“Neomenia megatrapezata spec. nov.
(Fig. 1B, 6–8, 13B, 15)
Diagnosis
Large animals (up to 18 cm in length) with paired dorsal and lateral longitudinal ridges (lacking special sclerites); sclerites without a spear-shaped distal end; with pre-pallial spines; pharyngeal foregut with four regions, the first one with lateral “clefts” (ventral or lower “lip”), terminally with a shincter; midgut terminally with a pair of latero-ventral blind pouches enclosing the sheaths of the copulatory stylets; pallial cavity surrounded by dense muscle fibres; pericardium with dorso-ventrally organised heart, in suprapallial position; spawning ducts fused only at their opening; with paired spawning duct glands;
copulatory stylet gland with two outlets. South Shetland Islands.
Material
Three specimens, two very big animals (18 cm and 16.5 cm; decalcified) and a third, small, not yet mature individual (1.8 cm), were collected during the USARP from Stn Hero 824/32-1 close to the South Shetland Islands, 64°37’–64°38’S, 62°50’48"–
62°51’35"W, at 640–670 m.
The small specimen was serially cross-sectioned (designated as holotype), and one of the big specimens (18 cm) was anatomically dissected.
Holotype Smithsonian Institution (National Museum of Natural History) Washington, DC, United States, USNM no. 1016987; serial sections on slides.
Paratype 1 = large-sized animal of 16.5 cm length in alcohol (Fig. 15), no. 1016988.
Paratype 2 = dissected specimen of 18 cm in alcohol, no. 1016989.
Etymology
Greek mega = big, large; trapeza = table, geometrically a trapezoid. Referring to the large-sized body with its ridges, which give the animals a trapezoid cross-section, and also pointing to the external similarity to N. trapeziformis.
Description
Appearance Animals “reddish” in life, exhibiting stout shape of most neomeniids (Fig. 15), measuring up to 18 cm in length and with an average diameter (without ridges) of c. 2.5 cm. Externally characterized by four prominent, longitudinal edges or ridges—a pair of dorso-lateral and a pair of latero-ventral ones—which give the big animals a trapezoid
cross-section; in the small specimen (holotype) the dorsal ridges are not very prominent (more rounded) anteriorly and the lower ridges are nearer to the venter. The surface in front of the atriobuccal funnel and behind the mantle cavity is characterised in all three specimens by radially arranged clefts.
Foot is distinctly visible as mid-ventral groove; body openings retracted.
The measurements of the three specimens were: 18 cm (length) × 2 cm (dorsal breadth) and 3.4–4 cm (ventral breadth) × 2 cm (height); 16.5 cm (length) × 1.8 cm (dorsal breadth) and 4.4 cm (ventral breadth) × 2.5 cm (height); 1.8 cm (length) × 3.5–3.2 mm (dorsal) and 8.2–8.8 mm (ventro-lateral breadth) × 7–6.3 mm (height).
Mantle Cuticle of the histologically sectioned specimen uniform in thickness (only 30–60 μm), enclosing large papillae (up to 80 μm × diam. 70 μm) of the epidermis (25–30 μm) as well as the predominantly radially arranged sclerites. Sclerites of 18 mm individual (Fig. 1: B) consist of solid, basally slightly curved needles in two sizes (125 ×
8 μm and 220 × 8 μm), and of groove-shaped elements (100 × 15 μm and 180 × 20 μm) without spear-shaped distal end; no specialised sclerites on the ridges.
Terminally, the foot bears a 350 μm-deep groove on both sides; each houses about 12 pre-pallial spines (visible in sections only), measuring 200–250 μm × diam. 25 μm.
Foot Contracted pedal pit of the histologically sectioned specimen with two regions: anterior region of high glandular cells with poor ciliation, receiving along its entire length the intercellular openings of the cells of the pedal gland. Gland consisting of a diffuse mass of large cells with mucous secretion and extending around the pedal pit up to the side of the foregut (Fig. 6). Posterior region of pedal pit separated by a transverse bulge, with ciliated and longitudinally folded epithelium. In the small, sectioned specimen only 11 of the initially 19 pedal folds run through the pedal groove; the large animals still have 19–21 folds in the midbody. Near the transition into the pallial cavity, the folds in the groove of the small specimen decrease down to a single fold connected with the pallial cavity; in the large animal 13 folds continue further and end in front of the mantle cavity. Sole glands along the foot have granulated contents.
Pallial cavity Pallial cavity mainly occupied by respiratory structures: posteriorly and posterior-laterally, 28 dorso-ventral septa of connective tissue and muscle fibres protrude from the cavity wall, which is covered by a folded, partly ciliated epithelium; at ventral opening of cavity, the central septa are free, dorsally suspended lamellae of partly ciliated and glandular epithelium. Dorso-frontally, they diminish in size and number, and the region is modified to become the strongly folded rectal opening with a large, mid-dorsal fold. In the large, dissected specimen c. 50 respiratory lamellae are differentiated.
Pallial cavity extending further anteriorly, bearing up to 28 peripherally arranged respiratory folds there; ventro-frontally, the wall forms a transverse septum (Fig. 7) that roofs the opening of the pallio-genital pouch. This pouch is latero-frontally continuous with the stylet sheaths and includes dorsally the unpaired opening bulb of the paired spawning ducts; this bulb is short and of moderate size in the small specimen, but it is a voluminous organ bulging into the pallio-genital pouch in the large animal. In the small specimen the pallio-genital pouch widens anteriorly into a dorso-ventrally flattened space and mid-frontally a narrow duct (diam. 100 μm) continues as a blind pouch (Fig. 7 and 8; see also N. trivialis, below), in part flanked by the paired ventro-lateral midgut sac. No subvaginal epithelial gland.
Sense organs As usual (Salvini-Plawen 1985), a horseshoe-shaped ciliary tract delimits the sensory area of the atrium; this tract is dorso-frontally paired and subsequently unites into a uniform field before it divides to curve latero-ventrally as a wide band and to encompass the atrium. Sensory area with slender, long papillae, basally united in groups.
The terminal sense organ is located close to the terminal (ventrally shifted) mantle rim. It is sunk in a median groove and formed of narrow cells without mantle cuticle. Supplied by unpaired nerve.
Musculature Below the epidermis, the matrix is 580–930 μm thick. It also forms the longitudinal ridges, reaching a thickness of 1860 μm dorsally and a thickness of up to 3700 μm ventro-laterally; in front of the atrium and behind the mantle cavity, however, it has radially arranged clefts.
The matrix, staining pale blue with Azan, is traversed by numerous muscle fibres and lacunae; the longitudinal fibres are condensed ventrally, especially at both sides of opening of mantle cavity. Matrix medially delimited by circular musculature, strengthened around the mantle cavity. Distinct inner longitudinal musculature present only anteriorly, where bundles extend along the dorsal and dorso-lateral periphery of the foregut; midgut directly connected to circular musculature.
Dorso-ventral musculature distinct, serially constricting midgut. Particularly strong circular musculature surrounds foregut (see below).
Nervous system Cerebral ganglion (900 μm broad, 350 μm high, 200 μm long) dorsal to the frontal foregut bulge, giving off three pairs of frontal nerves which subdivide several times towards the atrial region. The three connectives separate laterally. The lateral ganglia (500 μm × diam. 200 μm) adjoin the body wall.
The buccal system (see Fig. 6) corresponds to that of N. naevata: a pair of pharyngeal ganglia (diam. 160 μm) with dorsal commissure (ring?) in the anterior second foregut region, buccal ganglia (diam. up to 220 × 160 μm) with a nerve ring (second
region) c. 450 μm further back, and a pair of minute
swellings (diam. 100 μm) with two nerve rings close
to the end of the foregut.
Cerebro-ventral connectives delicate; ventral ganglia (500 μm × diam. 150 μm) below the buccal ganglia and behind the pedal pit are interconnected by at least two commissures. Other ventral swellings with commissures are present at distances of 150–
200 μm, in contrast to the lateral swellings with latero-ventral connectives at distances of 200–250 μm; near the buccal ganglia, both the ventral swellings and the latero-ventral connectives are thickened.
In posterior body, the lateral cords have several swellings near the seminal receptacles and spawning ducts, the two posterior ones at each side forming connectives to the terminal ventral ganglion (ganglia posteriora inferiora; Fig. 7); they run alongside the pericardioducts and innervate the adjacent genital apparatus with numerous nerves. Subsequently, the lateral cords give off several nerves posteriorly along the pallial cavity. The terminal ganglia posteriora superiora (up to diam. 300 × 200 μm) are interconnected by a medullary suprarectal commissure (diam. 130 μm) . The commissure extends dorso-posteriorly above pallial cavity and gives rise to a median main nerve to the sense organ as well as to some paired small nerves to the respiratory lamellae.
Alimentary tract Mouth opening within common atriobuccal opening well separated from atrial area. As in Neomenia labrosa Salvini-Plawen (1978), the anteriormost foregut—the buccal area—forms a highly lobed circular fold which, in the serially sectioned animal, protrudes somewhat into the common atriobuccal cavity between the ciliary tracts. The entire buccal wall (inside and outside the fold) consists of a low epithelium. No separate oral sphincter; beginning of the pharyngeal foregut marked on both sides by a lateral epithelial cleft: in the sectioned small specimen the anteriormost ventral pharyngeal portion forms a short (c. 450 μm) free lower lip (the ventral foregut lumen still being delimited by the buccal epithelium; see N. labrosa in Salvini-Plawen 1978).
The pharyngeal foregut possesses a cuticularised epithelium and in both the serially sectioned and dissected animals has about 15 strong longitudinal folds. Histologically, four regions can be distinguished: the first (including the clefts and the lip) lacks circular musculature but has colourless, granulated subepithelial gland cells, whose secretion is discernible in the gut lumen; the large lateral and ventral pouches may allow protrusion of the foregut. The second region is characterised by a strong circular musculature (600–800 μm thick) traversed by red-stained subepithelial gland cells which may form compact packs laterally and ventrally; red secretion clearly visible in gut lumen. Third region with few gland cells, circular musculature diminished (100–150 μm), but radial dilators markedly reinforced. Third and fourth regions in large animal very elongate and representing most of the pharynx. Fourth region in serially sectioned specimen shortly intruded into third one and marked by thickened circular musculature (600–1000 μm) that is not obvious in large animal; radial musculature restricted to ventral area. Terminal portion of fourth foregut region opens frontally into midgut; circular musculature diminished here (500– 200 μm), representing a poorly delimited sphincter (in both specimens this terminal portion is intruded into the midgut by contraction).
Midgut with both a mid-dorsal and mid-ventral ciliary tract. Remainder consists of glandular cells intruding folds reflecting constrictions by the serially arranged dorso-ventral musculature. Midgut does not always extend to lateral body wall, resulting in wide haemocoel spaces. Gut empty.
At transition to hindgut, at both sides of mid-ventral ciliary tract above frontal area of pallio-genital pouch, a short, folded connection (duct without sphincter; absent from the dissected animal) continues into a large latero-ventral, empty pouch with simple, ciliated epithelium. This termino-ventral blind midgut pouch or sac at each side extends anteriorly for a short distance at same level as pallio-genital pouch; posteriorly it enlarges and mostly surrounds anterior copulatory stylet sheath reflecting condition in Archaeomenia prisca (above) and N. trivialis (below).
Hindgut narrow and ciliated throughout. Shortly before opening medio-dorsally into the mantle cavity, the strongly folded rectum is characterized by a mid-dorsally intruding, voluminous fold.
Circulatory system In both animals, heart ventricle connected with roof of pericardium only anterio-dorsally, just before the dorsal sinus; auricle paired and ventro-posteriorly continuous with suprapallial lacunae; single atrio-ventricular opening. Strong posterior contraction of small specimen displaces ventricle and atria in a dorso-ventral position (Fig. 13: B). Dorsal sinus continuing anteriorly between gonadial tubes. Ventral sinus splits in front of mantle cavity into two sinuses before opening into ventrolateral pallial lacunae.
Blood cells as disk-shaped haemocytes (16– 20 μm × 12 μm) with distinct nucleus and vacuoles, and as globular, light-refracting granulocytes (diam. c. 8 μm).
Gonopericardial system (Fig. 7 and 8) Paired gonad of simple tubes with no trace of developing germinal products (small specimen). Large animal has densely packed eggs (average diam. 300 μm); here, tube-like gonads bear numerous ventral germinal bags. Gonads continuous with short, narrower and glandular gonopericardioducts which open dorsally into pericardium. The latter gives rise to ciliated pericardioducts latero-ventrally (Fig. 13: B). They form superimposed loops adjacent to body wall and possess a regularly folded epithelium. Vesiculae seminales absent in sectioned and dissected animals.
Each pericardioduct medially continuous with voluminous, elongated receptaculum seminis, and posterio-ventrally opening into spawning duct; in large animal, receptacula form a flattened pouch each anterior to pericardioduct opening, and spawning ducts extend almost dorso-ventrally. Epithelium of spawning ducts and beginning of receptacles with glandular cells but no cilia. Short terminal portion of each spawning duct narrowed, receiving duct of spawning duct gland (Fig. 8: sdg); gland located between spawning duct itself and midgut sac, with glandular epithelium but no cilia. Slender interconnecting duct also opens near narrowed spawning duct. Spawning duct gland is a voluminous, internally much-folded organ extending from medial of spawning duct to far anteriorly of receptaculum.
Paired copulatory apparatus consists of a lateral groove-shaped element and a spine within a common sheath. In the small animal a small gland (stylet gland, “penis” gland) is differentiated at distal third of sheath; this gland also sends a narrow duct to end of spawning duct. In the large specimen, the gland developes several thin-walled lappets. Each stylet sheath surrounded proximally by a midgut sac, distally embedded in fibrous connective tissue continuous with matrix. Each sheath curves slightly and is continuous with lateral portion of palliogenital pouch of mantle cavity.
Comparative discussion
With regard to the accessory genital apparatus, the histologically sectioned small (18 mm) animal reflects an adult organisation in a not yet mature condition. Identical appearance, presence in the same sample, and coinciding anatomical organisation support
conspecificity of the dissected large specimen with the submature individual. In addition, the state of the genital apparatus (preceeding maturity) allows full comparison with the organisation of other Neomenia species (see also variable correlation in Archaeomenia
nova: Scheltema 1999).
The present animals are extremely large representatives of Neomenia similar to N. yamamotoi Baba, N. herwigi Kaiser, and N. permagna S.-Plawen. Yet, an external similarity (four longitudinal ridges) exists with N. trapeziformis only, and a closer relationship between the two species is also suggested by the similar configuration of the anteriormost pharynx and the genital apparatus (cf. Salvini-Plawen 1978). In both species, the pharyngeal epithelium does not begin with a circular change of histology, but exhibits a more anterior pharyngeal differentiation dorsally and ventrally, thus presenting in its first region a paired lateral cleft as in N. labrosa. In both the present species and N. labrosa the freely protruding ventral pharyngeal epithelium forms a lower lip in the buccal space, not present in N. trapeziformis. On the other hand, N. carinata and N. crenagulata S.-Plawen differentiate a similar pharyngeal cleft medio-ventrally (Salvini-Plawen 1978), whereas the unnamed species from the South Shetland Islands (Garcia-Alvarez & Urgorri 2003) exhibits three clefts, a paired lateral one delimiting a ventral lip and a mid-dorsal cleft. Other distinguishing features from N. trapeziformis include the foregut regionation, the pericardium with heart, the latero-terminal blind midgut pouches, and the mantle covering sclerits (short harpoon-shaped sclerites absent in present specimens).
Besides the longitudinal body ridges, the pericardium-heart condition and the latero-terminal midgut sacs likewise serve to distinguish N. megatrapezata spec.nov. from other Neomenia species (Table 1; cf. Wirén 1892; Heath 1918; Salvini-Plawen 1978), including N. labrosa with its similar foregut (Salvini-Plawen 1978).”
(Salvini-Plawen & Paar-Gausch, 2004: 147-152)
