A new species each of <i>Sicafodia</i> Just, 2004 (Amphipoda: Sicafodiidae) and of its parasite <i>Rhizorhina</i> Hansen, 1892 (Copepoda: Nicothoidae) from the deep bathyal off northeastern Japan

Abstract A new species of Sicafodiidae (Amphipoda) and a new associated copepod parasite of the family Nicothoidae are described from bathyal (200–3,500 m) depths in the Northwest Pacific Ocean. The sicafodiid can be distinguished by several features, including left and right mandibles respectively having a bi-dentate and tri-dentate incisor and an acute lacinia mobilis; the inner palp of maxilla 1 having two slender setae; coxa 4 being slightly longer than wide with a subacute ventral margin; and the telson having three slender setae and a deep notch. The associated parasite, Rhizorhina sicafodiaesp. nov., is the tenth known species of Rhizorhina Hansen, 1892 and the third to parasitise amphipods. Only a single female of the parasite was recovered, and it can be distinguished from all congeners by its pyriform shape, body size, length to width ratio, and geographical location. The mitochondrial cytochrome c oxidase subunit I (COI) and 16S rRNA genes were sequenced for Sicafodia pentagonumsp. nov. The Kimura 2 parameter distances of 0.267–0.271 for COI were calculated between S. pentagonumsp. nov. and S. iceage Campean &amp; Coleman, 2018. The 18S rRNA gene of R. sicafodiaesp. nov. and a phylogenetical analysis performed with existing sequences formed a robust clade with the other known species of Nicothoidae, Diexanthema hakuhomaruae Kakui, Fukuchi &amp; Ohta, 2023, Rhizorhina soyoae Kakui, 2016, and R. ohtsukai. Kakui, 2016.