No abstract

Abstract. In this study, we investigated ostracod assemblages from the Co To Islands in northeastern Vietnam. We identified 77 ostracod species belonging to 46 genera in nine surface sediment samples and recognized three biofacies (I, II, and III) based on Q-mode cluster analysis. The dominant species of biofacies I and II were Aurila hataii, Loxoconcha japonica, and Xestoleberis hanaii, which commonly occur in seaweed beds from southern China to Japan. This is the first report on the ostracod assemblage from the open-sea area in northeastern Vietnam. We clarified that the ostracod assemblages in the Gulf of Tongking, including northern Vietnam, have a strong biogeographical relationship with East Asia seas. A new species, Loxoconcha cotoensis sp. nov., was described herein from the Co To Islands (http://www.zoobank.org/NomenclaturalActs/41d3fb9f-ae17-4215-82c1-0874a8bf1a30, last access: 3 June 2019).

Five species of decapod crustaceans, including Calliax nishiki sp. nov. (Axiidea, Eucalliacidae) and Carcinoplax longimana (de Haan), are described from the upper part of the Moeshima Silt Bed (latest Pleistocene to Holocene: ca 13,000 to 8,000 cal BP). Ten decapods, including Laticallichirus grandis (Karasawa & Goda) and Ebalia tuberculosa (A. Milne-Edwards) are described from the Moeshima Shell Bed (Holocene: ca 8,000 to 2,000 cal BP). Calliax nishiki is abundant and Carcinoplax longimana is common in the decapod assemblage of Moeshima Silt Bed. The new species of Calliax seems to have dispersed around the deep marine setting with a reducing environment. The decapod assemblage of the Moeshima Shell Bed is characterized by a predominance of L. grandis. The present record suggests that L. grandis is abundant under sandy and gravelly bottoms mixed with shells of the lower sublittoral to subtidal zones during the Quaternary.