 9 diagnostic sites between P. rudis and P. nobilis, whereas hybrids showed both parental diagnostic alleles at the diagnostic loci. Regarding the multilocus genotypes from the 8 microsatellite markers, the segregation of two Pinna species was clearly detected on the PCoA plot and the 3 hybrids showed intermediate positions.This is the first study evidencing the existence of hybrids P. nobilis x P. rudis, providing molecular methodology for a proper identification of new hybrids. Further studies testing systematically all parasite-resisting isolated P. nobilis should be undertaken to determine if the resistance is resulting from introgression of P. rudis into P. nobilis genome and identifying aspects related to resistance.

Host Diversity Affects Parasite Diversity: A Case Study Involving Unionicola spp. Inhabiting Freshwater Mussels.

Water mites of the genus Unionicola are common parasites of freshwater mussels, living on the gills or mantle of their hosts and using these tissues as sites of oviposition. Although surveys of this mite fauna among North American unionid mussels indicate that these mites represent highly diverse assemblages, we know very little regarding the determinants of Unionicola species diversity among their molluscan hosts. The present study addresses the relationship between host diversity and mite diversity for Unionicola assemblages associated with unionid mussels of North America. The results of this study found a significantly positive relationship between host species richness and mite species richness, adding to a growing body of evidence that host diversity is an important determinant of parasite diversity. In recent years, molecular sequence data have discovered cryptic biodiversity among unionid mussels, yielding revisions in the nomenclature and systematic taxonomy of the group. DNA sequence variation has also revealed cryptic species complexes among Unionicola mites. Collectively, these findings suggest that the results of the present study may be underestimating species richness among mites and their host mussels. Unfortunately, human perturbations are known to have caused high recent rates of extinction in the mussel and mite faunas of North America and could play a major role in influencing patterns of species richness for this host-parasite system moving forward.

Comparative mitogenomic analysis of the superfamily Tellinoidea (Mollusca: Bivalvia): Insights into the evolution of the gene rearrangements.

The superfamily Tellinoidea is widespread and contains approximately 180 living species, which is one of the most diverse and representative groups among the bivalves. In order to extend our knowledge on evolution of tellinoidean species, we newly determined five tellinoidean mitochondrial genomes (mitogenomes). The newly determined mitogenome vary in size from 16,333 to 16,986 bp. The results show that the genome size and genome organization are conserved in tellinoideans. However, gene arrangement and the location of the major non-coding region (NCR) show diversity. The atp8 gene presents in all the five new mitogenomes. Two trnK and trnP genes were detected in Gari togata mitogenome. Phylogenetic analysis supports the monophyly of Tellinoidea, however, it's family Psammobiidae is polyphyletic. CREx analysis suggests that the gene order of Nuttallia olivacea is assumed as the most primitive condition of Tellinoidea. We map the gene order onto the phylogeny and infers the possible gene rearrangement scenarios among tellinoidean mitogenomes. The mitochondrial gene rearrangement is a useful information that help reassessing the phylogeny of Tellinoidea. Phylogenetic relationship and gene arrangement analyses suggest that a careful review for the current taxonomy of the family Psammobiidae is required.

Genome-wide identification, characterisation and expression analysis of the ALAS gene in the Yesso scallop (Patinopecten yessoensis) with different shell colours.

Porphyrins, one of the most common shell pigments, are by-products of the haem pathway. 5-Aminolaevulinate synthase (ALAS) is the first and rate-limiting enzyme in this pathway and has been well studied in vertebrate species. However, the function of ALAS in shell colouration has been poorly studied in molluscs, which are renowned for their colourful shells. In the present study, an ALAS gene, named PyALAS, was identified through whole-genome scanning in the Yesso scallop (Patinopecten yessoensis), an economically and evolutionarily important bivalve species in which the shell colour represents polymorphism. Two conserved domains were detected in the PyALAS protein sequence, including a Preseq-ALAS domain and a 5-ALAS domain, confirming the identification of PyALAS. Phylogenetic analysis of the ALAS proteins among various invertebrate and vertebrate species revealed a high consistency between the molecular evolution of ALAS and the species taxonomy. PyALAS was ubiquitously expressed in most adult tissues of the Yesso scallop. The left mantle expressed a significantly higher level of PyALAS than the right side in brown scallops, whereas there was no significant difference in white scallops. Significantly different expression levels of PyALAS was also detected between the two different shell colour strains. These data indicate that PyALAS plays an important role in shell colouration in Yesso scallops and the present study provides new insights into the molecular mechanism of shell colouration in molluscs.

Massive bioconstructions built by Neopycnodonte cochlear (Mollusca, Bivalvia) in a mesophotic environment in the central Mediterranean Sea.

The present paper provides a multidisciplinary fine-scale description of a Mediterranean mesophotic new habitat dominated by the bivalve Neopycnodonte cochlear (Poli, 1795), building large and thick pinnacles on vertical cliffs at two study areas along the southern Italian coast. The pinnacles, constituted by a multilayered aggregation of living and dead specimens of N. cochlear, were interconnected with each other to form a framework of high structural complexity, never observed before for this species. The bioconstruction, considerably extended, resulted very complex and diversified in the associated community of structuring organisms. This latter included 165 taxa attributable to different ecological groups occurring in different microhabitats of the bioconstruction. Among the secondary structuring taxa there were scleractinians, serpulids and bryozoans, all contributing to the deposition of calcium carbonate, and poriferans, helping to bind shells together or eroding carbonate by boring species. In comparison with coralligenous sensu stricto and the recently described Mediterranean mesophotic coral reef, the Neopycnodonte bioconstruction showed peculiar features, since it lacked the major contribution of encrusting coralline algae and scleractinians as reef builders, respectively.

Embryo and larval development of the yellow clam Mesodesma mactroides (Reeve, 1854) (Mesodesmatidae) in laboratory.

The yellow clam Mesodesma mactroides (Reeve, 1854) is a sand mollusc with historical and socioeconomic importance in Brazil, Uruguay and Argentina. A guaranteed form to access a successful reestablishment of the species in their natural environment is directly linked to their reproduction biology. Then, our report introduces the embryonic and larval development of the yellow clam reared in laboratory for such purposes. M. mactroides broodstock were selected as specimens who possess a mean total shell length and weight of 66 ± 3.82 mm and 27.15 ± 4.07 g for an afterwards spawn induction through stripping technique. Regarding the embryonic development, newly fertilized oocytes exhibited a mean diameter of 51.20 ± 6.64 µm. The first polar corpuscle, trochophores and D-veliger appeared at 20 min, 18 and 24 h after fertilization, respectively. Umbonate and pediveliger larvae were noticed on the 8th and 25th day, respectively, with complete metamorphosis occurring only at the 27th day, when all larvae were retained in a 200 µm nylon mesh. Therefore, with that basic understanding of the embryonic and larval development of M. mactroides in the laboratory, forwards studies will focus in establish a technological package for this species.

New freshwater mussel taxa discoveries clarify biogeographic division of Southeast Asia.

While a growing body of modern phylogenetic research reveals that the Western Indochina represents a separate biogeographic subregion having a largely endemic freshwater fauna, the boundaries of this subregion are still unclear. We use freshwater mussels (Unionidae) as a model to reconstruct spatial patterns of freshwater biogeographic divides throughout Asia. Here, we present an updated freshwater biogeographic division of mainland Southeast Asia and describe 12 species and 4 genera of freshwater mussels new to science. We show that the Isthmus of Kra represents a significant southern biogeographic barrier between freshwater mussel faunas of the Western Indochina and Sundaland subregions, while the Indian and Western Indochina subregions are separated by the Naga Hills, Chin Hills, and Rakhine Yoma mountain ranges. Our findings highlight that the freshwater bivalve fauna of Southeast Asia primarily originated within three evolutionary hotspots (Western Indochina, Sundaland, and East Asian) supplemented by ancient immigrants from the Indian Subcontinent.