Inopinatamenia calamitosa, Cobo & Kocot, 2021

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 : 86-89

Inopinatamenia calamitosa sp. n.

( Figure 7 , Table 2 , 3 )

Type material. Holotype: ZSM Mol 20171269 (Zoologische Staatssammlung München). Serial sections (eight slides) and sclerites (one SEM stub, four slides). Brazil Basin, DIVA 3 Me 79/1 area 2, station 561 (26º 34.78’S, 035º 13.90’W), 4484.7 to 4503 m depth.

Derivatio nominis. From Latin calamitosus-a-um: calamitous. Due to the overcoming of difficulties during the first author’s PhD studies.

Diagnosis. Elongate and narrow body with sharp ends. Sclerites as lanceolate, leaf-shaped scales. Atrium with simple, pedunculate papillae. Flat pedal fold. Monoserial radula with six denticles per tooth: each tooth consists of a dumbbell-shaped base with three denticles on each side. Ventrolateral foregut glands of type A, with very short, wide and muscular simple ducts with inner musculature and extraepithelial gland cells. Midgut without lateral constrictions. With respiratory folds.

Description. Habitus: Elongate animal (2.18 mm long, 0.2, 0.15, 0.10 mm wide in the anterior, middle and posterior part respectively), with pointed anterior and posterior ends ( Figure 7 A ). White in 96 % ethanol but with a translucent cuticle, especially, in the posterior region where the imbricated arrangement of the scales and the absence of intestinal constrictions can clearly be seen. Pedal groove not externally evident.

Mantle: The cuticle and epidermis are displaced and quite deteriorated in most of the sections obtained, so the measurements (7.5 to 8 μm cuticle, 1.5 to 4 μm epidermis) are not very precise. Epidermal papillae are absent and the sclerites are embedded in a single layer. Sclerites as lanceolate, leaf-shaped scales ( Figure 7 B, C ) that are 94 to 100 μm long and 20 to 25 μm wide with a narrowing in the proximal end.

Pedal groove and mantle cavity: The pedal pit is a short slit (17.5 μm long, 15 μm wide, 7.5 to 10 μm high; Figure 7 D-3), with almost no cuticular division between it and the opening of the common atrio-buccal cavity. The pedal fold is flat and does not change in shape or size (6.25 to 7.5 μm long, 2.5 μm high) along the length of the body until the mantle cavity. Whether or not it enters the cavity is unclear because of the quality of the histological sections. The anterior follicular pedal glands are bulky and extend towards the dorsal region of the body.

The opening of the mantle cavity (35 to 37 μm wide, 20 to 40 μm high) is almost terminal ( Figure 7 E ). The epithelium of the mantle cavity is folded. The folds are interpreted as respiratory folds because of their position, shape, and the presence of ciliated cells. There are six or eight folds. The rectum (dorsal) and the spawning duct (ventral) open centrally into the mantle cavity. Remains of a posterior mass of glandular cells were found but their function could not be determined.

Digestive system: The mouth (located within the atrium) gives way to an initially narrow foregut (15 μm long, 22.5 μm wide, 5 to 15 μm high), which becomes slightly wider (25 to 30 μm long, 20 to 25 μm in diameter). In this pre-radular region, the foregut runs almost parallel to the pedal groove. The foregut has a very glandular epithelium, and it is surrounded by longitudinal musculature (5 to 7.5 μm). In the radular region, the longitudinal musculature layer surrounding the foregut is reinforced with a layer of circular musculature ( Figure 7 D-4).

The ventrolateral foregut glands consist of a pair of short and wide, simple ducts (15 μm long, 12.5 to 20 μm in diameter) with inner musculature walls ( Figure 7 D-4, 4’) and extraepithelial gland cells (type A). The gland cells are of two types (according to the shape and the staining). Most of the gland cells, which stained deep red, discharge into the ducts along their entire length, although with a greater concentration in their anterior region. In this area and dorsally, a cluster of a second type of gland cells, which are more rounded and stained intensely orange, also discharge into the ducts.

The radular sac is short (15 μm long, 5 μm in diameter) and runs very close to the foregut, flanked by the ven-trolateral foregut glands ( Figure 7 D-4, 4’). The monoserial radula consists of teeth having a plate with a dumbbellshaped outline (18.9to 19.2 μm long, 4.74 μm wide at the ends and 3.22 μm wide in the center; Figure 7 D-4’, F) with three denticles on each side. The outer-most denticles are the largest (8.8 to 9 μm long and 1.63 to 1.7 μm wide) and the innermost denticles are the smallest (6.41 to 6.8 μm long, 1.45 to 1.53 μm wide). The denticles are narrow and equidistant from each other. The inner denticles are slightly curved while the outer ones are straight.

The esophagus is narrower than the foregut (22.5 to 25 μm wide, 5 to 12 μm high) and very glandular. The longitudinal musculature surrounding the foregut disappears in the esophageal region. The esophagus terminates centrally into the midgut without forming a sphincter. The midgut has tenuous lateral constrictions in its anterior region. The ciliated rectum emerges centrally in the mantle cavity.

Nervous system and sense organs: The cerebral ganglion (32.5 μm long, 25 to 37.5 μm in diameter) is nearly circular in cross-section and is located dorsal to the middle region of the foregut ( Figure 7 D-3). The buccal ganglia (10 μm in length, 5 to 7.5 μm in diameter) surround the anterior half of the ventrolateral foregut glands. The pedal ganglia (20 μm in length 7.5 to 10 μm in diameter) are located beyond the posterior end of the pedal pit (after about 25 μm).

The opening of the atrium is wide (20.5 μm; Figure 7 D-1,2) and located far to the anterior of the body. It gives way to a large horseshoe-shaped cavity (52.5 μm in length, 45 μm in width, up to 50 μm in height) that is surrounded by a compact glandular tissue layer. There are numerous long, pedunculated papillae (12.5 μm long, papilla head 2.5 to 3 μm wide, <2.5 μm peduncle; Figure 7 D-2), which take up almost all the space of the cavity ( Figure 7 D-2). The anterior-most region of the atrium ( Figure 7 D-1) is occupied by a characteristic compact cellular accumulation that represents a central fold

Gonopericardial system: Oocytes were observed in the gonads. The pericardium is wider in the middle region (12.5 to 15 μm in diameter) before re-narrowing (5 μm in diameter) where the pericardioducts arise ( Figure 7 E ). A complete heart was not observed, although some blood cells were observed in the lumen of the pericardium. The pericardioducts are narrow (55 μm long, 5 to 7.5 μm in diameter) and join to the dorsal surface of the spawning duct in its middle region. The spawning ducts are completely fused into a single tube. In its anterior region, the lumen of the fused spawning duct is wide (12.5 to 22.5 μm in diameter) and surrounded by a thick glandular layer (20 μm). As the spawning duct narrows, the glandular layer becomes thinner, as does the outer layer of longitudinal musculature (2.5 to 3.25 μm), before discharging centrally into the mantle cavity. No copulatory stylets or other accessory reproductive structures were observed.

Remarks. The description of Inopinatamenia gen. n. is based on the combination following characters: 1) Monoserial radula with six denticles per tooth. 2) Presence of a single type of scales. 3) Ventrolateral foregut glands (type A) with two different gland cells opening into the ducts. 4) No copulatory stylets or accessory abdominal spicules. 5) No seminal vesicles or receptacles. 6) With six or eight respiratory folds. 7) With a posterior gland, of unknown function, that connects with the mantle cavity.

The new genus Inopinatamenia is mainly based on the unique type of radula. The number of denticles of the radula teeth is fundamental to differentiation of the genera of Dondersiidae (according to the current taxonomy, at least) and was one of the main characters in the diagnoses of the genera Helluoherpia and Squamatoherpia ( Handl & Büchinger 1996; Büchinger & Handl 1996). Inopinatamenia calamitosa sp. n. is unique among Dondersiidae in that it has a monoserial radula with six denticles per tooth, where each tooth consists of a dumbbell-shaped base with three denticles on each side.

The family Dondersiidae was originally defined by the presence of at least two types of sclerites. However, the dondersiid Squamatoherpia Büchinger & Handl, 1996 only has one type, which justifies the inclusion of Inopinatamenia gen. n. also has only one type of sclerites, in this family. The scales (lanceolate leaf-shaped) resemble those of Micromenia amphiatlantica but they are smaller, M. amphiatlantica also has acicular sclerites and the internal characters of the two species are very different ( Cobo & Kocot, 2020).

The ventrolateral foregut glands in Dondersiidae are of type A. In Inopinatamenia calamitosa n. sp. they are very short, wide and muscular, with two different cell types opening into the ducts. One type opens along the entire extension of the ducts while the other exoepithelial glandular cells connect with the ducts just in their anterior region. Neither of the glandular cell types have posteriorly bent necks, so, according Handl and Todt (2005) they could be classified as Pararrhopalia - type.

Figures

FIGURE 7. Inopinatamenia calamitosa sp. n. A. Habitus (holotype). B. Arrangement of the leaf-shaped scales. C. Regular lanceolate leaf-shaped scales. D. Anatomical reconstruction of the anterior body based on manual reconstructions of the holotype. E. Anatomical reconstruction of the posterior body based on manual reconstructions of the holotype. 1-4. Serial sections; location in the body is indicated in D and E. F. Reconstruction of the radula. *radula. Arrows indicate the two different gland cells of the ventrolateral foregut glands.