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Three new deep-sea species of Thyasiridae (Mollusca: Bivalvia) from the northwestern Pacific Ocean
(2023)
The Thyasiridae is one of the species-richest families in the abyssal and hadal zones of the northwestern Pacific Ocean. Many thyasirid species dominate benthic communities in terms of abundance and play an important role in the functioning of deep-sea ecosystems. Most of the thyasirid species in the region are new to science and have not been described. Based on the material collected from 1954 to 2016 by seven deep-sea expeditions, three new species of Thyasiridae (Parathyasira coani sp. nov., P. pauli sp. nov., and Thyasira kharkovensis sp. nov.) are described from the abyssal and hadal zones (3210–7540 m depth) of the Sea of Okhotsk, the Bering Sea, as well as the Kuril-Kamchatka and Japan trenches. The new species are remarkable among their congeners due to the combination of the following characters: an obliquely-rhomboidal shell with a weak and shallow posterior sulcus and a large prodissoconch with sculpture of lamellated folds. Comparisons with related species are provided.
During the Census of Marine Life Polarstern ANDEEP I-III and Meteor M79/1 DIVA-3 expeditions, autonomous baited trap systems were employed to sample the mobile, necrophagous amphipods from abyssal depths. Within DIVA-3 (July 10–August 26 2009), a free-fall baited trap was used successfully at three stations in the southwest Atlantic, once in the Argentine Basin and twice in the Brazilian Basin. A total of twenty-one stations were sampled by baited traps during the ANDEEP I-III (2002, 2005) cruises in the Southern Ocean. Trap sets recovered large numbers of scavenging lysianassoid and alicelloid amphipods, including specimens of the widespread and commonly considered cosmopolitan uristid species Abyssorchomene abyssorum (Stebbing, 1888). During examinations of these and other North Atlantic collections of A. abyssorum, two similar new species A. patriciae sp. nov. and A. shannonae sp. nov. were discovered. Important morphological characters which differentiate the two new species from their congeners are found in the shape of the head lobe, coxa 1, gnathopod 2, coxa 5, pereopod 7 basis and uropod 3 rami length. The new species are fully figured and an identification key is provided. Abyssorchomene abyssorum is redescribed and for the first time, the female is fully described and illustrated from new material. The Southern Ocean endemic A. scotianensis (Andres, 1983) is also described and illustrated from new collections to complement the original description.