Parasabella columbi Kinberg 1867

Parasabella columbi (Kinberg, 1867), redescription

( Figs 13, 14, 32 C)

Sabella columbi Kinberg, 1867: 353; 1910: 72, pl. 28, fig. 3.— Johansson 1925: 10, fig. 3 (1–4).— Johansson 1927: 128.— Hartman 1949: 15.— Hartman 1969: 559.

Demonax columbi.— Knight-Jones & Perkins 1998: 404.

Parasabella columbi.— Tovar-Hernández & Harris 2010: 3.

Material examined. ARGENTINA, UANL 8063: Campaña SAO I, Golfo de San Matías, St. 46, 41°27’S, 64°57’W, 36 m depth, gravel, February 1971, coll. J.M. Orensanz, 2 specimens. UANL 8064: Golfo de San José, Isla de los Pájaros, 42°25’S, 64°31’W, sand flat, low intertidal, in patches of Phyllochaetopterus, coll. J.M. Orensanz, 18 January 1975, 1 specimen. UANL 8065: Campaña Sanjo II, Golfo de San José, off Las Covachas, E of Punta Tehuelche, 42°24’S, 64°19’W, scallop ground, May 1976, coll. J.M. Orensanz, 1 specimen regenerating posterior abdomen. UANL 8066–8067: Puerto Deseado, Dos Hermanas, 10 March 1981, coll. P. Knight-Jones & W.Knight-Jones, on bed rock at foot of waterfall, 20 specimens, and on holdfasts and rocks, 10 specimens, respectively. UANL 8068: Golfo de San José, Banco de Enfrente, El Riacho, intertidal mussel bed on sand, December 2005, coll. J.M. Orensanz, 44 specimens, 12 specimens regenerating.

Redescription. Trunk length 17 mm; width 1.5 – 3 mm. Branchial crown length 5 – 6 mm, with 7 – 11 pairs of radioles. Thorax with 5 – 8 segments (17 specimens with 6 segments, 2 with 5 segments, 1 with 8 segments). Abdomen with 101 – 103 segments for complete specimens. Semicircular branchial lobes, fused dorsally (Fig. 13 A), separate ventrally (Fig. 13 B). Radioles with brown pinnules along entire radiolar length (Fig. 13 A, B). Palmate membrane and radiolar eyes absent. Radioles with narrow flanges along entire lengths (Fig. 13 D), wider towards radiolar tips (Fig. 13 C). Longest pinnules at mid-radiole lenght. Radioles with short, flattened, digitiform tips, as long as equivalent space of 8 – 9 pinnules (Fig. 13 C). Dorsal lips triangular (Fig. 13 F), as long as 1/4 branchial crown length, erect, with distinct longitudinal brown radiolar appendage forming two lateral lamellae and one dorsal pinnular appendage. Ventral lips broadly rounded (Fig. 13 E), brown, with two ventral pinnular appendages. Parallel lamellae present. Mid-dorsal collar margins not fused to faecal groove, exposing anterior peristomial ring (Fig. 14 A, B). Ventral collar lappets triangular with rounded distal margins (Fig. 14 C, D), not overlapping when contracted (slightly overlapped when retracted). Lateral collar margins even, covering basal union of radioles. Ventral shield of chaetiger 1 swollen, quadrangular with rounded antero-lateral corners and slight antero-medial indentation, two times longer than second thoracic shield (Fig. 14 B, C). Collar with double row of elongate narrowly hooded chaetae. Body uniformly slender. Faecal groove wide and depth in thorax (Fig. 14 A, B).

Thoracic ventral shields rectangular (Fig. 14 C, D). First thoracic torus slightly shorter than others, not contacting ventral shield. Second thoracic neuropodial torus contacting or not contacting ventral shield. Third to sixth thoracic tori always contacting shields (Fig. 14 C, D). Superior thoracic notochaetae narrowly hooded, two times longer than inferior group, hood 1/8 times wider than shaft. Inferior thoracic notochaetae narrowly hooded (Type B: slender hoods and a progressively tapering distal tip according to Capa & Murray, 2015) (Fig. 13 G), shorter than superior notochaetae, hood 1/4 times wider than shaft. Thoracic uncini with 5 – 7 rows of equal size teeth above main fang, occupying one-half of main fang length, hood absent, breast well developed, handles as long as two times the distance from main fang to breast (medium size). Companion chaetae with denticulate bulbous head with one tooth longer, with elongate membranous tip (Fig. 13 K). Abdominal neurochaetae with two rows of elongate narrowly hooded (all chaetae equal in size). Abdominal uncini with dentition similar to those in thorax but with handles short, as long as main fang. Pygidium bilobed (Fig. 14 E, F). Pygidial eyes absent. Tubes amber colored, covered with sand grains.

Type locality. La Plata, Argentina.

Remarks. Parasabella columbi was described as Sabella by Kinberg (1867) from La Plata (Argentina) and illustrated posteriorly in Kinberg (1910: pl. 28, fig. 3). Johansson (1925) examined the holotype (NRM 1084), that is broken into four parts: branchial crown, thorax, anterior abdomen and posterior abdomen, considering it as a member of Sabella. Due to bad preservation and rupture of the type, Johansson did not add other characters to Kinberg’s description (1867) and drawings (1910), except for some chaetae. Later, Knight-Jones & Perkins (1998: 404) grouped it with Demonax based on the examination of the holotype but a description was not provided. The species was only known for the type locality. In this study, we report and redescribe P. columbi from several localities in Golfo de San José (Argentina).

Parasabella columbi has ventral shield of chaetiger 1 quadrangular, twice as long as the length of 2nd thoracic shield (in P. leucaspis complex and P. yonowae sp. nov., it is rectangular, 1.5 longer than the length of 2nd thoracic shield). In P. columbi, P. leucaspis complex and P. yonowae, radiolar tips are digitiform versus lanceolate, leaf-like as in P. fernandezensis. The ventral lappets of collar are triangular, long in P. columbi (rounded, short in P. leucaspis complex and P. yonowae) Parasabella columbi does not have pygidial eyes whereas they are present in P. fernandezensis and P. yonowae) (Table 2).

Regarding the shape of broadly hooded inferior thoracic notochaetae, Capa & Murray (2015) show that a range of forms are present, from those with broader hoods and distal ends narrowing abruptly (referred to as type A), and those with more slenderer hoods and a progressively tapering distal tip (referred to as type B). Type A is found in P. leucaspis (fide Capa & Murray 2015), whereas type B is present in P. columbi, P. fernandezensis and P. yonowae sp. nov. (Table 2).

Lengths of handles of thoracic uncini in Parasabella were also classified by Capa & Murray (2015) as short (less than the distance from the main fang to breast), medium (1–2 times distance from main fang to breast), and long (2.5 to 3.5 times distance main fang and breast). All species from South America (P. leucaspis P. columbi, P. fernandezensis and P. yonowae sp. nov.) have handles of medium length.

Transverse fission and regeneration is a common phenomenon in P. columbi (Fig. 13 H, J). This process was found in 13, out of 45 specimens (28.8%). In these worms, branchial crowns exhibit some degree of regeneration: radioles may be short, thin (Fig. 13 H, I), or rudimentary (Fig. 13 J). The number of thoracic segments is variable in worms where branchial crowns are at an advanced regeneration stage, with 5–6 thoracic segments being the most common condition (Fig. 13 I). In these regenerating worms, the first thoracic neuropodial torus does not contact the ventral shields (Fig. 13 I), in comparison to non-regenerating worms, in which the first thoracic tori always contacts shields (Fig. 14 D). In other regenerating worms, there are abdominal segments immediately posterior to the regenerated branchial crowns, indicated by 1) the presence of ventral abdominal shields divided longitudinally by the faecal groove; 2) chaetal arrangement and reorganizing chaetigers (some anterior segments lost chaetae and uncini; others have two pairs of parapodia in a single segment; or the most anterior segments have neuro and notochaetae (as regular abdominal chaetal arrangement) and the following posterior segments have noto- and neurochaeta (as regular thoracic chaetal arrangement), but with ventral abdominal shields. Thus, tori contacting (or lack of contact) with the ventral shields is not a diagnostic feature for species level identification, and must be used with carefully.