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      “TUBULAR GILLS” Extreme gill modification in the Thyasiroidea with the description of Ochetoctena tomasi gen. et sp. nov. (Bivalvia: Thyasiroidea)

      Zoosystematics and Evolution

      Pensoft Publishers

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          Abstract

          Three dimensional tubular structures of the ctenidium of some thyasirid bivalves are described for the first time. The classification of the thyasirid gill is modified accordingly into five types based on the number of demibranchs, reflection of the filaments and shape of the filaments, either rod, laminar or tubular. The tubular structure is seen in its most modified form in a chemosymbiotic abyssal species from the south-east Atlantic, which is described here as Ochetoctena tomasi gen et sp. nov.

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          Most cited references 16

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          On the Basic Form and Adaptations to Habitat in the Lucinacea (Eulamellibranchia)

           J Allen (1958)
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            Gill anatomy and the evolution of symbiosis in the bivalve family Thyasiridae.

            Among families of bivalves with chemoautotrophic symbionts, the Thyasiridae may vary the most in their anatomical characters and in the extent of their nutritional reliance upon symbionts. Since only a fraction of thyasirid species are symbiotic, and the symbionts are mostly observed to be extracellular, this group may be representative of early stages in the evolution of bacterium- bivalve symbioses. To better understand the distribution of symbiosis among thyasirid genera, and the relationships between gill structure and symbiont occurrence, the gills of 26 thyasirid species were studied by light and electron microscopy. Observations revealed three gill types, which are generally constrained within genera or subgenera. Symbionts were found in two gill types: the most simple, homorhabdic filibranch morphotype, and the most derived and thickened morphotype, which resembles the gill structure of other chemosymbiotic bivalves. In all observable cases, the symbionts were located extracellularly among the microvilli of the bacteriocytes. Among individuals of the species Thyasira (Parathyasira) equalis, the quantity of symbionts varied. The results suggest an evolutionary sequence: a homorhabdic filibranch gill structure with few symbionts among the epithelial cell microvilli eventually thickened abfrontally, and thereby offered a larger surface for colonization by symbionts. Eventually, the symbionts persisted and grew in vacuoles within epithelial cells.
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              Sulphur-oxidising bacteria and haemoglobin in gills of the bivalve mollusc Myrtea spinifera

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                Author and article information

                Journal
                Zoosystematics and Evolution
                ZSE
                Pensoft Publishers
                1860-0743
                1435-1935
                October 01 2014
                October 01 2014
                : 90
                : 2
                : 121-132
                Article
                10.3897/zse.90.8323
                © 2014

                http://creativecommons.org/licenses/by/4.0/

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