Dondersia Hubrecht 1888

Dondersia ? foraminosa sp. n.

(Figure 2, Table 2)

Type material. Holotype: ZSM Mol 20171266 (Zoologische Staatssammlung München). Serial sections (eight slides) and sclerite preparations (two slides). Brazil Basin, DIVA 3 79/, area 2, station 561 (26º 34.78’S, 35º 13.90’W), 4484.7 to 4503 m depth.

Derivatio nominis. From Latin foraminosus-a-um, with holes. With reference to the second posterior opening.

Diagnosis. Small, elongate animal (<2 mm). Woolly appearance. With a pointed anterior end and a finger-like projection at the posterior end. With two different types of leaf-shaped scales and one type of pallet-shaped scales. Atrium without papillae. With a single pedal fold. Monoserial radula. Ventrolateral foregut glands of type A with long, simple ducts. Foregut with a dorsal pocket. With a dorsoterminal sensory organ.

Description. Habitus: Small animal (1.6 mm long) with a woolly appearance. Although the anterior body region is wider than the posterior (0.175, 0.15, 0.12 mm wide in the anterior, middle, and posterior regions, respectively), the anterior end is pointed. The posterior region is narrower and more delicate, probably because of poor fixation of the specimen, but it can be stated that it has a finger-like projection. Pedal groove not marked externally. Grey-white in 96 % ethanol (Figure 2 A).

Mantle: Thin epidermis (6.5 to 11 μm) and cuticle (12.5 to 15 μm), in which two different types of leaf-shaped and one type of pallet-shaped (=oar-shaped) scales are inserted in a single layer: 1). Lanceolate leaf-shaped scales with a rounded base (Figure 1A1 *; Figure 2 B’, C) are the most abundant sclerite type (38 to 60 μm long, 15 to 24 μm wide). These vary somewhat in shape as some have a narrower distal end. 2) Irregular elongate leaf-shaped scales (Figure 1 A 2 *; Figure 2 B’’) (32.5 to 47.4 μm long, 10.5 to 12 μm wide). They appear mostly in the dorsal region of the body. In these scales a proximal rim is faintly visible. 3) Pallet-shaped scales (25 to 42.5 μm long, 8 to 10 μm wide) (Figure 2 D). Without special scales along the pedal groove.

Pedal groove and mantle cavity: The pedal pit (12.5 μm long, 15 μm wide, 35 μm high) is strongly ciliated and has a small internal extension (2.5 μm long; Figure 2 E-2). The anterior follicular pedal glands end latero-dorsally in the pedal pit and then continue as two separate packages that surround the foregut (Figure 2 E). The pedal glands have two types of cells that differ fundamentally by the color that they acquire with Mallory’s trichrome staining: the dorsal ones are stained an intense red and form a more compact package than the ventral cells, which are less intensely stained and are more dispersed. The pedal pit gives rise to a single triangular pedal fold that retains its shape and size (3.75 to 5 μm high, 5 μm wide) throughout the body. The posterior pedal glands, which are small and closely associated with the groove, are clearly distinguishable.

The mantle cavity opens posteriorly as a wide ciliated channel (25 to 30 μm; Figure 2 F). It is small (30 μm long, 28 μm wide, 40 μm high) without respiratory folds and with a posterior prolongation (70 μm long, 5 to 10.5 μm in diameter). The rectum opens dorsally, and the spawning ducts (fused to a single tube) open ventrally in the cavity.

Digestive system: The mouth opens at the posterior end of the atrium. The foregut (50 μm long, 12.5 to 15 μm in diameter) runs almost parallel to the pedal groove (60 μm long, 25 to 27.5 μm wide, 10 to 12.5 μm high) and in the middle region it widens significantly (30 to 32.5 μm in diameter) and forms an elongated dorsal pocket (12.5 μm long, 15 μm wide, 30 μm high) that is separated from the foregut by a thick muscle layer (10 to 12.5 μm thick; Figure 2 E-4).

The radula is monoserial with a posterior radular sac (30 μm long, 12.5 to 15 μm wide, 10 to 22.5 μm high in the middle region). Fragments of a single pair of denticles (Figure 2 G-G’), which are long and very straight (8 μm long, 0.6 to 0.8 μm), were observed inside the foregut above the ventral wall. Only a fragment of the distal end of a denticle was preserved, it has a pointed end, but it cannot be determined whether the denticles are curved distally or not. Remains of a rounded radular base were also observed (Figure 2 G’’).

The ventrolateral foregut glands open into the foregut on either side of the radular sac. They are formed by two long, simple ducts (approximately 80 μm in length, 15 to 22.5 μm in diameter) with inner musculature and extra-epithelial gland cells opening into the ducts along the entire length of the gland (Type A; García-Álvarez & SalviniPlawen 2007). The inner musculature of the ventrolateral foregut glands is reinforced in their posterior part where the cell necks of the gland cells run along the duct; the glandular cells have posteriorly bent necks (Acanthomenia type; Handl & Todt 2005). The midgut is without a dorsal caecum or marked lateral constrictions. The rectum is oval in cross section (17.5 μm wide, 20 μm high) and terminates laterally in the anterior wall of the mantle cavity.

Nervous system and sense organs: The cerebral ganglion (35 μm long, 50 to 75 μm wide, 45 to 52.5 μm high) is circular in cross section and located dorsal to the middle part of the foregut (Figure 2 E-3). The pedal ganglia (5 μm long, 12.5 to 15 μm in diameter) are located ventral to the pedal pit and are joined by a commissure (2.5 to 5 μm in thickness). A pair of buccal ganglia (10 μm long, 10 to 12.5 μm in diameter) are located on both sides of the radular sac.

The opening of the atrium (22.5 μm) gives way to a cavity that is much higher than it is wide (up to 85 μm in height, 10 to 15 μm in width) that is directed dorsally and narrows to resemble a duct (Figure 2 E). The atrium has folded and glandular walls, but no papillae are present. It also has a small anterior extension where the epithelium has fewer gland cells (10 μm long, 7.5 to 10 μm wide, 10 to 15 μm high).

At the posterior part of the body is a glandular structure that is interpreted as a dorsoterminal sensory organ. This organ is big (about 18 μm long, and 20 μm in diameter). The dorsoterminal sensory organ opens to the outside through a small pore at the posterior-most part of the body (7.5 μm aperture diameter; Figure 2 F-6). The inner an-terior region of the dorsoterminal sensory organ reaches the posterior prolongation of the mantle cavity.

Gonopericardial system: The studied animal is immature. The pericardium (182.5 μm long, 3 to 15 μm in diameter) has a straight, posterior region that extends past the point where the pericardioducts are attached (approximately 75 μm in length; 3 μm in diameter). The epithelium of the pericardioducts (diameter around 8 to 10 μm) is very thin and their connection to the spawning duct is not evident, but they seem to be attached to it in its mid-posterior region, making the length of the pericardioducts about 70 μm (Figure 2 F). The spawning ducts are fused as a single duct along their entire length (110 μm long, 8 to 10 μm wide, 7.5 to 8 μm high). This duct opens centrally in the anterior region of the mantle cavity. The opening is wide, and the epithelium is densely ciliated in this region. No accessory copulatory structures were observed.

Remarks. Dondersia ? foraminosa sp. n. is placed within Dondersiidae because it has at least two types of scales, a monoserial radula with elongated denticles, and type A ventrolateral foregut glands. If the radula can be fully seen, the distinction between the genera of Dondersiidae is simple (according to the current taxonomy; Table 3). Otherwise, this is more complicated due to the lack of consensus on other diagnostic characters, that appear combined between some genera (Table 3). During histological sectioning, solenogaster radulae may become fragmented, as was the case here. Thus, a complete characterization of the radula of Dondersia ? foraminosa sp. n. was not possible, making its classification into one or another genus of Dondersiidae challenging.

The fragments of the radula indicate that it is monoserial with apparently straight denticles (very close to each other) and that the base of the tooth is rounded. The radular teeth are similar in appearance to the drawings of fragmented teeth by Salvini-Plawen (1978) for D. laminata Salvini-Plawen, 1978 (for which the radula was also described to have two denticles) and their base is similar to the base of the radula of D. festiva Hubrecht, 1888 imaged and drawn by Scheltema et al. (2012) (although serrations were not observed in the new species). While the available information on the radula is consistent with a placement of Dondersia ? foraminosa sp. n. in the genus Dondersia, the following other genera cannot be ruled out: Lyratoherpia Salvini-Plawen, 1978, Nematomenia Simroth, 1893, Ichtyomenia (Pruvot, 1890), Stylomenia Pruvot, 1899 and Micromenia Leloup, 1948. However, the new species lacks the characteristic sclerites of Stylomenia (Handl et al. 2001) and it differs in external appearance from most species of Nematomenia and Lyratoherpia (Species of both genera usually have a tegumenal keel or keel made by sclerites). Moreover, Nematomenia lacks a posterior digitiform projection (Salvini-Plawen 1978; Scheltema et al. 2012) and Ichtyomenia has a characteristic rimmed posterior opening (Pruvot 1890; García-Álvarez et al. 2014). Dondersia and Micromenia differ from each other in radular morphology (Table 3) although this character is insufficiently known for Micromenia. In contrast to Dondersia, Micromenia lacks copulatory stylets and a digitiform projection (Leloup 1948; Salvini-Plawen 1972, 2003a, Cobo & Kocot 2020). In addition, of the four Micromenia species only M. amphiatlantica has an atrio-buccal cavity (Cobo & Kocot 2020). The sclerites or the presence/absence of a dorsoterminal sensory organ and seminal receptacles vary without a defined pattern between species of Dondersia and Micromenia. The inclusion of the new species in some of these other genera could not be completely discarded, but is not chosen here, based on its digitiform projection, the existence of atrio-buccal cavity and the following arguments.

Despite uncertainties about the radula, other characters suggest that Dondersia ? foraminosa sp. n. belongs to Dondersia. First, Dondersia ? foraminosa sp. n. has an elongated body (without a keel) that is pointed at the anterior end and terminates posteriorly as a finger-like projection, which was stated characteristic of the genus by Scheltema et al. (2012) based on D. namibiensis Scheltema et al., 2012, D. festiva Hubrecht, 1888, D. annulata Nierstrasz, 1902 and D. incali (Scheltema, 1999). According to these authors, and in accordance with the original descriptions of the nominal species D. festiva (Hubrecht 1888; Scheltema et al. 2012), animals of this genus have a posterior digitiform projection and a posterior lobe, although this may not be developed in immature specimens (Scheltema et al. 2012) as seems to be the case with the new species described here. Also, the sclerites of D. ? formaninosa (two types of leaf-shaped scales and one type of pallet-shaped scales) coincide with what is expected for Dondersia, where all the species have leaf-shaped scales and D. festiva, D. annulata, D. laminata Salvini-Plawen, 1978 and D. namibiensis have pallet-shaped scales (Hubrecht 1888; Nierstrasz 1902; Salvini-Plawen 1978; Scheltema et al. 2012). In addition, the scales show a proximal rim, a characteristic also described by Scheltema et al. (2012) for the scales of D. incali and D. namibiensis. The size and shape of the scales are, however, characteristic of D. ? foraminosa sp. n., which supports that it is a different species. D. ? foraminosa sp. n. has a common atrio-buccal cavity. This character is not uniform in Dondersia, although the existence of an atrio-buccal cavity seems to be the rule. D. festiva, D. namibiensis and D. incali have a common opening (Scheltema 1999; Scheltema et al. 2012) while mouth and atrium were described as separate in D. annulata, D. cnidevorans and D. stylastericola. Nevertheless, in the descriptions of D. annulata (Nierstrasz 1902, 1908; Thiele 1913c), the separation was determined by the external study of the sclerites’ disposition rather than histology (Scheltema et al. 2012). The description of D. cnidevorans is clear regarding the separation of atrium and mouth, but it was placed provisionally in Dondersia by Salvini-Plawen (1978). In D. stylastericola there is a certain separation between mouth and atrium, but this is not cuticular (SalviniPlawen 1978). For D. laminata it was not determined if it has a common atrio-buccal cavity or if there are two openings. Dondersia ? foraminosa sp. n. has a dorsoterminal sensory organ. Dondersia festiva and D. annulata have two dorsoterminal sensory organs (Scheltema et al. 2012) and D. cnidevorans has probably one (Salvini-Plawen 1978). Finally, D. ? foraminosa sp. n. lacks respiratory folds.

The studied specimen is immature, and it would be necessary to determine the existence of seminal vesicles and copulatory stylets of mature specimens for a certain assignment to Dondersia. In addition, the knowledge of a complete radula would be necessary to confirm its inclusion in Dondersia. However, due to the aforementioned suite of characters, it is provisionally placed in the genus. The combination of its internal characteristics compared with the existing species, habitus, sclerites and biogeographical distribution (Hubrecht 1888; Pruvot 1891; Nierstrasz 1902; Thiele 1913c; Salvini-Plawen 1978; Scheltema 1999; Scheltema et al. 2012; Klink et al. 2015) justifies its designation as a new species.