Description of Three New Species of the Family Xyalidae (Nematoda: Monhysterida) from Chinese Marine Waters

Three new species of free-living marine nematodes, which were collected from Chinese marine waters, are herein described and illustrated. Metadesmolaimus pilosus sp. nov. is distinguished by a colourless body bearing numerous long somatic setae; cephalic setae measuring 13–17 µm in length; arcuate spicules with a cephalated proximal end and a conical, pointed distal end, approximately 1.6 cloacal body diameters long; a boat-shaped gubernaculum without apophyses; and a conico-cylindrical tail with three terminal setae. M. sinicus sp. nov. is distinguished by a relatively large amphideal fovea (53% of corresponding body diameter) in males; cephalic setae measuring 12 µm in length; slender, almost straight spicules, with a cephalated proximal end and a conical distal end, approximately 1.9 cloacal body diameters long; gubernaculum hooked distally without apophysis; and tail conico-cylindrical with two terminal setae. Steineria gracilis sp. nov. is distinguished by outer labial setae comparatively thick; eight groups of subcephalic setae, each comprising four unequal setae, 15–49 µm long; two sublateral pairs of cervical setae posterior to each amphid; circular amphideal fovea located about one head diameter from the anterior end; spicules slightly bent ventrally, approximately 1.2–1.3 cloacal body diameters long; gubernaculum barrel-shaped with hooked dorso-caudal apophyses; and tails with two long terminal setae and two short terminal setae. Updated keys to all valid species of Metadesmolaimus and Steineria are provided.