Prochaetosoma marisalbi Fedyaeva, Neretina, Konovalova & Tchesunov, 2016, sp. n.

Prochaetosoma marisalbi sp. n.

Figs 4–6, Table 3 (morphometrics)

Material. Holotype male, six paratype males, seven paratype females. The type specimens are deposited in Senckenberg Institute, Frankfurt am Main (Germany), inventory numbers SMF 17047 (holotype) and SMF 17048– 17054 (paratypes).

Type locality. White Sea (Northern Russia), Karelian Coast of the Kandalaksha Bay, vicinity of the White Sea Biological Station (66°33'N, 33°06'E), Velikaja Salma Strait between the Veliky Island and Kindo Peninsula, depth 2–5 m, red algae. 18–27 August 2013.

Description. Body elongate and generally slim. Pharyngeal region slightly swollen, rest of body almost cylindrical in males and elongate spindle-shaped in females (swollen in area of gonads). Curvature of the body of fixed specimens rather variable. Rostrum (or cephalic capsule, anteriormost area with non-annulated cuticle) rounded anteriorly. Body cuticle annulation conspicuous and without ornamentations. Cuticular annulations of the pharyngeal region wider than those of rest of the body. Tail cuticle annulated except for terminal cone.

Mouth opening surrounded with six triangular lips and then with zone of rugose (crumpled) cuticle (Fig. 6 D). Inner labial papillae on the lip bases; outer labial papillae on the periphery of the rugose zone. Four cephalic setae rooted at the level of anterior margin of the amphideal fovea. Length of the cephalic setae 6.3–9.0 µ m in males and 6.0–9.0 µ m in females. Amphideal foveas spirally coiled in 1.5 turns in males and females, 0.7–1.0 µ m wide in males and 0.6–1.4 µ m wide in females.

Numerous somatic setae (about 260–280) along the body. Longest somatic setae (34-49 µ m in males and 32-52 µ m in females) located on pharyngeal region. Somatic setae in intestinal region of the body reach 26–45 µ m in length in males and 37–49 µ m in females with shorter setae 5.0–9.4 µ m in males and 3.5–9.0 µ m in females. One or two long subdorsal and several short latero-ventral setae on each side of the tail, 23–33 µ m long in males and 11–40 µ m in females.

Eight strong cephalic adhesive tubes (CATs) arranged in four longitudinal pairs (two dorsal and two subdorsal pairs) located dorsally on the head posterior to the rostrum, on 7th to 17th annule. The CATs slightly bent forward; their basal ends shaped as bulb-like butts and distal ends modified as cup-like swellings. Within each pair, posterior CAT slightly longer than anterior. Posterior adhesive tubes (PATs) present as thick, slightly bent to almost straight sticks, proximally swollen and gradually narrowing to bell-shaped distal ends. Latero-ventral PATs slightly longer than subventral ones. PATs arranged in four rows, two subventral and two latero-ventral on body between midbody and anal opening. Two subventral rows extended a bit longer posteriorly than latero-ventral rows. Within each row, length of the PATs gradually reduced posteriorly.

Buccal cavity narrow (2.2–5.5 µ m wide and 5.4–8.5 µ m long in males, 1.7–5.9 µ m wide and 6.8–12 µ m long in females), weakly sclerotized and armed with a small but conspicuous dorsal triangular tooth in the pharyngostoma. In some specimens, buccal cavity surrounded with accumulations of pigment granules in the epidermis. Pharynx rather short, nearly cylindrical or slightly swollen just anterior to nerve ring and posteriorly terminating as a rounded muscular bulb lacking a distinct valve and without cuticularised lumen.

Female reproductive system amphidelphic; ovaries antidromously reflexed and located ventral to the intestine; each uterus often contains one mature ovum.

Male reproductive system monorchic, anterior testis situated ventrally to the intestine. Spicules slightly arcuate, long, their proximal ends at the level of penultimate latero-ventral PATs or even at the third PATs from posterior end of the latero-ventral row. The spicules proximally ended with weak, ventrally turned knobs, and distally pointed. Gubernaculum a small but distinct bar thickened at anterior end. No supplementary organs evident. Ten rather short (8 Μ m) uncinate setae posterior to the PATs: three setae located anterior to anus and six to seven posterior to the anus.

Tail elongate conical, with distinct terminal cone of non-annulated but slightly rough cuticle.

Diagnosis. Prochaetosoma. Body length 555–839 µ m in males and 601–811 µ m in females; a 15–21.2 in males and 11.6–16.0 µ m in females. Somatic setae longest (32–52 µ m) on pharyngeal region and of approximately equal length on rest of body. Amphids spirally coiled with 1.5 turns in males and females. One small but distinct dorsal tooth in stoma; two smaller and less conspicuous subventral teeth may be visible. Four pairs of cephalic adhesive tubes. Eight to eleven latero-ventral posterior adhesive tubes in males and 9–13 in females; 8–11 subventral posterior adhesive tubes in in males and 8–12 in females. No cuticularised lumen in terminal bulb of the pharynx. Spicules 75–87 µ m, gubernaculum 13–18 µ m long.

Differential diagnosis. Prochaetosoma marisalbi sp. n. differs from all valid Prochaetosoma species with the possible exception of P. longicapitatum (Allgén, 1932) by lacking cuticular thickening of pharyngeal bulb internal lumen. Taking into account other characters, P. marisalbi sp. n. is more similar to P. longicapitatum (Allgén, 1932) (re-description of Allen & Noffsinger, 1978) and P. martensi Decraemer, 1989.

Prochaetosoma marisalbi sp. n. resembles P. longicapitatum in the shape of body and rostrum, and many measurements, but differs in having 8 vs 6 CATs, relative tail length (c 3.04–4.81 vs 7.8 in male), numbers of sublateral PATs (8–11 vs 18) and subventral PATs (8–11 versus 4–5), longer spicules (75–87 vs 45 µ m). Among Prochaetosoma species having distinctly thickened cuticle in the pharyngeal bulb, P. marisalbi sp. n. is to some extent related to P. martensi Decraemer, 1989 but differs by having a slimmer body (a 15–21 in males and 11.6–16 in females vs respectively 11–15.7 and 8.5–12.7, respectively), relatively longer tail (c 7.8–8.9 vs 12.5–20.1), fewer CATs (8 vs 10), and longer spicules (75–87 vs 37–49 µ m).