Geminofilum distichum (Eliason, 1962) Eliason, 1962

Capa, Maria, Nygren, Arne, Parapar, Julio, Bakken, Torkild, Meissner, Karin & Moreira, Juan, 2019, Systematic re-structure and new species of Sphaerodoridae (Annelida) after morphological revision and molecular phylogenetic analyses of the North East Atlantic fauna, ZooKeys 845, pp. 1-97 : 30-34

Geminofilum distichum (Eliason, 1962) comb. n. Figs 4 K–N, 5F, 8 B–C, 11, 12

Sphaerodorum distichum Eliason, 1962: 247-248, fig. 12.

Sphaerodoropsis distichum (Eliason, 1962). Hartmann-Schröder 1996: 236.

? Sphaerodoropsis chardyi Desbruyères, 1980: 115-117, fig. 4;? Böggemann 2009: 388-389, fig. 120, 123, 124, 137.

Type locality.

Skagerrak, 58°05'N, 8°32'E, 460 m.

Material examined.

Holotype: UUZM 203, Skagerrak, 58°05'N, 8°32'E, 460 m, 4 July 1933.

Holotype of Sphaerodoropsis chardyi : MNHN TYPE 1282, Bay of Biscay, 44°11.3'N, 4°15.4'W, 2430 m.

Additional material.

(6 specs) Iceland: DZMB-HH 28574 (1 spec. used for DNA sequencing, SPH 294), South Iceland, Iceland Basin, 62°33.50'N, 020°21.18'W, 1390 m, 02 Sep 2011; SMF 23898 (1 spec., used for SEM and DNA sequencing, SPH 064), Iceland Basin, 62°33.1'N, 20°23.71'W, 1384 m, 2 Sep 2011; SMF 23899 (1 spec. used for DNA sequencing, SPH 049) South Iceland, Irminger Basin, 61°36.19'N, 031°22.60'W, 2537 m, 07, Sep 2011. Norwegian Sea: ZMBN 127262 82.11.27.1 (1 spec.), 62°59.1'N, 3°13.1'E, 804 m, 27 Nov 1982; Skagerrak: ZMBN 127263 (1 spec. used for SEM and DNA sequencing, SPH295, photographed alive Fig. 8B), Drøbak, 59°38.664'N, 10°37.152'E, 106 m, 24 Oct 2014.

Diagnosis.

Body short and cylindrical, up to 2.5 mm long. Prostomial appendages smooth, lacking spurs or basal papillae. Dorsal macrotubercles sessile, hemispherical, arranged in two transverse rows per segment, with five and six macrotubercles each, from segment 2. Dorsum with 4-6 additional papillae per segment in mid body. Ventrum with 6-8 hemispherical papillae per segment, arranged in nearly Ʌ-shaped. Females with a pair of large ventral papillae, or sexual structures, between chaetigers 6 and 7. Parapodia without papilla. Acicular lobe from chaetiger 2. Stout hooks absent in anterior chaetigers. Compound chaetae in all parapodia, 4-7, with short blades (up to four times as long as wide).

Re-description of holotype.

Measurements and general morphology. Holotype short and cylindrical, 2.2 mm long and 0.4 mm wide, with 16 chaetigers. Dorsum convex, ventrum flattened. Segmentation inconspicuous, tegument smooth. Live specimens with some whitish macrotubercles (Fig. 8B); preserved specimen lacking pigmentation.

Head. Anterior end bluntly rounded. Prostomium and peristomium indistinct, appendages not observed in holotype, due to contraction of specimen. Additional material with small and digitiform prostomial appendages, without spurs or basal papillae (Figs 11A, B, 12 A–C). Antenniform papillae could be considered present (due to the position behind lateral antennae), but similar in shape and size to other prostomial papillae (Fig. 12B). Tentacular cirri, similar in size and shape to prostomial appendages. Either five or six rounded prostomial papillae between antennae (Figs 11B, 12B).

Tubercles. Dorsal macrotubercles sessile and ovoid (Figs 11C, 12D). First chaetiger with two anterior macrotubercles and a posterior with four (Figs 11B, 12B). Following chaetigers with double transverse rows of macrotubercles per segment, with five and six tubercles on each of the anterior and posterior rows, arranged in zig-zag pattern; lateralmost tubercles smaller (Fig. 4K). Few additional hemispherical papillae scattered on dorsum in no clear pattern, maximum of five or six papillae on mid segments (Fig. 12A). Ventrum with 6-8 small, hemispherical papillae arranged in a more or less Ʌ-shaped, on each segment (Figs 4L, 11D, 12E). One hemispherical papilla between parapodia, larger than ventral papillae, forming one row on each side on some segments (Fig. 11A).

Parapodia. Parapodia sub-conical, slightly longer than wide. Chaetigers with digitiform acicular lobe, present from chaetiger 2, projecting as long as ventral cirrus (Fig. 11B); ventral cirri bluntly rounded (Figs 11 D–F, 12F). Parapodia lacking papillae (Figs 5F, 11 D–F, 12F).

Chaetae. Compound chaetae present in all chaetigers, arranged in a straight or curved row posterior to acicular lobe, numbering 4-7 per fascicle (Fig. 11E, F). Shaft with slender distal end, blades slender, ranging 3-4 times longer than maximum width (Figs 11G, H, 12G).

Pygidium. Pygidium blunt with a pair of rounded terminal papillae (Fig. 11H).

Internal features. Eyes or muscular pharynx not seen in opaque holotype.

Reproductive features. Holotype female with few oblong eggs measuring 200 µm in length. A female with a flat tubercle (genital opening) between parapodia 6 and 7 (Figs 4L, 11F).

Variation.

Specimens studied measured between 1.5 and 2.5 mm long and 0.3-0.5 mm wide. Live specimens translucent with white spots in macrutubercles (Fig. 8B, C). Eyes seen (dark red) in one live specimen (Fig. 8B), pigmented (bright orange) nuchal organs observed in another similar specimen (Fig. 8C). Fixed specimens lacking any pigmentation pattern. Muscular pharynx, occupying ca. three segments, observed in translucent live specimens. All specimens identified as Geminofilum distichum comb. n. bear a similar pattern of dorsal hemispherical-oval macrotubercles (5+6), on each midbody segment, but in one individual, and also in the holotype of S. chardyi the lateral most tubercles are larger than the mid-dorsal ones, contrary to the holotype and some other specimens with smaller macrotubercles near parapodia (Fig. 4K). This specimen also presented a slightly different arrangement of ventral papillae, more aligned into four longitudinal rows (Fig. 4L).

Remarks.

Sphaerodoropsis distichum was described from 450 m depth in the Skagerrak ( Eliason 1962), and has never been found again. The single specimen acknowledged, the holotype, is not in optimal condition and some of the features referred to in the original description ( Eliason 1962) may be inaccurate. For example, the ante rior appendages were reported as absent even if the specimen looked like presenting a contracted anterior end. Re-examination of the holotype indicates parapodia lack papillae (although reported as bearing one papilla in the posterior surface), and instead there seem to be some papillae over the ventrum of the specimen, that was reported as smooth, except for a longitudinal row near parapodia. Additional material collected nearby the type locality confirmed these new findings: the specimen presents head appendages, that are short and digitiform; parapodia do not bear papillae; dorsum has a few scattered papillae in addition to the hemispherical macrotubercles and ventrum has 6-8 small spherical papillae. With these changes, the description of G. distichum comb. n. resembles that of S. chardyi Desbruyères, 1980 described from the Bay of Biscay, ca. 2500 m depth. Differences would be the presence of a curved acicula in S. chardyi , a feature that is herein questioned as it has not been observed in any sphaerodorid. Moreover, Desbruyères (1980) most likely unintentionally overlooked the existence of S. distichum and did not compare both species. It is the first time that sexual structures are reported in members of these two species.

Additional individuals found at 1400 m in Iceland (Fig. 12) also match the description of G. distichum comb. n. It is therefore here proposed the synonymy of S. chardyi and G. distichum (with some caution), based on morphological similarity between the types and additional material reported herein. There is, however, considerable genetic difference between specimens collected at different localities and depths (Fig. 1) that may indicate a large population structure within the species, or else that we are dealing with a species complex with clear geographical or bathymetrical boundaries between them. Furthermore, this species was reported from the Angola Basin also lacking parapodial papillae, apart from a globular papilla near anterior base ( Böggemann 2009). In order to assess the species boundaries of G. distichum comb. n., and the distribution range more material, and DNA sequences, will be needed.

Geminofilum distichum comb. n. is clearly recognised from other congeners, by the arrangement of macrotubercles in the first segment (2+4), the scarce and randomly arranged additional papillae over dorsum and the lack of parapodial papillae.

Distribution.

This is the first record for this species in Iceland and the Norwegian Sea. It had previously been reported from Skagerrak and English Channel ( Eliason 1962, Desbruyères 1980). Species reported also in Angola and Guinea Basins, 3900-5500 m ( Böggemann 2009).

Habitat.

Sediments from 100 to 2500 m (at least) ( Eliason 1962, Desbruyères 1980, and present study)