Three new deep-sea species of Thyasiridae (Mollusca: Bivalvia) from the abyssal plain of the northwestern Pacific Ocean and hadal depths of the Kuril-Kamchatka Trench

Abstract

The Thyasiridae is the most species-rich family of bivalves in the abyssal and hadal zones of the northwestern Pacific Ocean. In recent years, with at least 14 thyasirid species found in that region at depths exceeding 3,000 m. Some of them are the numerically dominant species in bottom communities. However, all members in that family have not yet been identified to the species level. Based on the material collected from 1953 to 2016 by five deep-sea expeditions, three new species of Thyasiridae (Mollusca: Bivalvia) are described from the abyssal and hadal zones of the northwestern Pacific. “Axinulus” roseus sp. nov. was found in the Kuril-Kamchatka Trench at 9,000–9,583 m depth. This species has a large rhomboidal shell with strong commarginal sculpture, a well defined, long and deep lunule and escutcheon without an auricle, a ctenidium consisting of a single demibranch, extensively lobed lateral pouches, and a large prodissoconch with specific sculpture. It is one of the dominant species in terms of abundance in macrobenthic communities in the deepest basin of the Kuril-Kamchatka Trench with a population density of up to 396 ind. m−2. The species has a shell length of up to 9.0 mm and it is the largest thyasirid with a single demibranch. “Axinulus” oliveri sp. nov. was found in a vast region of the northwestern Pacific on the abyssal plain adjacent to the Kuril-Kamchatka Trench, on the abyssal slope of the Kuril Islands, and in the Kuril-Kamchatka Trench at 4,648–6,168 m depth. This species is characterized by its ovate-rhomboidal shell, a well defined, deep and long escutcheon with a distinct auricle, a ctenidium with a single demibranch, and extensively lobed lateral pouches. It is widespread in the northwestern Pacific and forms populations with a density of up to 36 ind. m−2. Scanning electron microscopic observation of the gills of “A.” roseus sp. nov. and “A.” oliveri sp. nov. revealed that these species are not chemosymbiotic. “Axinulus” roseus sp. nov. and “A.” oliveri sp. nov. are provisionally assigned to the genus Axinulus, because they differ from the type species of the genus in a number of morphological and anatomical features. Parathyasira fragilis sp. nov. was found on the abyssal plain adjacent to the Kuril-Kamchatka Trench at 5,249–5,399 m depth. This species is distinguished by its very thin, fragile, dorsoventrally elongated, rhomboidal shell with very long anterodorsal margin and a long, wide, flat lunule. The taxonomic position of the new species is discussed.

Three new deep-sea species of Thyasiridae (Mollusca: Bivalvia) from the northwestern Pacific Ocean

“…In recent years, a study of these species was started by the author of the paper. As a first step, three new species were described from the abyssal and hadal zones of the Kuril-Kamchatka Trench region (Kamenev 2020). This paper is a continuation of the previous study and addresses another three, morphologically similar deep-sea species which are described herein as new.…”

“…Thyasirids are widespread in all world oceans, from shallow waters to depth exceeding 10 000 m (e.g., Filatova 1961;Belyaev 1989;Payne & Allen 1991;Coan et al 2000;Oliver & Kelleen 2002;Allen 2008Allen , 2015Zelaya 2009Zelaya , 2010Coan & Valentich-Scott 2012;Valentich-Scott et al 2014, 2020Kamenev 2015Kamenev , 2019Kamenev , 2020Oliver 2015). Recent studies have shown that thyasirids are one of the most species-rich families of bivalves in the Atlantic and the northwestern Pacific oceans at depths of more than 500 m (Allen 2008;Kamenev 2015Kamenev , 2018aKamenev , 2019.…”

“…Recent studies have shown that thyasirids are one of the most species-rich families of bivalves in the Atlantic and the northwestern Pacific oceans at depths of more than 500 m (Allen 2008;Kamenev 2015Kamenev , 2018aKamenev , 2019. They are often the dominant species in terms of abundance in deep-sea areas (Kamenev 2013(Kamenev , 2019(Kamenev , 2020Kamenev et al 2022). However, despite their significance in the deep-sea benthic communities, many thyasirids species have not been studied in detail (Allen 2008;Kamenev 2013Kamenev , 2015Kamenev , 2018aKamenev , 2019.…”

The Environmental Drivers of Benthic Fauna Diversity and Community Composition

“…Crustacea are widely reported as one of the most dominant components of species richness in the deep-water macrobenthos of the NW Pacific (Birstein, 1971;Kussakin and Vasina, 1990;Kussakin, 1999Kussakin, , 2003Brandt and Malyutina, 2015;Elsner et al, 2015;Golovan et al, 2018;Brandt et al, 2020) in addition to Polychaeta (Brandt et al, 2018) and Mollusca (Kamenev, 2019). Kamenev (2020) discovered that the Thyasiridae (Mollusca: Bivalvia) is the most species-rich family of bivalves in the abyssal and hadal zones of the NW Pacific; 14 species were recently found from depths exceeding 3,000 m with the deepest report between 9,000 and 9,583 m in the Kuril-Kamchatka Trench belonging to Axinulus roseus Kamenev, 2020 (Kamenev, 2020). Polychaetes, in general, are one of the most dominant and diverse groups of the macrofauna of the Pacific Ocean and play a major role in the functioning of benthic communities (Jumars et al, 2015;Bonifácio et al, 2020).…”

Macrofauna and Nematode Abundance in the Abyssal and Hadal Zones of Interconnected Deep-Sea Ecosystems in the Kuril Basin (Sea of Okhotsk) and the Kuril-Kamchatka Trench (Pacific Ocean)

“…The main goal of these expeditions was to study the species composition and distribution patterns of bottom animals, as well as to assess the faunal relationships in the deep-sea ecosystems of the Sea of Okhotsk and the Kuril-Kamchatka Trench separated by such a natural barrier as the Kuril Islands Brandt et al, 2019). As a result, numerous species from different taxonomic groups of animals were described, the composition of macro-and meiofauna and the patterns of distribution of the main taxonomic groups of animals were studied, the faunal relationship and levels of endemism of these deep-sea areas were estimated (e.g., Maiorova, 2018, 2019;Alalykina, 2018;Brandt et al, 2018Brandt et al, , 2019Polyakova, 2018, 2019;Fukumori et al, 2018Fukumori et al, , 2019Kamenev, 2018aKamenev, ,b, 2019Kamenev, , 2020Maiorova and Adrianov, 2018a,b;Brandt, 2018, 2020;Mironov et al, 2018Mironov et al, , 2019bBorisanova and Chernyshev, 2019;Brandão et al, 2019;Mordukhovich et al, 2019;Schmidt et al, 2019;Alalykina and Polyakova, 2020;Maiorova and Adrianov, 2020;Saeedi and Brandt, 2020). However, the analysis of the quantitative distribution of macro-and meiofauna in the Kuril Basin has not been completed, and only preliminary results of the macrofauna abundance study have been reported for the Kuril-Kamchatka Trench (Lins and Brandt, 2020).…”