Paraedwardsia hadalis, Sanamyan & Sanamyan, 2018

Paraedwardsia hadalis View in CoL sp.n.

Table 1 View Table 1 ; Figs. 1–5 View Fig View Fig View Fig View Fig View Fig .

MATERIAL EXAMINED. Holotype: ZMMU Ec-114, R/V “Vityaz”, cruise 45, station 6145 , depth 7250 m, 51°09.7′N, 174°35.5′E, 20 June 1969, one specimen GoogleMaps . Paratypes: ZMMU Ec-115, same data, two specimens GoogleMaps .

DESCRIPTION. Three available specimens were in a very good condition ( Fig. 1A View Fig ). The largest specimen is 16 mm in height and 9 mm in diameter was designated as holotype. Two other specimens, 10 × 5.5 mm and 13.5 × 7 mm were designated as paratypes. The latter specimen had better preserved cuticle and ectoderm and was used for histological sections. The body is pear-shaped with wide round aboral end, divisible into physa, scapus, scapulus and very short capitulum. The distal part of the column is contracted and deeply invaginated; the tentacles, the capitulum, the scapulus and the distal part of the scapus are not visible from exterior in all specimens. The body wall is thin and eight mesenterial insertions are visible. Contracted retractors with filaments and gonads are located in proximal part of the body ( Fig. 1B View Fig ). The mesogloea of the scapus and physa is about 30–45 µm in thickness. The endoderm is about 25 µm in thickness and the thickness of the ectoderm varies from 1–2 to 50–70 µm in numerous “protuberances” and patches of thickened ectoderm ( Fig. 1C View Fig ). Some of these high protuberances of the ectoderm are supported by mesogloeal strands and resemble tenaculi ( Fig. 1D View Fig ). The scapus is covered by thin (about 1 µm) gray-brown cuticle encrusted by fine sediment, but not by sand grains and other solid particles. Nemathybomes are not present. The cuticle and the ectoderm is partly abraded ( Fig. 1A View Fig ) and because of this the border between the physa and the scapus is not clear. However, on histological sections, the physa is differentiated from the scapus by smooth surface of the ectodermal side of the mesogloea. A ring of eight (?) tiny apertures were observed in one specimen (intact paratype) but we failed to demonstrate them on histological sections. The scapulus is not covered by cuticle, it has eight high, up to 800–900 µm, mesogloeal ridges between the insertions of the macrocnemes ( Figs. 1B View Fig , 2A View Fig ). These mesogloeal ridges extend to invaginated part of the scapus. Very short thin-walled capitulum (the thickness of the mesogloea is 3–6 µm) is just at the bases of the tentacles ( Fig. 1B View Fig ). Circular endodermal columnar muscles are well developed.

The tentacles ( Fig. 2B View Fig ), 12 in number, arranged into two cycles, 9 tentacles are in the outer ring and 3 tentacles are in the inner ring, one of which arise from endocoel of ventral directives (not visible on Fig. 2B View Fig ) and two from endocoels of the dorso-lateral pairs of mesenteries. Longitudinal muscles of the tentacles ( Fig. 1E View Fig ) and radial muscles of the oral disc are ectodermal. The mesogloea on the tips of the tentacles is very thin.

The actinopharynx is rather short, has no recognizable siphonoglyphs. It has eight high longitudinal mesogloeal ridges corresponding to insertions of the macrocnemes. Four highest ridges, up to 250 µm, correspond to lateral macrocnemes and four smaller, up to 150 µm, correspond to directives. Insertions of the microcnemes are marked by small ridges, up to 50 µm ( Fig. 2C View Fig ). At aboral end of the actinopharynx the mesogloeal ridges continue into cnidoglandular tracts of the trilobate filaments, while the ectoderm of the adjacent regions of the actinopharynx form ciliate tracts ( Fig. 2F View Fig ). Reticular tract of the trilobate filaments is derived from endoderm of the actinopharynx. The filaments are present on all macrocnemes.

Eight macrocnemes are arranged as in all Edwardsiids and present along the whole length of the body. Four microcnemes paired with lateral macrocnemes are present in the distal part of the body. They appear at the most distal part of the scapus and extend through the whole length of the scapulus ( Fig. 2A View Fig ), capitulum and present at the distal part of the actinopharynx ( Fig. 2C, D View Fig ). On the transverse sections on the level of the distal part of the actinopharynx the microcnemes are rather long ( Fig. 2C View Fig ) and have a layer of fine non branched longitudinal muscles on their ventral sides (i.e. facing to exocoels) ( Fig. 4E View Fig ). In the region of the scapulus and capitulum the microcnemes have flap-like mesogloeal thickening on the ventral side. In these regions the microcnemes are lined by fine muscle fibers which are expanded on the body wall ( Fig. 4D View Fig ).

Retractor muscles on macrocnemes are restricted, with 17–22 muscle processes, few of them are weakly branched but many are not. There are no pennons or free flaps on retractors. The retractors are situated closer to the actinopharynx ( Fig. 2C–F View Fig ) and attached to the body wall by thin lamellae. At the region of physa the retractors become smaller and the mesogloeal lamella connecting the retractors and the parietal muscles become shorter ( Fig. 4B View Fig ) and disappears completely in the most proximal part of the mesenteries ( Fig. 4C View Fig ). The parietal muscles have different appearance in the different regions of the body ( Figs. 3 View Fig , 4A–D View Fig ). They first appear at the region of capitulum as a thickening of the mesenterial mesogloea lined with longitudinal muscle fibers ( Fig.3A View Fig ). Toward the scapulus tiny mesogloeal muscle processes become apparent. In this region the parietal muscles are not symmetrical, better developed on the same side as retractor ( Fig. 3B View Fig ). In the scapulus the parietal muscles are symmetrical and fan shaped ( Fig. 3C View Fig ). In the middle part of the scapus muscle processes are more branched ( Fig. 3D View Fig ) and parietal muscles are triangular on transverse sections ( Fig. 3E View Fig ). In the proximal part of the body parietal part of the mesentery covered by parietal muscles gradually become longer (on transverse sections), its mesogloea become thicker and muscle processes become shorter ( Fig. 3F View Fig ). At the region of physa parietal muscles form small, up to 10 µm, but recognizable flap from the side opposite to retractor ( Fig. 4A–B View Fig ). Parietal muscle fibers are expanded on the body wall.

Two sectioned specimens were males. On the smaller specimen gonads are present on four lateral macrocnemes but we failed to find them on directives. On the larger specimen (holotype) gonads are present on directives too. Spermatozoa are about 3–3.5 × 2–2.5 µm, tip headed, symmetrical, with wide mitochondrial complex ( Fig. 4F View Fig ).

Cnidom includes spirocysts, holotrichs, basitrichs, p-mastigophores A ( Tab. 1 View Table 1 , Fig. 5 View Fig ). Small sparse holotrichs in the ectoderm of the scapus ( Fig. 5B View Fig ) were detected only on histological sections but we failed find them on macerated samples of ectoderm from the scapus of all three specimens. Basitrichs of the capitulum and endoderm of all regions of the body ( Fig. 5J View Fig ) were seen in histological sections only (in paratype). Ectoderm of the oral disc contains spirocysts and basitrichs (recorded on histological sections). We failed to find p-mastigophores in the actinopharynx of two dissected specimens (distal, middle and proximal parts of actinopharynx were examined).

REMARKS. The most significant features of the described here species are the absence of nemathybomes, weak development of tenacules and the presence of only 12 tentacles. The arrangement of the tentacles is identical to those shown by Manuel (1988, Fig. 72) for Edwardsia ivelli Manuel, 1975 . The present record from 7250 m is a most deep water known record of a species belonging to the family Edwardsiidae . Edwardsiidae is a diverse family comprising about 70 species most of which are known from shallow and moderate depths. Very few abyssal edwardsiids are known, two of them belong to the genus Edwardsia ( E. mcmurrichi Daly et Ljubenkov, 2008 and E. sojabio Sanamyan et Sanamyan, 2012 ) and three to Paraedwardsia ( P. abyssorum Carlgren, 1951 , P. lemchei Carlgren, 1956 and P. heia Daly et Ljubenkov, 2008 ).

The genus Paraedwardsia comprises six valid species. Its members differ from the members of much more diverse genus Edwardsia by the presence of tenaculi in the scapus and by the absence of nemathybomes, nematocyst bearing structurescharacteristic for Edwardsia and Scolanthus .

Two most deep-water Paraedwardsia species, P. abyssorum and P. lemchei , differ from other members of the genus by weak development of tenaculi. Carlgren (1951, 1956) failed to find clear tenaculi in these species and referred them to Paraedwardsia provisionally because he has not seen any tenaculi, but wrote that “the numerous mesogloeal papillae indicate that such may have been present” ( Carlgren, 1956: 10). The species described here also has poorly developed tenaculi but ectodermal side of the mesogloea forms numerous high strands (“papillae” sensu Carlgren, 1956) and these three species obviously constitute closely related group. In the present paper we follow Carlgren (1951, 1956) and despite poorly developedtenaculi, referthespecimensfromAleutian Trench to the genus Paraedwardsia .

COMPARISON WITH OTHER SPECIES OF THE GENUS. Paraedwardsia lemchei is known only from the original description based on numerous specimens from Java Trench, 7160 m. It differs from P. hadalis sp.n. by possessing 14–16 tentacles and by the presence of large p-mastigophores in the actinopharynx.

Paraedwardsia abyssorum is known only from one poorly preserved specimen from Tropical Atlantic, 5610 m. The number of the tentacles, 12, is the same as P. hadalis sp.n., but retractor muscles appear to be weaker and have recognizable flap (see Carlgren, 1951, Fig. 2 View Fig ), while in P. hadalis sp.n. the flap is totally absent. It has small basitrichs in the filaments (24–28 × 3 µm) which not present in the specimens from Aleutian Trench. Although the variability of reported features and the significance of these differences cannot be accessed without examination of additional specimens, we prefer to treat the specimens from Aleutian Trench as distinct from P. abyssorum : the specimens come from too distant locations (North Pacific and tropical Atlantic) and presumably from different environment (abyssal plain for P. abyssorum and deep-water trench for P. hadalis sp.n.).

Third deep water species of Paraedwardsia , P. heia , described from 2650–3065 m off California ( Daly, Ljubenkov,2008), differsfrom P. hadalis sp.n., by well developed tenaculi (photo of intact specimen shows attached sand grains, see Daly & Ljubenkov, 2008, Fig. 9A), presence of 16 tentacles and much smaller basitrichs in the tentacles.

Other three hitherto known species of Paraedwardsia – P. arenaria Carlgren in Nordgaard, 1905, P. cretata (Stimpson, 1856) and P. sarsii (Dueben et Koren, 1847) – are relatively shallow water and do not occur in abyssal depths.