Nematomenia, Simroth, 1893

Cobo, M. Carmen & Kocot, Kevin M., 2021, On the diversity of abyssal Dondersiidae (Mollusca: Aplacophora) with the description of a new genus, six new species, and a review of the family, Zootaxa 4933 (1), pp. 63-97 : 75-78

Nematomenia ? guineana sp. n.

( Figure 4 , Tables 2 , 4 )

Type material. Holotype: ZSM Mol 20171265 (Zoologische Staatssammlung München). Serial sections (four slides) and sclerite preparations (one SEM stub, five slides). Guinea Basin, DIVA 2 Me 63/2 area 4, station 89 (00º 42.95’N, 05º 31.29’W), 5142 m depth.

Derivatio nominis. Spanish epithet, from Guinea.

Diagnosis. Small, elongate body (<2 mm long). Woolly appearance. Sclerites as leaf-shaped scales and two different types of laminar scales. With a small ciliated pedal fold that diminishes until disappearing just before the opening of the mantle cavity. Very small atrium without papillae, mouth opening in the centre of the atrium. Ventrolateral foregut glands of type A with long, simple ducts. Monoserial radula; teeth with at least two denticles. With subradular pouch. Midgut with an unpaired dorsal caecum and without lateral constrictions. Without dorsoterminal sensory organ. Without accessory copulatory structures.

Description. Habitus: Small animal (1.50 mm long, 0.2 mm wide in the middle) with elongate body, slightly narrower towards the truncated posterior end (0.18 mm wide) and a slightly narrower towards the sharper anterior end (0.14 mm wide). Velvety, woolly appearance and with a small, almost imperceptible dorsal keel made of sclerites in the anterior region. White in 70 % ethanol ( Figure 4 A ).

Mantle: Epidermis with secretory cells (intense orange color when stained) but without epidermal papillae. Scales are radially inserted in a thin cuticle (6.75 to 9 μm thick). One type of leaf-shaped scales, two types of laminar scales, and one type of scales of the pedal groove are present: 1) Leaf-shaped scales ( Figure 4 B, C ) characterized by the angular shape of their proximal end and regular size (34.4 to 42.5 μm long, 17.6 to 22.05 μm wide) present throughout the specimen are the most common sclerite type. 2) Straight, laminar scales ( Figure 4 D ) that are located mainly in the dorsal region (38 to 38.5 μm long, 8 to 8.5 μm wide). 3) Lanceolate, laminar scales ( Figure 4 E ) are the least abundant and the smallest (28 to 30 μm long, 6.5 to 7 μm wide) sclerite type. 4) Knife-shaped scales of the pedal groove, which are small (8.5 to 9 μm long, 6 to 8 μm wide) and strongly inserted into the cuticle.

Pedal groove and mantle cavity: The epithelium of the pedal pit is strongly ciliated ( Figure 4 F-3). Of the total length of the pedal pit (30.5 μm long, 20 to 22.5 μm wide and 15 to 27.5 μm high), 15 μm corresponds to a narrow anterior extension (9 to 15 μm wide, 3.5 to 10 μm high). The pedal groove, which is barely externally distinguish-able, contains a single very small ciliated fold (1 to 2 μm), which decreases in size toward the back of the body until it disappears, just anterior to the opening of the mantle cavity. The pedal glands are bulky ( Figure 4 F-3, 4, 5) and extend dorso-posteriorly towards the middle region of the body and to the midgut caecum.

The terminal mantle cavity is small (37.5 μm high, 25 μm wide) and without respiratory folds, sacs, spicules, or other accessory copulatory structures.

Digestive system: The mouth (12.5 μm diameter in the aperture) opens into the atrium and continues as a circu-lar foregut whose dimensions remain more or less constant (65 μm long, 35 to 40.5 μm in diameter) throughout the pre-radular region ( Figure 4 F-2,3), except in a small central part where it narrows significantly (10 μm long, 12.5 μm in diameter; Figure 4 F-4). It is surrounded, in all its extension, by a layer of radial and circular musculature that widen slightly in the narrow section (8 to 12.5 μm of thickness). The epithelium of the foregut is glandular with well-differentiated, almost cubic, gland cells occupying part of the lumen ( Figure 4 F-3). These cells stain purple with a brown-colored nucleus.

This species has a monoserial radula with a radular sac (45 μm long, 10 to 17.5 μm wide and high) and a ventral radular pouch (35 μm long, 7.5 to 22 μm wide, 7.5 to 8 to 17 μm high; Figure 4 F-6). The monoserial radula was not entire in the sections. Each tooth consists of at least two thin slightly curved denticles (4 to 5 μm high, 1.2 to 1.8 μm wide). No junction of the distal end of the denticles was observed (but this cannot be ruled out) and the precise location on the base of the tooth where the denticles attach could not be located. Most of the fragments preserved in the sections are remains of the tooth base ( Figure 4 G’’) and tips of the denticles ( Figure 4 G’). There are sixteen rows of teeth.

The ventrolateral foregut glands are fused into a narrow duct (7.5 μm in diameter) where they attach ventrally to the foregut anterior to the subradular pouch. For most of their length, however, these glands are paired (20.5 to 30 μm in diameter) and extend posteriorly on either side of the foregut ( Figure 4 F-6). The two simple ducts have a thick layer of inner musculature (7.5 to 10 μm thick) and gland cells opening into the tubes in their entire extension (type A). In the distal regions of the ventrolateral foregut glands, which are widened, the extraepithelial gland cells are most numerous. The glandular cells have posteriorly bent necks (Acanthomeni a-type). The esophagus, which is almost triangular in cross-section, is narrower than the foregut (30.5 μm long, 45 μm wide, 17.25 to 20 μm high) and joins the midgut just below the formation of the intestinal caecum. The midgut has an unpaired dorsal caecum (112.5 μm long, 30 to 35.5 μm wide, 50 to 60 μm high; Figure 4 F-2, 3, 4, 5) and is without lateral constrictions.

Nervous system and sense organs: The cerebral ganglion (35.5 μm long, 82.5 μm wide and 30 μm high) sur-rounds the anterior region of the foregut dorsally ( Figure 4 F-3). The paired pedal ganglia (5.7 μm long, 15 μm wide, 5 to 10 μm high) are located dorsal to the posterior part of the pedal pit and are joined by a thin commissure (5 μm thick; Figure 4 F-3). The buccal ganglia (12.5 μm long, 10 μm wide, 5 to10 μm high) are located on both sides of the anterior region of the radular sac. The opening of the common atrio-buccal cavity (7.5 μm) is anterior and gives way to a strikingly small atrium (10.5 μm long, 25 μm wide and high), which is without sensory papillae but has a rather glandular epithelium in its central region ( Figure 4 F-1).

Gonopericardial system: A precise reconstruction of the posterior region of the body was not possible to perform, but the general arrangement of the reproductive apparatus could be determined, from which no remarkable characteristics were observed. The spawning ducts are completely fused as a single duct (12.5 μm high and wide), with a thin musculature it opens centrally into the mantle cavity. The pericardioducts are small, without seminal vesicles, and ventrally connected with the posterior part of the pericardium.

Accessory reproductive structures were not present. As noted, the anatomical structures of the posterior body were difficult to study due to the preservation of the specimen, but the appearance of the posterior sections lacks definition of organs and absence of developed gonads suggest that the studied animal is an immature specimen.

Figures

FIGURE 4. A. Habitus (holotype). B. Leaf-shaped scales and laminar scales. C. Leaf-shaped scales. D. Laminar scales. E. Lanceolated laminar scales. F. Anatomical reconstruction of the anterior body based on manual reconstructions of the holotype. 1-6. Serial sections; location in the body is indicated in F. G. Reconstruction of the radula: G’ denticle fragment, G´´ radular base.