Alexandromenia gulaglandulata, Salvini-Plawen, 2008

Salvini-Plawen, Luitfried von, 2008, Contributions to West European Cavibelonia (Mollusca, Solenogastres) with two new species, Zoosystema 30 (4), pp. 873-897 : 889-896

Alexandromenia gulaglandulata n. sp. ( Figs 16-19 ; 20A ; 21-24 )

HOLOTYPE. — Sclerites, section series on slides; midbody in alcohol ( MNHN 9835 ).

TYPE LOCALITY. — Thalassa Cruise 73 Z, stn Z 435.

MATERIAL EXAMINED. — West European Basin. Thalassa Cruise 73 Z ( CENTOB), stn Z 435, 48°40’N, 09°53’W, 1050 m, 26.X.1973, 1 specimen, 21 mm long, without keel formation ( Fig. 16 ). Th e preserved animal has a rounded, coiled body, somewhat higher (2.8-3.5 mm) than broad (2.4-2.8 mm), and is thicker anteriorly.

Both body ends of the specimen were serially cross sectioned in paraffi n of 10 µm and stained with Azan.

ETYMOLOGY. — Latin: gula, gullet, throat, pharynx; glandula, small gland; -atus, provided with. Referring to the different types of foregut glands.

DIAGNOSIS. — 21 × max. 3.5 mm, without keel formation. Mantle papillae long-stalked with most distal swellings at the same level. With 18-11 (terminally 4) pedal folds that do not enter the mantle cavity; mantle cavity posteriorly with 16 respiratory folds, anteriorly with irregular, slender sacculations; with suprapallial glands. Transverse body wall musculature embedded in matrix. Atrial papillae slender, single. With four sets of foregut glands: pharyngeal glands, pre-radular packages of glands, paired lateroventral foregut glandular organs with duct leading into pouch through ductule-less cone and not opening on a papilla, and oesophageal glands. Radula plates 55-65 µm wide, with one denticle at each side; radula support (bolster, odontophore) symmetrically arranged turgescent cells; midgut caecum short, unpaired throughout. Fused section of spawning ducts anteriorly still with glands, muscular opening ventral at anterior border of mantle cavity opening. Suprarectal commissure long; dorsoterminal sense organ above mantle cavity.

DESCRIPTION

Mantle and body wall

The 150-250 µm thick cuticle with densely arranged, long-stalked epidermis papillae, whose distal swellings are nearly all at the same level ( Fig. 18 ). The sclerites are mostly eroded, but fragments and negatives in the cuticle show two kinds of acicular, slightly curved, hollow spicules ( Fig.17 ): small, 60-90 × 4-8 µm long, tangential needles, densely arranged in several layers,

cu pap lm sp

and strong radial spicules ( Fig. 18 , sp 2) measuring 120-200 × 10-15 µm (the shorter and more slender ones towards ventral), some up to 225 × 25 µm. The low epidermis (only 4-8 µm) is underlain by a 70 µm thick matrix. This contains the transverse (“circular”) musculature ( Fig. 18 ), which is rather scattered in the anterior body. The longitudinal musculature of the body wall is indistinctly grouped into bundles; bundles somewhat enlarged ventrally without,however,forming a distinct,paired musculus longitudinalis ventralis.

Foot and mantle cavity

The posterior half of the pedal pit shows 18 slender folds which enter the pedal groove and extend along the oesophageal region; in the posterior body, 11 of these folds are reduced to eight below the anterior mantle cavity,ultimately to four folds directly in front of the opening of the mantle cavity.Th e mantle cavity forms 16 symmetrically arranged respiratory folds. Anterior to the ventral opening, these are replaced by peripheral, irregular, slender diverticula (up to 200 µm long) or sacculations of the roof ( Fig. 24 );

they delimit the mantle cavity against the circular hindgut; the 800 µm long suprarectal commissure is located near the end of the pericardium. The muscular terminal portion of the unpaired spawning duct opens at the anterior border of the mantle cavity opening.From the suprarectal commissure backwards, the whole mantle cavity is coated by a dense mass of suprapallial glands. The terminal sense organ lies above the central mantle cavity ( Fig. 23 ).

Alimentary tract

The anterior sense organ is located within the common atriobuccal cavity and is delimited by the horseshoe-shaped ciliary tract; the atrial papillae are single and slender. Th e cerebral ganglion (400 µm broad, 200 µm long, 200 µm high) is located above the buccal cavity between atrium and mouth opening; it has three pairs of cerebral nerves, each with a small basal swelling and a separate lateral origin of the connectives. Th e wide anterior pharynx shows several irregular foldings and is surrounded by distinct, but loosely arranged musculature as well as by indistinct pharyngeal glands. More posteriorly, the lumen becomes X-shaped and the musculature more distinctly circular; ventro-laterally, the paired foregut glandular organ opens pre-radularly

A B

( Fig.19 ). Th e region between these openings and the radula is widened and characterised by peripheral packages of an additional type of strongly stained glands; the cells of each package empty together via slender stalks into the pharynx ( Figs 19 ; 21 ). These packages differ distinctly from those of the paired lateroventral foregut glandular organs; both types partially overlap in the radula region ( Fig. 21 ). The ducts of the paired foregut glandular organs show a median ridge and are encircled by musculature. The anterior-most portion of each duct curves medially and widens to a pouch. Th e median ridge of the pouch is enlarged to form a cone ( Fig. 20A ) which, in this species, has no ductule. Both pouches open directly into a deep, slender cleft (“duct”) of the pharynx; no protruding opening papilla is developed. Each of the paired ducts of the glandular organs is, in the radula region, continuous with the dorsal and ventral limb of a collecting duct. Th is duct, in turn, receives the often ramified outleading ductules of the gland clusters; these clusters are thus more or less semicircularly arranged at both sides of the postradular foregut.

The radula bends into an unpaired anterio-ventral sack containing the radula plates. Th e plates are basally 65 µm, distally 50 µm broad and about 30 µm high with a 27-30 µm long denticle at both lateral borders ( Fig. 22 ); no central denticles were detected. The radula support consists of a paired bolster of at least five turgescent cells each. The buccal ganglia (diam.150 × 100 µm) closely adjoin (distance 50 µm) and are located ventro-posterior to the radula sheath. The postradular foregut (oesophagus) initially narrows without structural change and is provided with circular musculature as well as several strong radial muscle bundles. It is surrounded by densely arranged, small groups of long-necked oesophageal glands. More posteriorly, this portion telescopically intrudes into the wide second oesophageal region, which has a weak circular musculature and groups of gland cells. The oesophagus narrows again and axially intrudes into the midgut. Th e anterior caecum of the midgut is short, extending above the oesophagus only. The midgut enlarges laterally to flank both sides of most of the oesophagus.Both the midgut and caecum have a middorsal ciliary tract. The dorsoventral muscle bundles, about 300 µm apart, cause regular ventrolateral midgut pouches. Below the pericardium and between the spawning ducts, the midgut continues into the hindgut, which is ciliated throughout; the anterior hindgut is surrounded by loose longitudinal muscles,posteriorly by both longitudinal and circular musculature.Th e 800 µm long, medullar suprarectal commissure (diam. 90 µm) is located below the end of the pericardium ( Fig. 23 ).

Gonopericardial system

The animal is mature, with the paired gonad extending to the second oesophageal region; the eggs are up to 150 µm in diameter. The gonopericardioducts are continuous with the dorso-frontal pericardium, which shows a paired anterior pouch. Both the ventricle and the single auricle of the heart are a free tube within the pericardium; the connection

rs

ro

of both portions is strongly narrowed. The two pericardioducts emerge lateroterminally. There are no vesiculae seminales. A small, posteriorly directed receptaculum seminis is laterally differentiated at the opening of each pericardioduct into the anteriomost, lateral spawning duct ( Fig. 23 ). The ciliated, simple spawning ducts are paired along most of their length and fuse near the end region of the pericardium. The subepithelially arranged, elongate and slender spawning glands are aggregated; they surround the ducts as a dense mass and open intercellularly. The unpaired spawning duct is provided with circular musculature and anteriorly (region of rectum) still shows openings of glands. Th e posterior portion of the unpaired spawning duct (below anterior mantle cavity) has a sphincter and radial muscles within a bulge at the bottom of the mantle cavity ( Fig. 24 ); the ventro-terminal outlet opens into the beginning mantle cavity opening.

COMPARISONS

All organizational characters of the single specimen classify it as a member of the Amphimeniidae . The key features here include the configuration of the lateral foregut glandular organs, the monoserial radula and the organization of the spawning ducts (see Salvini-Plawen 1972, 1978). Except in Plathymenia Schwabl, 1961 , and Alexandromenia (?) crassa Odhner, 1921 , all members of the family show a pre-radular outlet of the ventro-lateral foregut glandular organs.In most species this organ possesses a terminal cone within a pouch or sheath ( Fig. 20 ) that opens by formation of a papilla intruding into the pharynx ( Fig.20B ). Th e details of the outleading duct, however, differ:this may open directly, without cone formation, into the pharynx (with or without a papilla), such as in Alexandromenia pilosa Handl & Salvini-Plawen, 2002 ; the duct may open into the pouch (sheath) of the cone ( Fig. 20A ), such as in the present species (no papilla); the outleading duct may be directly continuous with a (somewhat eccentric) internal ductule of the cone ( Fig. 20B ), such as in A. antarctica Salvini-Plawen, 1978 . Several species exhibit both a connection with the pouch and a ductule within the cone (Salvini-Plawen 1978); in some species, several slender ductules pass through the cone ( Heath 1911).

Within the Amphimeniidae , the present specimen clearly belongs to the genus Alexandromenia (see diagnosis above and the comparative tables in Salvini-Plawen 1978 as well as García-Álvarez et al. 2000). An identity with the incompletely known Meromenia ( M. hirondellei Leloup, 1949 , from the Bay of Biscay) can be excluded based on the lack of respiratory organs and midgut pouches. Eight Alexandromenia species are currently known. The present specimen differs from these by the packages of additional foregut glands in the region between the outlets of the lateral glandular organs and the radula apparatus. Th is also includes the three known Atlantic representatives: A. crassa Odhner, 1921 , from the Hjelteŋord (Bergen area, Norway) at 100-200 m, A. grimaldii Leloup, 1946 , from off the Azores at 1250 m, and A. pilosa Handl & Salvini-Plawen, 2002 , from the Trondheimsŋord ( Norway) at 180- 240 m.

The poorly described A. crassa , apart from its “pair of small salivary glands opening into the radula sac” ( Odhner 1921: 22) – which casts doubt on the generic classification – also differs from the animal at hand by the dorso-median keel of spicules, by the fairly stout body (10 × 3-3.3 mm) with 9 pedal folds, by the 160 µm broad radula plates of different outline, by the long midgut caecum extending beyond the cerebral ganglion, and by the dorsally extended receptacula seminis.

Alexandromenia grimaldii measures 20 × 2.3 mm with an about 140 µm thick cuticle; it differs from the present specimen in the number (7-3) of pedal folds, whereby the median fold connects with the mantle cavity, in the voluminous midgut caecum, in the hindgut being continuous with the ventral midgut only, as well as in the receptacula seminis being enlargements of the distal pericardioducts ( Leloup 1946). It is particularly characterised by the U-shaped radula teeth (Salvini-Plawen 1972).

Alexandromenia pilosa , measuring 9 × 1.2-1.4 mm, has only 5-3 pedal folds, club-shaped epidermis papillae, sclerites of one type only, a subepithelial matrix outside the transverse musculature, an anteriorly bilobed midgut caecum, a short suprarectal commissure (about 100 µm), and anteriorly directed receptacula seminis; it lacks suprapallial glands and no turgescent cells of the radula support or bolster are mentioned (Handl & Salvini-Plawen 2002).

The present specimen is thus organizationally well separated and represents a proper, new species: A. gulaglandulata n. sp.

Figures

FIG. 16. — Alexandromenia gulaglandulata n. sp., holotype in preserved state (anterior end upper left), 21 mm long.

FIG. 17. — Alexandromenia gulaglandulata n.sp., mantle sclerites. Scale bar: 50 μm.

FIG. 18. — Alexandromenia gulaglandulata n. sp., cross section through mantle. Abbreviations: ca, anterior caecum of midgut; cu, cuticle;gl, lateroventral foregut glands;lm, longitudinal musculature;pap, epidermal papillae;rm, matrix with transverse musculature of body wall; sp2, radial spicules. Scale bar: 150 μm.

FIG. 19. — Alexandromenia gulaglandulata n. sp., organization of the anterior body viewed from left. Abbreviations: ao, atrial sense organ; es, oesophagus; gb, buccal ganglion; gc, cerebral ganglion; gl, lateroventral foregut glandular organ; go, gonad; ma, mantle; mg, midgut; pi, pedal pit; pgl, peripheral packages of pre-radular glands; ph, pharynx; vrs, ventral radula sack. Scale bar: 1000 μm.

FIG. 20. — Two types of the outlet of the lateroventral foregut glandular organs with terminal cone (and opening papilla) within Amphimeniidae (schematised): A, simple cone as in Alexandromenia gulaglandulata n. sp.; B, cone with internal ductule such as in A. antarctica Salvini-Plawen, 1978. Abbreviations: con, cone; cp, cone pouch (sheath); dg, duct of glandular organ; du, ductule of cone; pa, opening papilla; ph, pharyngeal cavity; ri, median ridge of duct.

FIG. 21. — Alexandromenia gulaglandulata n. sp., cross section through posterior pharynx, just anterior to radula. Abbreviations: ds, dorsal sinus; gl, lateroventral foregut glands; pgl, pre-radular packages of glands; ph, pharynx. Scale bar: 150 μm.

FIG. 22. — Alexandromenia gulaglandulata n. sp.,one radula plate. Scale bar: 20 μm.

FIG. 23.— Alexandromenia gulaglandulata n.sp.,organization of the posterior body viewed from left.Abbreviations:co, suprarectal commissure;div, diverticula;fo, pedal folds (foot);ma, mantle;mg, midgut;pc, pericardium;ro, respiratory organs;rs, receptaculum seminis; sd, spawning duct; sdg, spawning duct glands; so, dorsoterminal sense organ; spg, suprapallial glands. Scale bar: 500 μm.

FIG. 24.— Alexandromenia gulaglandulata n. sp.,cross section through anterior mantle cavity with irregular,slender diverticula.Abbreviations: div, diverticle; gl, suprapallial glands; mc, mantle cavity; sd, spawning duct; sdg, spawning duct glands. Scale bar: 150 μm.