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Image of Neomenia trivialis Salvini-Plawen & Paar-Gausch 2004
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Neomenia trivialis Salvini-Plawen & Paar-Gausch 2004

Description

provided by NMNH Antarctic Invertebrates

Neomenia trivialis spec. nov.

(Fig. 1C, 9–12, 13C, 16–18)

Diagnosis

About 1 cm long (submature), mid-dorsally with a slight ridge; solid sclerites of three types, without sclerites with spear-shaped distal end; with pre-pallial spines; pedal folds not continuous with pallial cavity; pharyngeal foregut with three regions, with terminal sphincter; midgut terminally with pair of latero-ventral blind pouches enclosing sheaths of the copulatory stylets; spawning ducts terminally fused with unpaired secondary genital opening; paired spawning duct glands present; stylet gland with two outlets; 39 respiratory folds. South Shetland Islands.

Material

One specimen from the same sample as N. megatrapezata close to the South Shetland Islands, USARP Stn Hero 824/32-1, 64°37’–64°38’S, 62°50’48–62°51’36"W, 640–670 m.

Holotype Smithsonian Institution (National Museum of Natural History) Washington, DC, United States, USNM no. 1016986; serial sections on slides.

Etymology Latin trivialis = simple, usual, insignificant; referring to that lack of any exceptional, unique species-specific character.

Description

Appearance Body measures c. 1 cm (preserved 9.6 mm), with somewhat club-shaped outline (Fig. 9): anterior body 3.3 mm across, posterior body 2.6 mm. Slight ridge mid-dorsally, becoming more distinct posteriorly. Foot is a longitudinal groove.

Mantle Cuticle 90–120 μm thick anteriorly and up to 140 μm thick posteriorly. It encloses globular to club-shaped epidermis papillae as well as the predominantly radially arranged sclerites. These consist of three types (Fig. 1: C): groove-shaped elements (105 μm × 12 μm and 130 μm or more × 12 μm) without spear-shaped distal end; solid, slightly bent needles, mostly with a basally opposite bend (180 μm × 8 μm); and solid, straight and basally thickened needles (up to 140 μm × 12 μm). At end of pedal groove, a mantle infolding on both sides houses 14 pre-pallial spines in two rows of 7 one above the other.

Foot Pedal pit with two successive areas, anteriorly with ciliated epithelium of cuboid cells, posteriorly with high, folded, and strongly ciliated epithelium. Pedal glands consist of large mucus cells in a frontally paired arrangement and open intercellularly into both areas.

Thirteen longitudinal folds at transition to pedal groove proper, nine of which pass into the posteriorbody; only median fold reaches area between the pre-pallial spines, but does not enter mantle cavity. Accompanying sole glands distinctly smaller than pedal gland cells, with granulose contents.

Pallial cavity Pallial cavity extending frontally beyond anal opening; posteriorly, up to 39 radially arranged respiratory folds, ciliated distally only. Anteriorly, pallial wall serves as septum ventrally separating the pallio-genital pouch (Fig. 11–12: se and pp). This pouch receives opening of terminally fused spawning ducts dorsally and is latero-frontally continuous with paired sheath of copulatory stylets; glandular, anteriorly bifurcate blind sac present mid-frontally (Fig. 12: ppd). Mantle cavity opens ventrally somewhat behind opening of pallio-genital pouch. No subvaginal epithelial gland.

Sense organs Atrial sensory area with bundles of slender, basally united papillae. Horseshoe-shaped ciliary tract (cf. Salvini-Plawen 1985) forms a wide field dorsally, ventro-laterally developed as a fold.

Terminal sense organ behind opening of pallial cavity, forming a cuticle-free epithelial pit; innervated by unpaired nerve.

Musculature Ground substance or matrix below epidermis on average between 350 μm (anterior) and 500 μm thick (posterior body), but mid-dorsally (slight ridge) and in part also laterally thickened up to 580 μm, with embedded haemocoel lacunae and muscle fibres. Longitudinal fibres strengthened above both sides of pedal groove. Circular musculature medially delimiting matrix; inner longitudinal muscle layer not very prominent, forming more compact layer only ventrally and laterally in second foregut region.

Dorso-ventral muscle bundles close to body wall are weak, midgut constrictions absent. Foregut and genital apparatus with special musculature.

Nervous system Cerebral ganglion voluminous (550 μm broad, 300 μm long, 250 μm high) in relation to body size; three paired frontal nerves supply atrium. Connectives emerging separately laterally. The thick lateral ones short (100 μm); at each side a first lateral ganglion (250 μm × diam. 90 μm) is followed at some distance by another one (diam. 100 μm). Cerebro-ventral connectives traverse most of pedal glands, forming ventral ganglia (diam. 160 μm) with two commissures behind the pedal pit. Buccal system developed above this, in third foregut region; its ganglia (up to diam.

150 μm × 100 μm) interconnected by commissural ring and each releases a delicate nerve to end of foregut.

Ventral and lateral nerve cords have irregularly placed swellings; ventral commissures at intervals of c. 100 μm, as are latero-ventral connectives, although displaced by 30–40 μm. Lateral cords with multiple swellings in genital region, ending in marked ganglia posteriora superiora (diam. 200 × 100 μm). Each of latter connected by thick, medullary connective (diam. 45 μm) with respective posterior inferior ganglion (diam. 100 μm; with commissure), and interconnected by medullary suprarectal commissure (diam. 90 μm) overspanning frontal pallial cavity. Commissure gives off several paired nerves posteriorly and median nerve to terminal sense organ.

Alimentary tract Foregut begins, behind a buccal fold, without oral sphincter or lip (in contrast to N. megatrapezata). Pharyngeal epithelium cuticularised and longitudinally folded throughout. It has three regions (Fig. 10): First region characterised by thick peripheral layer of greyish to reddish stained, granulated gland cells, interspersed with a few circular and radial muscle fibres. Second region features strong circular musculature (c. 200 μm thick) and a peripherally adjacent layer of transversing gland cells (red secretion visible in lumen). Third region with moderate layer of circular musculature (50–90 μm thick) and distinct ventral radial bundles, but only few gland cells; its terminal portion intrudes into midgut, and musculature forms a sphincter (200–250 μm thick).

Midgut lacks a frontal caecum. With mid-dorsal and mid-ventral ciliary tract. Remaining epithelium glandular, increasingly forming intruding bulges posteriorly, most with distal ciliation. Midgut lumen contains scattered cnidae, including spirocysts, and some cuticular material. Ventro-lateral midgut pouches formed on each side at transition into hindgut; each pouch interconnected dorsally with sac around stylet sheath and muscle of proximal copulatory stylets (Fig. 16); these sacs are empty (see also N. megatrapezata and Archaeomenia prisca) with a low, partly ciliated epithelium, ending at initial part of bifurcated lumen of pallio-genital pouch. Rectum with longitudinally folded, densely ciliated epithelium and opens dorso-frontally into pallial cavity.

Circulatory system Heart ventricle entirely connected to roof of pericardium, atrium paired and extending freely, having formed from paired pericardial wall (Fig. 13: C). Wide lumen at end of ventral sinus. Blood cells: disk-shaped haemocytes (15 μm × 10 μm) and globular, lumpy granulated cells (up to diam. 7.5 μm).

Gonopericardial system (Fig. 11 and 12) Gonads of this submature individual still as simple tubes with slightly glandular epithelium devoid of cilia, extending to above foregut and surrounded by connective tissue, without trace of germinal products. Paired gonopericardioduct narrow, opening dorsally into pericardium. Pericardioducts with folded, slightly glandular epithelium with few cilia, emerging ventro-terminally from posteriorly divided pericardium, without vesiculae seminales (not yet developed?), and opening laterally into spawning ducts.

Spawning ducts with frontal, tubiform extension, each with low, scarcely ciliated epithelium (receptaculum seminis) and lined by weakly glandular cells devoid of cilia. Ducts paired until their short, common opening into genital pouch, each receiving duct of spawning duct gland medially in its distal portion (Fig. 12: sdg) and terminally connected to stylet sheath. At each side, this latter connection has a small, vesicular enlargement which, based on its position and glandular epithelium, represents a rudimentary stylet gland (Fig. 12: csg). Both spawning duct glands, located medially of the remaining genital system, with a well-differentiated glandular epithelium (in contrast to the spawning ducts, for example). The adult but submature stage of the animal allows the mutual relation of this organ complex to be traced in a sequence of only few sections (Fig. 17 and 18).

Copulatory stylets at each side as a groove-shaped element and a solid spine, both within a common sheath. Sheath accompanied by longitudinal muscle bundle (protractor) and proximally (from bifurcated pallio-genital pouch to below receptaculum seminis) anchored laterally by retractor; elsewhere sheath embedded in latero-ventral blind midgut pouch (Fig. 16). Distally, each sheath extends within mesenchyme and connective tissue towards ventrolateral region of pallio-genital pouch.

Comparative discussion

The present specimen has all essential organs and characters of the gonopericardial system (adult organisation), but was not yet fully developed (submature). Comparing it with the similarly submature N. megatrapezata described above and coming from the same sample, several characters appear to be identically differentiated, particularly the latero-ventrally extended blind pouches of the terminal midgut and the genital apparatus with the pallio-genital pouch. In detail, however, specific differences emerge. These include the development of the longitudinal body ridges, the types of sclerites, the foregut organisation, the configuration of the pericardium and heart, the mantle cavity with the arrangement of the respiratory organs, as well as the peri-pallial thickening of musculature in N. megatrapezata. Moreover, the 1.8 cm specimen of N. megatrapezata has 28 respiratory formations, whereas the present, less than 1-cm-long animal already possesses 39 respiratory folds.

Neomenia labrosa Salvini-Plawen (1978) is the unique congeneric species in close geographic proximity (61°19’S, 56°10’W, at 230 m) to N. trivialis. It exhibits great similarities in several characters, including the indication of a dorso-median ridge in the posterior body. The main differences, however, are the types of sclerites (no needles), the foregut structure (paired lateral cleft and lip formations, no terminal sphincter; cf. Salvini-Plawen 1978), as well as the lack of termino-ventral midgut sacs surrounding the copulatory stylet sheaths. Except for N. trapeziformis (see above) and for N. crenagulata Salvini-Plawen (1978) from near the Kerguelen Islands, which differ largely in the foregut structure (long ventral cleft) and the pallio-genital pouch, all other Neomenia species lack the spawning duct glands and/or differ in other characters (Table 1; see Wirén 1892; Heath 1918; Baba 1975; Kaiser 1976; Salvini-Plawen 1978). Each single, specific character, however, is known elsewhere among Neomenia species, and only their different combination separates the present specimen. Thus, a new species N. trivialis spec. nov. is defined.”

(Salvini-Plawen & Paar-Gausch, 2004: 152-161)

license
cc-by-nc
author
Salvini-Plawen & Paar-Gausch, 2004: 152-161
project
Smithsonian National Museum of Natural History - Antarctic Invertebrates

Depth range

provided by World Register of Marine Species
640-670 m.
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cc-by-4.0
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WoRMS Editorial Board