On the diversity of abyssal Dondersiidae (Mollusca: Aplacophora) with the description of a new genus, six new species, and a review of the family Author Cobo, M. Carmen 0000-0002-8481-2086 Department of Zoology, Genetics and Physycal Antrhopology, University of Santiago de Compostela. Rúa Lope Gómez de Marzoa, s / n. Campus Vida. 15782 Santiago de Compostela, Spain. & Department of Biological Sciences and Alabama Museum of Natural History, University of Alabama. 300 Hackberry Lane, Tuscaloosa, AL 35487, USA. & kmkocot @ ua. edu; https: // orcid. org / 0000 - 0002 - 8673 - 2688 mariadelcarmen.cobo@usc.es Author Kocot, Kevin M. 0000-0002-8673-2688 Department of Biological Sciences and Alabama Museum of Natural History, University of Alabama. 300 Hackberry Lane, Tuscaloosa, AL 35487, USA. & kmkocot @ ua. edu; https: // orcid. org / 0000 - 0002 - 8673 - 2688 kmkocot@ua.edu text Zootaxa 2021 2021-02-18 4933 1 63 97 journal article 8067 10.11646/zootaxa.4933.1.3 1ec7cff3-1bd4-4038-87fc-8a6d87f4c7ef 1175-5326 4547986 303F97F8-463C-4A52-B5D7-28154E492493 Helluoherpia vieiralaneroi sp. n. ( Figure 6 , Table 2 ) Type material. Holotype : ZSM Mol 20171268. (Zoologische Staatssammlung München). Serial sections (seven slides) and sclerites (one SEM stub, five 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. Male genitive in honor of Dr. Vieira Lanero (University of Santiago de Compostela). Diagnosis. Small animal (< 2 mm ) with elongate body. Appearance slightly scaly. With a depression around the atrio-buccal cavity. With lanceolate, leaf-shaped scales as the main type of sclerites, pedunculate, lanceolate scales, and solid, curved acicular sclerites. With a single pedal fold. Common atrio-buccal cavity is very small and without papillae. Helluoherpia - type ventrolateral foregut glands. Radula with three denticles. Mantle cavity very small. With seminal vesicles. Description. Habitus : Small animal ( 1.35 mm long, 0.2 mm wide in the middle) with the anterior end rounded and somewhat wider than the rest of the body ( 0.3 mm ). With a small ventral depression where the atrio-buccal opening is located ( Figure 6 A ). The posterior end was externally damaged, but good histological preparations were still obtained. The pedal groove is evident externally. Yellowish-white in 96 % ethanol. The imbricate arrangement of the sclerites is easily observed, although it does not have a very scaly appearance. Mantle : Thin epidermis (1.5 to 2.5 μm thick) without epidermal papillae. Thin cuticle (5 to 7.5 μm) with three types of sclerites arranged in a single layer: 1) Regular lanceolate, leaf-shaped scales ( Figure 6 B, C ) that cover the entire body (60 to 80.5 μm long, 20 to 12.5 μm wide). The other sclerites appear intermittently between them. 2) Irregular, pedunculate, lanceolate leaf-shaped scales ( Figure 6 D ) are the second most common type of sclerites (34 to 36 μm long, 10 to 12 μm wide). 3) Solid, acicular, sclerites ( Figure 6 E, D ) are present, concentrated mainly in the dorsal region. These are curved sclerites with a narrow proximal region and are somewhat flat (but rounded in cross-section) in the middle region, with a pointed end (60 to 65 μm long, 0.8 to 1.5 μm wide). These acicular sclerites are concentrated mainly in the dorsal region. They are the least abundant type . No sclerites specific to the pedal groove were found. FIGURE 6. Helluoherpia vieiralaneroi sp. n. A. Habitus (holotype). B. Leaf-shaped scales. C. Regular lanceolate leaf-shaped scales. D. Irregular pedunculate lanceolate leaf-shaped scales. E. Solid acicular sclerites. F. Anatomical reconstruction of the anterior body based on manual reconstructions of the holotype. G. Anatomical reconstruction of the posterior body based on manual reconstructions of the holotype. 1-6. Serial sections; location in the body is indicated in F and G. H. Reconstruction of the radula. H’ Denticles in the position they were found. H’’ Radular base. Pedal groove and mantle cavity : The pedal pit is very glandular ( Figure 6 F-2). It appears just posterior to the atrium and continues internally (25 μm long with an opening that is 10 μm in diameter, 10 to 32 μm wide, 5 to 10.5 μm high). The anterior pedal glands connect with the dorsal wall of the pedal pit. They are very voluminous ven-trally and reach the mantle cavity. The pit originates as a single, triangular pedal fold in the anterior region (7.5 μm wide 8 μm high), which flattens in the medial and posterior body regions (10.5 to 17.5 μm wide, 7.5 μm high). The mantle cavity opening is ventro-posterior and very narrow (5 to 6 μm; Figure 6 G-5). In the sections it looks like a folded duct as it is small (20 μm long, 27.5 to 30 μm wide, 32.5 μm high) and is virtually confined to the opening of the spawning duct ( Figure 6 G-5). Digestive system : The mouth, which is located at the posterior end of the atrium, continues as a foregut that is narrow and circular in cross-section (32.5 μm long, 5 to 15 μm in diameter; Figure 6 F-2) and surrounded by a thin layer of longitudinal musculature (<2.5 μm). It also has a small dorsal pouch (12.5 μm long, 15 μm wide, 10 to 15 μm high). Radula with three straight, identical narrow denticles per tooth (<1 to1.2 μm wide, fragments of 10 μm long) ( Figure 6 H-H´). Fragments of a straight radular base were also observed (2 to 8 μm anterior to posterior and 2 μm high) ( Figure 6 H-H´´). The fragmented tooth was observed in the radular sac. In this area, the ventrolateral foregut glands ( type A) open ventrally in the foregut as a single duct (45 μm long). The ducts of the ventrolateral foregut glands are long and surrounded by a strong layer of longitudinal and circular musculature (approximately 5 μm thick). Most of the glandular cells discharge into the middle of the ducts, with the posterior region of the ducts free of glandular cells. The ventrolateral foregut glands are characteristic and were described for the first time for the type species of the genus Helluoherpia ( H. aegiri ) ( Handl & Büchinger 1996 ) so they can be referred to as Helluoherpia - type . The midgut has a short dorsal caecum (10 μm long, 20 μm wide), just below where the esophagus ends. This caecum is circular in cross-section and similar in diameter to the foregut. The rectum emerges dorsally in the mantle cavity (5 to 5.5 μm in diameter) and its wall is made up of sparsely ciliated. Nervous system and sense organs : The cerebral ganglion (32.7 μm long, 62.5 to 82.5 μm wide, 32.5 to 70 μm high; Figure 6 F-2) and most of the ganglia were easily observed. The pedal ganglia (5 μm long, 5 μm in diameter) are located on both sides of the pedal pit and are joined by a small (<2 μm wide) commissure. The supra-rectal com-missure is short and narrow (7.5 μm long, 5 to 6 μm thick). The atrium is very small (15 μm aperture and full length, 25 μm wide and 10 to 15 μm high) with no atrial papillae, but with a glandular wall ( Figure 6 F-1). Gonopericardial system : Well-formed gonads with the reproductive cells (lateral oocytes and spermatozoa in the central area) concentrated in their anterior region. The pericardium is long and narrow (217 μm long, 22.5 μm wide, 7 to 30 μm high; Figure 6 G-3), with a short heart attached to its dorsal wall. The pericardioducts arise in the mid-posterior region of the pericardium; they form a large seminal vesicle (50 μm long, 25 to 30 μm wide and 30 to 55 μm high; Figure 6 G-4) that is directed toward the posterior end, and although it has an unpaired origin, the posterior region is bi-lobed. The pericardioducts run parallel to the pericardium and the midgut, on both sides of the spawning duct as a pair of circular tubes (120 μm long and 7.5 to 12.5 μm in diameter; Figure 6 G-3). The spawning duct is unpaired (175 μm long) in all its extension. The pericardioducts are centrally connected to its anterior region (52.5 μm wide, 30 to 45 μm high: Figure 6 G ), which is narrower than the middle region (70 to 75 μm in diameter). The spawning duct narrows again in its posterior region (30 to 40 μm in diameter) and opens into the mantle cavity as a wide ciliated duct (opening 25 to 30 μm wide and high), which loses the surrounding musculature layer (2.5 to 4 μm). Without copulatory stylets. Remarks. The classification of Helluoherpia vieiralaneroi sp. n. within Dondersiidae is determined by the type of mantle sclerites, the ventrolateral foregut glands ( type A), the radula, and the absence of respiratory folds. The description of the radula of H. vieiralaneroi sp. n. is based on fragments found in the radular sac and it is possible to affirm the presence of three denticles, which have an arrangement on the radula consistent with the genus Helluoherpia ( Handl & Büchinger 1996 ) . The inclusion of the new species in this genus is due to: 1) the sclerites (leaf-shaped scales and solid needles); 2) the radula; 3) the lack of a dorsoterminal sensory organ, copulatory stylets and respiratory folds; 4) the presence of a seminal vesicle, and 5) the peculiarities of the ventrolateral foregut glands ( type A; Helluoherpia - type ). Helluoherpia vieiralaneroi sp. n. is considered a new species based on its geographical distribution and a combination of anatomical characters that set it apart from the only other known species of the genus, Helluoherpia aegiri . Externally it resembles H. aegiri although the specimen studied here is much smaller ( 1.3 mm ) than the holotype of H. aegiri ( 6 mm ; Handl & Büchinger 1996 ). The main type of leaf-shaped scales of the two species are similar in shape and size, but in the new species the solid acicular sclerites are curved and somehow flattened, while those of H. aegiri are straight ( Handl & Büchinger 1996 ), and it also has fewer pedunculated leaf-shaped scales. The basic internal anatomical characteristics of H. vieiralaneroi sp. n. and H. aegiri are similar but there are some important differences: a) H. vieiralaneroi sp. n. has a very small atrium, with no atrial papillae, whereas in the atrium of H. aegiri , there are large globular papillae ( Handl & Büchinger 1996 ); b) In H. vieiralaneroi sp. n. the ducts of the ventrolateral foregut glands are shorter, and they run parallel to the foregut and then to the ventral region, and not to the dorsal region as in H. aegiri ( Handl & Büchinger 1996 ) ; c) H. vieiralaneroi sp. n. has a small midgut caecum that is absent in H. aegiri ; d) In H. vieiralaneroi sp. n. the seminal vesicles are exceptionally bulky and extended toward the posterior end, whereas in H. aegiri , they are smaller and extend toward the anterior region; e) The spawning duct in H. vieiralaneroi sp. n. is much longer and bulkier than in H. aegiri ( Handl & Büchinger 1996 ) ; f) The pericardium is markedly larger in H. vieiralaneroi sp. n. than in H. aegiri , which has a very short pericardium ( Handl & Büchinger 1996 ) and it also extends far posteriorly from where the pericardioducts fuse with it.