Arcticidae

Abstract: Mitochondrial DNA (mtDNA) is strictly maternally inherited in metazoans. The major exception to this rule has been found in many bivalve species which allow the presence of different sex-linked mtDNA molecules. This mechanism, named doubly uniparental inheritance (DUI), is characterized by the presence of two mtDNAs: The female mtDNA is found in somatic tissue and female gonads, whereas the male mtDNA is usually found in male gonads and sperm. In this study we highlight the existence of two divergent mitochondrial haplotypes with a low genetic difference around 6–8% in Arctica islandica, a long-lived clam belonging to the Arcticidae, a sister group to the Veneridae in which DUI has been found. Phylogenetic analysis on cytochrome b and 16S sequences from somatic and gonadic tissues of clams belonging to different populations reveals the presence of the “divergent” type in male gonads only and the “normal” type in somatic tissues and female gonads. This peculiar segregation of divergent mtDNA types speaks for the occurrence of the DUI mechanism in A. islandica. This example also highlights the difficulties to assess the presence of such particular mitochondrial inheritance system and underlines the possible misinterpretations in phylogeographic and phylogenetic studies of bivalve species linked to the presence of two poorly differentiated mitochondrial genomes.

Abstract: In a recent review, Morton (2011, ‘The biology and functional morphology of Arctica islandica (Bivalvia: Arcticidae) – A gerontophilic living fossil’, Marine Biology Research 7:540–53) provides a detailed description of the function of the anatomy of the long-lived Arctica islandica. In the abstract, Morton concludes that the indolent lifestyle and tissue antioxidant levels sustained into gerontocy predisposes it to negligible senescence and states stable tissue antioxidant levels may slow senescence and extend lifespan. While briefly reviewing only a sub-section of the literature on antioxidants in bivalves, Morton further states that high antioxidant capacities may explain the long lifespan of A. islandica. Recent research demonstrates that such statements are misleading or unsubstantiated. Genetic manipulations of antioxidant expression in a range of species and comparisons of antioxidant activities in longer-lived species compared to shorter-lived relatives produces inconsistent results, but ...

Abstract: Molecular analyses of 13 species of the marine bivalve family Thyasiridae, using sequences from 18S rRNA and 28S rRNA genes, showed that the family is monophyletic despite the anatomical disparity and inclusion of both chemosymbiotic and asymbiotic species. This new analysis also confirmed that the three families (Thyasiridae, Lucinidae and Ungulinidae), previously included in the Lucinoidea, were not closely related. Four species of Ungulinidae grouped within a clade containing Veneridae, Arcticidae and Mactridae. In relation to a range of other heterodont bivalves, Thyasiridae occupied a near basal position, apart from a clade comprising Carditidae/Astartidae/Crassatellidae. The earliest thyasirid recognized in the fossil record is a species from the Lower Cretaceous of England. Within the Thyasiridae, some groups can be identified but relations between these are weakly supported. Amongst the taxa analysed, those with symbiotic bacteria and two ctenidial demibranchs belong to at least three groups, while there is some support for a clade of asymbiotic taxa with single demibranchs. In recognition of the monophyletic status of the Thyasiridae, distinct from all other heterodont bivalves, we elevate the rank to superfamily Thyasiroidea.

Abstract: Bivalves of the families Arcticidae, Isocyprinidae n. fam., and Veneridae are described from the Middle‐Late Jurassic of New Zealand and New Caledonia. Morphology of shells from several genera in Arcticidae, Isocyprinidae, Veniellidae n. fam., and Veneridae is analysed in detail. These families are shown to be closely related in terms of morphological shell characters, especially ontogenetic and evolutionary hinge development. This is highlighted by identical ranges in commissural outline and shell form together with similar strength and form of ornamentation, and the fact that hinge plate development is an uninterrupted evolutionary process between Arcticidae and Isocyprinidae to Veneridae. These converging lineages show that Veneridae is at least diphyletic. This same evolutionary process is also shown to be an ontogenetic function of shells, with the final state of hinge development limited by maximum shell size. The family Corbiculidae is considered to be a major offshoot from one of the two ...