Nematomenia divae sp. n.

(Figure 3, Tables 2, 4)

Type material. Holotype: ZSM Mol 20171262 (Zoologische Staatssammlung München). Serial sections (12 slides) and sclerite preparations (one SEM stub, five slides). Guinea Basin, DIVA 2 Me 63/2, area 4, station 90 (00º 40.49’N, 05º 29.71’W), 5144 m depth. (Table 2).

Derivatio nominis. divae feminine genitive from DIVA, acronym of the expedition name in which the specimen was collected.

Diagnosis. Long and narrow body. Slightly scaly appearance. Sclerites as two types of leaf-shaped scales and two types of oar-shaped scales; oar-shaped scales less abundant than leaf-shaped scales. Single pedal fold. Bulky anterior pedal glands. Atrium with long digitiform papillae. Ventrolateral foregut glands of type A with short and wide ducts (blister-shaped) with inner musculature and exoepithelial gland cells opening mostly in the anterior region of the ducts. Monoserial radula; teeth with four denticles on a rounded, non-serrated base, neighboring denticles joined at the apex. Midgut without caecum or lateral constrictions. Without dorsoterminal sensory organs. Without accessory copulatory structures.

Description. Habitus: Wormlike animal with rounded ends (3.77 mm long, 0.23 mm wide in the mid-body region), narrowing towards the posterior end (0.13 mm). The pedal groove is externally evident. It is white in 70 % ethanol, with a shiny and scaly appearance, and slightly translucent (Figure 3 A).

Mantle: Mantle formed by a very thin epidermis (1.4 μm) without epidermal papillae. Thin cuticle (8.3 to 15.9 μm thick) with two leaf-shaped and two oar-shaped scale types: 1) Oblong leaf-shaped scales (Figure 3 B, C) with very straight sides and with a slightly mucronated distal end (45.8 to 77.8 μm long, 26 to 32.3 μm wide); this is the most common type. 2) Oval leaf-shaped scales (Figure 3 D) that are smaller than the previous type (40 to 46 μm long, 18 to 25 μm wide) and with rounded sides and base. 3) Small cuneiform oar-shaped scales (Figure 3; 33.2 to 37.6 μm long, 8 to 8.9 μm wide). 4) Larger and slightly more elliptical oar-shaped scales (Figure 3 E; 55.4 μm long, 9.6 μm wide). Without a distinct type of pedal groove scales.

Pedal groove and mantle cavity: Pedal pit (15 μm long, 35.6 μm wide, 20 to 25 μm high) in a very anterior posi-tion (Figure 3 G-1). Anterior pedal glands discharge dorsally into the pedal pit through a muscular layer (4 to 6 μm thick). These glands surround the atrium ventrally, run parallel to the anterior region of the foregut, and surround it in its medial region. They continue posteriorly up to the mantle cavity. The inconspicuous pedal groove contains a single fold (9.4 to 11.5 μm wide, 8 to 11.5 μm high) that is well defined in the anterior region but becomes less prominent (almost disappearing) in the mid-posterior region of the body (1 to 2 μm wide and high). The mantle cavity (55 μm long, 35 to 40 μm wide, 60 μm high in the middle region) is almost terminal and decreases in size towards the posterior end (15 to 20 μm in diameter; Figure 3 H-6) and has a ventro-anterior pouch (Figure 3 H; 40 μm long, 10.5 to 12.4 μm wide and high). Without respiratory folds.

Digestive system: The mouth opens at the end of the atrium and continues as a narrow foregut (22.1 μm wide, 37.2 μm high) surrounded by a glandular epithelium and a thin muscular layer (2 to 4 μm; Figure 3 G-2). In the middle region, the foregut extends ventrally, and the layer of musculature thickens (longitudinal musculature: 8.2 to 8.5 μm; circular musculature: 20 to 27 μm; Figure 3 G-3). The radular sac extends towards the posterior body region (32 μm long, 25.5 to 28 μm wide and high; Figure 3 G-3). The monoserial radula is formed of about seven rows of single teeth. Each tooth is composed of a broad, non-serrated base (9 to 11 μm long, 2.6 to 3 μm high) bearing four long and narrow denticles (7 to 10 μm high, <1 to 2 μm wide). Neighboring denticles are joined at the apex (Figure 3 G-3’, I).

The ventrolateral foregut glands are blister-shaped: they consist of two short and wide, simple ducts (34 μm in length, lumen 20 to 30 μm in diameter; Figure 3 G-3,3’), with inner musculature and extraepithelial gland cells (type A). Most of the gland cells open into the ducts and are restricted to their anterior region. The posterior regions of the ducts are almost free of glandular cells, but no posteriorly bent necks of the glandular cells were observed. Thus, they can be classified as the Pararrhopalia - type (Handl & Todt, 2005). The ducts open ventrally into the foregut via a common papilla (i.e. unpaired opening into the foregut).

The esophagus (95 μm long, 35 to 40 μm in diameter) forms a sphincter as it joins the midgut centrally. The midgut has no caecum or lateral constrictions. The rectum (10 to 11.25 μm in diameter) discharges dorsally into the mantle cavity.

Nervous system and sense organs: Cerebral ganglion of quadrangular shape in cross section (80 μm long, 42 to 83 μm wide, 34 to 50 μm high; Figure 3 G-2). The atrium (86 μm long, 81.5 to 90.2 μm wide, 50 to 75 μm high) opens ventrally (opening is 35 μm wide) and has six to eight large digitiform atrial papillae (18.7 to 22.5 μm long, 12 to 13.5 μm wide; Figure 3 G-1) in its posterior region.

Gonopericardial system: The gonads are small (7.5 to 10 μm in diameter), suggesting that it is an immature specimen. The pericardium (approximately 312.5 μm long, 7.5 to 25 μm diameter) is significantly narrow in its posterior region (7.5 μm). The heart runs through the center of the pericardium. The pericardioducts (232.5 μm long, 5 to 7.5 μm diameter) are parallel to the pericardium: they rise laterally in the posterior region of the pericardium and connect to the anterior end of the spawning ducts. The paired region of the spawning ducts is much shorter (<10 μm in length) than the fused part (180 μm in length; Figure 3 H-4), which narrows to become a muscular duct that is circular in cross-section (75 μm long, 20 to 25 μm in diameter; Figure 3 H-5) that terminates into the anteroventral pouch of the mantle cavity. Without seminal vesicles, copulatory stylets, or other accessory reproductive structures.