Pycnophyes abyssorum sp. n. (Kinorhyncha: Homalorhagida), the deepest kinorhynch species described so far

A new homalorhagid kinorhynch, Pycnophyes abyssorum sp. nov., found in brown silt from the abyssal depth at the Kuril-Kamchatka Trench is described and illustrated using differential interference contrast (DIC) microscopy and scanning electron microscopy (SEM). This is the ninth species of the genus to be described from the North–West Pacific and the first representative of the Kinorhyncha described from the Kuril-Kamchatka Trench. P. abyssorum sp. nov. constitutes the 16th species of Pycnophyes known from the Pacific and 55th species of the genus described based on adult specimens. P. abyssorum sp. nov. is the only second abyssal species of kinorhynchs and the deepest one described so far. The species is characterized by a unique combination of characters. P. abyssorum sp. nov. is distinguished from other congeners by the presence of long and sharp middorsal processes on trunk segments 1–10; strongly denticulated anterior margin of first tergal plate with wide submarginal area of reticulate cuticle; single paradorsal seta on trunk segments 2–4 and 7–9(8–9); two pairs of lateroventral setae closely adjacent to tergal–sternal articulation on trunk segment 10; two pairs of flosculi-like sensory papillae and one pair of terminal tubular papillae on posteriormost tergal plate. Anteromesial thickenings of ventral pachycycli absent in both sexes.