Stygodesmodora amphidialis sp. n.

(measurements in Table 2, Figs 2–4).

Type material. Holotype male, adult MNRJ 367. Station ME2C4 (05°02’10”S, 036°23’02”W). Paratype female, adult, MNRJ 368. Station CME3 (05°01’54”S, 036°11’15”W). Other paratypes, 12 females (188 and 192 LMZOO-UFPE), eight males (193–197 LMZOO-UFPE) and eight J1 and two J3 (198 LMZOO-UFPE), data as for holotype.

Etymology. The species epithet is given for the size of the fovea amphidialis, the largest in the genus.

Description. Holotype (Figs 2–3). Body relatively short, cylindrical, yellowish brown, tapering slightly toward each end. Cuticle annulated except in final portion of tail, no lateral differentiation. Ventral ala absent. The somatic setae are arranged in eight longitudinal rows: two dorsal, four lateral and two ventral; in some specimens these are difficult to see. Head region with annulations completely surrounding fovea amphidialis (Fig. 2 A). In some specimens, labial region folded inward. Anterior sensilla arrangement: six outer labial papillae (difficult to see), four cephalic setae, and below, 16 cervical setae near cephalic circle. Cervical setae arranged in four circles: four at level of cephalic setae, four at anterior edge of fovea amphidialis, four at mid-level of fovea, and four at level of posterior half of fovea (Fig. 2 A). Cryptospiral fovea amphidialis longitudinally elongated, occupying 83% of diameter of head, located on cuticularized amphideal plate. Oral cavity with one dorsal tooth and one ventral tooth. Cylindrical pharynx with muscular, pyriform endbulb. Lumen simple (Figs. 2 E and 3D). Nerve ring located 82 µm from anterior region. Ventral gland and excretory-secretory pore not observed. Cardia well developed, inserted in intestine. Reproductive system monorchic, with single anterior testis situated to left of intestine, outstretched. Sperm cells globular. Spicules tiny, with pointed ends. Broad velum present, gubernaculum well developed. Seven tubular precloacal supplements (Fig. 2 C). All specimens with anterior group of four closely set supplements, remaining supplements more widely separated from each other. Caudal papillae present. Tail conical, with three small caudal glands and spinneret.

Paratype female (Figs 3–4). Similar to male in body size, with minor differences: fovea amphidialis smaller than in male and a different cephalic arrangement (i.e. two sexual dimorphisms). Female with 12 cervical setae. Cervical setae arranged in two circles: four at mid-level of fovea and eight just below fovea amphidialis . In all specimens, nerve ring not visible. Reproductive system with paired ovaries, opposite and reflexed, situated to right of intestine. Vulva as transverse slit. Vagina vera short, vagina uterina with sphincter muscle. No egg found. Vulva comprising 60% of total body length (595 µm distant from anterior end). Tail conical.

Sex Male Female Juvenile Juveniles. Very similar to adults, except for lack of some characteristics such as supplements, the immature reproductive system, and the size of the fovea amphidialis (smaller than males). Juveniles in stage 1 lack cervical setae and have very few somatic setae along the body. Juveniles in stage 3 have four cervical setae and some somatic setae along the body. Ten juveniles were found, eight in stage 1 and two in stage 3.

Diagnosis. Stygodesmodora amphidialis sp. n. is characterized by the cephalic arrangement, with four cephalic setae and four distinct circles of cervical setae (for males). In females, the cervical setae are located in different circles (see Fig. 4 A). Fovea amphidialis large (larger in males than in females, sexually dimorphic in size). 6–8 tubular supplements, group of anteriorly aggregated supplements, remaining supplements more widely separated from each other.

Differential diagnosis. Stygodesmodora amphidialis sp. n. is similar to Stygodesmodora bacillicauda (Gerlach, 1963) in the annulated cuticle, buccal cavity and size of the spicules. It differs from S. bacillicauda in the absence of supplements, caudal papillae and cervical setae. Stygodesmodora bacillicauda has numerous somatic setae in the tail region, which are absent in S. amphidialis sp. n.

Stygodesmodora amphidialis sp. n. is similar to Stygodesmodora epixantha Blome, 1982 in having a velum, cephalic setae, and the buccal cavity with one dorsal and one ventral tooth. It differs in the shape of supplements (setiform in S. epixantha) and the size of the fovea amphidialis (larger in S. amphidialis, 83%, vs. 55% in S. epixantha).

Stygodesmodora amphidialis sp. n. is comparable to Stygodesmodora confusa Leduc & Verschelde, 2015 in the presence of a well-developed capitulum and velum. It differs in the number of somatic setae (many somatic setae in S. confusa), cephalic arrangement (cervical setae absent in S. confusa) and shape of the supplements (tubular in S. amphidialis sp. n. and setiform in S. confusa).