Commensodorum commensalis ( Luetzen , 1961)

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 : 22-24

Commensodorum commensalis ( Luetzen, 1961) Figs 4E, F, 5C, 9

Sphaerodoridium commensalis Lützen, 1961: 409-416, fig. 1.

Type locality.

Blåbergsholmen Island, Gullmarfjord, Sweden, 35 m.

Material examined.

Holotype: ZMUC-POL-1984, Sweden: Gullmarfjord, Blåbergsholmen Island, 35 m, 30 Oct 1960.

Additional material.

(2 specs) Skagerrak, NTNU-VM 73780 (1 spec. on SEM stub) (1 spec.), Lindön, 58°47.90'N, 11°09.52'E, 46 m, 7 May 2008. ZMBN 127260 (1 spec.), Tvedestrand, 58°33.929'N, 9°0.215'E, 34 m, 27 May 2011.

Diagnosis.

Body ellipsoid, up to 2.5 mm long. Head with short appendages, without spurs or basal papillae; antenniform papillae absent. Tubercles sessile, conical or pear-shaped, small, arranged in four longitudinal rows on dorsum, one transverse row per segment; except for first chaetiger with only two. Additional epithelial papillae minute over dorsal and ventral surfaces, in ca. 5-6 transverse rows per segment. Parapodia lacking papillae, acicular lobe, and ventral cirrus small and ellipsoid. Stout hooks in anterior chaetigers absent. All chaetae simple, unidentate, with broadened distal end, and serrated edge.

Re-description of holotype.

Measurements and general morphology. Holotype 2.5 mm long, 0.7 mm wide, with 17 chaetigers; body ellipsoid, with convex dorsum and flattened ventrum; segmentation slightly noticeable, especially on ventral side (Fig. 9A, F). Pigmentation absent on preserved material.

Head. Head fused to first chaetiger (Fig. 9A, B). Prostomial appendages conical, slightly longer than wide. A pair of palps, bigger than lateral and median antennae (Fig. 9B). Dorsal antenniform papillae absent. Few small hemispherical papillae scattered on head surface (only noticeable under SEM, Fig. 9B). Tentacular cirri ellipsoid, smaller than palps, similar in size and shape to median antenna (Fig. 9B). Eyes not observed.

Tubercles. Dorsum with four longitudinal rows of larger tubercles; in one transverse row per segment (Figs 5E, 9A, C). First chaetiger only with two tubercles. Distance between dorsalmost rows larger than there and lateral rows of tubercles. Tubercles sessile, ellipsoid or pear shaped (Fig. 9 C–E), with some pores on distal end (Fig. 9D). Dorsal papillae minute, hemispherical, arranged in ca. six transverse rows per segment (Figs 4E, 9C, E). Ventrum with fewer and larger papillae, more abundant near parapodial bases, than in mid-ventral line; with no clear arrangement pattern (Figs 4F, 9F).

Parapodia. Parapodia conical, as long as wide in three or four anterior chaetigers, 2-3 times longer in medium and posterior chaetigers. Acicular lobes ellipsoid, from first chaetiger (Fig. 9G, H). Ventral cirri similar in shape and size as acicular lobe (Fig. 9H). Parapodial papillae absent (Fig. 5C).

Chaetae. All parapodia with 4-5 simple chaetae (Fig. 9 G–I); blade serrated on cutting edge and slight recurved distal tip; distal framed by thickened edges (Fig. 9 I–K). One straight acicula per parapodium. Large, recurved hooks in the first chaetiger absent.

Pygidium. Pygidium with two dorsolateral and one mid-ventral ellipsoid cirri, similar in size.

Reproductive features. Holotype, gravid female, with oocytes measuring ca. 200 µm. Largest specimen also with oocytes, but smaller. Sexual structures or genital openings not observed.

Variation.

Largest specimens examined 4 mm long, 0.9 mm wide and 22 chaetigers. Smallest specimens with 19 chaetigers, 0.5 mm long. General morphology is homogenous among material studied. All specimens bear short prostomial appendages, ellipsoid or pear-shaped dorsal tubercles and minute epithelial papillae barely noticeable under stereomicroscope. One specimen (ZMUB 127260) is a gravid female with spheroid eggs occupying most of the body. Genital openings not observed in specimens examined.

Remarks.

Epithelium described as transparent in live specimens ( Lützen 1961), allowing the observation of the nuchal organs (as pharyngeal glands) and the coiled gut. These are not visible in currently preserved specimens, with opaque epithelium. Original drawings of specimen with 22 chaetigers, does not correspond with the holotype. The body shape is ellipsoid, with blunt anterior and posterior ends, unlike the original description with tapering anterior end, probably due to contraction of specimens after fixation. Head appendages are short and ellipsoid in all specimens examined, not digitiform as in original description ( Lützen 1961), also probably due to contraction. The main dorsal tubercles, four per chaetiger, were described as clavate ('forme de massue’), additionally other mid-dorsal ‘capsules’ were described in anterior chaetigers, but they were not spotted in the material examined. Additional minute papillae are displayed forming ca. six transverse rows per segment.

Commensodorum commensalis differs from other sphaerodorids in the presence of a unique combination of morphological features: four longitudinal rows of small dorsal tubercles, arranged in a single transverse row per segment, and the presence of simple chaetae. Sphaerodorids with simple chaetae comprise members of Euritmia , including the recently synonymised Amacrodorum ( Capa et al. 2016b), lacking large epithelial tubercles but with epithelium covered with small papillae. In addition, members of Sphaerodorum typically bear simple chaetae, but macrotubercles are arranged in two longitudinal rows and have clearly defined terminal papillae, absent in Commensodorum , and bear in addition two longitudinal rows of dorsal microtubercles (small tubercles also provided with a terminal papilla).

Distribution.

Skagerrak,? United Kingdom ( Lützen 1961, Howson and Picton 1997, Frid et al. 2009, present study).

Habitat.

Originally described as commensal of Terebellides stroemii Sars, 1835 ( Lützen 1961), at 35 m. The species has only a few times been reported in the literature since and is not necessarily associated exclusively with Terebellides ( Frid et al. 2009).