Veromenia singula Gil-Mansilla, García-Álvarez & Urgorri, 2008, new species

Veromenia singula new species

Body to 2.3–3.5 mm x 0.75 mm in its anterior region and 0.4–0.55 mm in its posterior region. Thin cuticle, without epidermal papillae. Several types of sclerites in one layer, with oblique insertion, protruding from the cuticle: hollow acicular sclerites; scale­shaped sclerites with folded side rims; solid rimmed scale­shaped sclerites; blade­shaped sclerites of scale type, only in the ventral part of the specimen. Common atrio­buccal cavity. Pedal groove without fold. Without radula. Ventrolateral foregut glandular organs type A (according to Salvini­Plawen 1978) or Acanthomenia (according to Handl & Todt 2005). Midgut without constrictions. With a pair of seminal receptacles. Unpaired secundary genital opening. Without respiratory folds. Without copulatory stylets. With a dorsoterminal sense organ.

Types: Holotype and paratype 1 and 2 (cut in serial sections) are deposited in the Zoologische Staatssammlung Mnchen (ZSM Mol 20070750).

Type locality: Abyssal Angola Basin (SW Africa) (station 344 – Me48/1 DIVA 1; 17° 06'12'' S 04°41'42'' E – 17°07'30'' S 04°42'18'' E; 5415 m depth).

Other localities: Abyssal Angola Basin (SW Africa): Paratype 1 (station 348 – Me48/1 DIVA 1; 16°18'6'' S 05° 27'12'' E – 16°19'18'' S 05°27'12'' E 5390 m depth); Paratype 2 (station 340 – Me48/1 DIVA 1; 18°18'18'' S 04° 41'18'' E – 18°19'24'' S 04°41'54'' E; 5395 m depth).

Etymology: Latin: singulus, simple, what makes reference to the simplicity of its internal anatomy.

Description:

Habitus: 2.3–3.5 mm long specimens, 0.75 mm thick in their anterior region and 0.4–0.55 mm in their posterior region. Elongated body with a circular transverse section, without lumps or keels. Marked pedal groove. White specimens after fixation and preserved in 70º ethanol (figure 4A).

Mantle: Thin cuticle (8 μm), without epidermal papillae, with several types of sclerites in one layer, with oblique insertion and protruding from the cuticle (figure 4B, 4C, 4D): curved hollow acicular sclerites, with pointed ends (320 μm x 7­8 μm) (the hollow represents 81­89% of their length); scale­shaped sclerites with pointed distal and blunt proximal ends, with folded lateral rims forming a groove (maximal width 160 μm x 25 μm); rimmed solid scale­shaped sclerites, with pointed distal and blunt proximal ends (maximal width 70 μm x 15 μm); blade­shaped sclerites of scale­type (maximal width 80­85 μm x 25 μm) only in the ventral part of the specimen.

Pedal groove and pallial cavity: Pedal pit (90 μm x 70 μm x 85 μm) with long cilia on all its surface; two large pedal glands that open into its anterior region and stretch from the anterior part of the body to the posterior part of the pedal pit (figure 5A). The pedal groove presents no folds. The pallial cavity is small, its opening is posteroventral and has a high and ciliate epithelium. The anus opens into its dorsoanterior region, whereas the spawning duct opens into the ventroanterior region (figure 4F). It lacks any respiratory folds.

Digestive system: The mouth opens into the dorsal region of the common atrio­buccal cavity. It continues with a narrow pharynx, in which medioventral part the radular sac opens. There is no radula. The ventrolateral foregut glandular organs consist of a pair of 160 μm long tubes encircled by musculature, into whose terminal region a mass of glandular cells comes out (type A according to Salvini­Plawen 1978, or type Acanthomenia according to Handl & Todt 2005). These organs come out through a duct into the terminal region of the radular sac (figure 4E, 5A, 5B). The pharynx continues in the midgut, it lacks oesophagus. The midgut lacks constriction and takes up most of the corporal space. The rectum is narrow and ciliate (figure 5D). The anus opens in the dorsoanterior region of the pallial cavity.

Nervous system and sense organs: The cerebral ganglion (350 μm x 250 μm x 70 μm) is situated above the atrial region. It is oval and bilobar in its ventroanterior region. The ventral ganglia (70 μm x 80 μm x 60 μm) are situated on both sides of the pedal pit. The atrial sense organ or atrium (125 μm x 200 μm x 180 μm) is completely ciliated and has numerous simple papillae (figure 4E, 5A). A dorsoterminal sense organ is present.

Reproductive system: The gonads are tubular, long and are situated above the midgut (figure 4F). In the front area, they contain precursor cells of the gametes. Ova and spermatozoids can be seen together in the middle area of the gonad. Spermatozoids can be observed also in the end part of the gonad. The ova (32 μm diameter) get together on the walls that separate the gonads. The spermatozoids are situated on the side walls of the gonads. The gonopericardioducts are two very narrow ducts that end to the pericardium. The pericardium is small and presents a tubular heart, whose ends join to the dorsal wall of the pericardium. Both pericardioducts come out in the posteroventral part of the pericardium, their diameter increases from their beginning to their end region. Each pericardioduct ends into the paired part of the spawning duct (figure 5C), where a seminal receptacle also joins through a narrow tube (figure 4F, 5D). These receptacles are situated on both sides of the rectum. The spawning duct presents an epithelium of high ciliate cells (figure 4F, 5D). Its posterior region is narrower and presents a ventral pouch (figure 4F, 5D), into which a glandular mass comes out. The spawning duct opens in the middle anterior region of the pallial cavity through the unpaired opening.

Discussion:

Veromenia singula n. sp. belongs to the order Cavibelonia Salvini­Plawen, 1978 because of the presence of hollow acicular sclerites on its cuticle. Within this group, it is included in the family Acanthomeniidae Salvini­Plawen, 1978, due to its thin cuticle, scale­shaped sclerites on all its surface and the type of ventrolateral foregut glandular organs (type A according to Salvini­Plawen 1978, or type Acanthomenia according to Handl & Todt 2005). The two genera of this family known so far (Acanthomenia Thiele, 1913 and Amboherpia Handl & Salvini­Plawen 2002) present a monoserial radula with two denticles (Thiele 1913; Handl & Salvini­ Plawen 2002). As Veromenia n. gen. lacks radula, the definition of the family Acanthomeniidae should be modified as follows: Thin cuticle. With hollow acicular sclerites and scale­shaped sclerites. Monoserial or absent radula. With ventrolateral foregut glandular organs of type A (according to Salvini­Plawen 1978) or Acanthomenia (according to Handl & Todt 2005). Differences among the three genera are shown in table 2.

Veromenia singula n. sp presents some differences with the species of the family Acanthomeniidae. The species of the genus Acanthomenia, Acanthomenia gaussiana Thiele, 1913, and Acanthomenia arcuata Scheltema, 1999, have the atrium and the mouth partially separated, they present an anterodorsal midgut caecum and respiratory folds in the pallial cavity (Thiele 1913; Scheltema 1999), features that are not present in Veromenia singula n. sp. In addition, Acanthomenia arcuata presents a fold in the pedal pit and its spawning duct is bilobed in the anterior region (Scheltema 1999). On the other hand, the species of the genus Amboherpia, (Amboherpia heterotecta and Amboherpia dolicopharyngeata) present a fold in the pedal pit, a dorsal midgut caecum, a ventral bag that is previous to the pallial cavity with prepallial spicules and a pair of seminal vesicles (Handl & Salvini­Plawen 2002), whereas Veromenia singula n. sp. has a pair of large seminal receptacles that the species of Amboherpia lack. Besides, there is a sphincter in the pharynx and several associations of glands in the ventrolateral foregut glandular organs of Amboherpia heterotecta whereas the pharynx has a much greater length and a sphincter in the posterior third of the spawning duct in Amboherpia dolicopharyngeata.

It should be pointed out that Veromenia singula n. sp presents solid scale­shaped sclerites with folded lateral rims forming a groove and a ventral pouch situated under the posterior region of the spawning duct. None of the species of the genera Acanthomenia and Amboherpia present these features.