Leucothoid amphipod and terebellid polychaete symbiosis with description of a new species of the genus <i>Leucothoe</i> Leach, 1814 (Crustacea: Amphipoda: Leucothoidae)

The order Amphipoda is one of the largest orders in the Crustacea, many species of which are involved in symbiotic relations with other animals. Despite the considerable diversity of the Amphipoda both in number of species and ecology, polychaete-commensalism has been poorly known and described from few species. In particular, there has been little discussion of the evolutionary origins of polychaete-commensalism relationships. Amphipods in the family Leucothoidae are known as commensal inhabitants of filter-feeding invertebrates, where they utilize the feeding current produced by their hosts. Leucothoids are typically found from three types of filter-feeding hosts: sponges, ascidians, and bivalve molluscs. Relatively little is known about leucothoids that associate with other types of hosts. An undescribed species of the genus <i>Leucothoe</i> associated with burrows of terebellid polychaetes from Japan has been found. We herein describe this species as <i>Leucothoe vermicola</i> sp. nov., providing COI mtDNA and 18S rDNA sequences for DNA barcoding. This is the first record of a symbiotic association between Leucothoidae and Terebellidae. We also provide a hypothesis of the phylogenetic position of <i>L. vermicola</i> sp. nov. and evolution of the polychaete-commensalism in this species. The polychaete-commensalism in the present new species may have resulted from the entry of generalist species into polychaete hosts, rather than from host-conversion from a specialist species. http://zoobank.org/urn:lsid:zoobank.org:pub:4EBA42DD-EC19-4278-B712-8BEFBC519F8F端足目（Amphipoda）是甲壳纲（Crustacea）中最大的类群之一，多数物种与其他动物存在共生关联。尽管端足目在物种数量与生态习性上均呈现出显著的多样性，但学界对多毛类共栖（polychaete-commensalism）的认知仍较为薄弱，相关类群的记述仅见于少数物种。尤为值得关注的是，目前针对多毛类共栖关系的演化起源相关探讨极为匮乏。亮虾科（Leucothoidae）的端足类为滤食性无脊椎动物的共栖物种，可借助宿主产生的摄食流获取食物。亮虾类通常依附于三类滤食性宿主：海绵动物、被囊动物（ascidians）与双壳类软体动物（bivalve molluscs）。而针对依附其他类型宿主的亮虾类，目前的研究仍相对不足。本研究在日本海域发现了一种未被正式描述的亮虾属（Leucothoe）物种，该物种与蛰龙介科多毛类的栖管存在共生关系。本文将该物种正式命名为蠕形亮虾（*Leucothoe vermicola*）sp. nov.，并提供了用于DNA条形码（DNA barcoding）的细胞色素C氧化酶亚基I线粒体DNA（COI mtDNA）与18S核糖体DNA（18S rDNA）序列。本研究为亮虾科与蛰龙介科之间共生关系的首次正式记录。此外，本文还提出了蠕形亮虾（*L. vermicola*）sp. nov.的系统发育位置假说，以及该物种多毛类共栖习性的演化路径。本新种的多毛类共栖习性，可能源自广食性物种直接入驻多毛类宿主，而非由专食性物种通过宿主转换演化而来。http://zoobank.org/urn:lsid:zoobank.org:pub:4EBA42DD-EC19-4278-B712-8BEFBC519F8F