Promastobranchus variabilis, Lin & García-Garza & Mou & Lin, 2023

Lin, Jun-Hui, Garcia-Garza, Maria E., Mou, Jian-Feng & Lin, He-Shan, 2023, Description of a new Promastobranchus species (Annelida, Capitellidae) from Chinese coasts, with molecular evidence for intraspecific variation in the number of thoracic chaetigers, ZooKeys 1174, pp. 1-14 : 1

publication ID	https://dx.doi.org/10.3897/zookeys.1174.106624
publication LSID	lsid:zoobank.org:pub:34358AFB-8EF7-41AC-9A9F-258F13AADF64
persistent identifier	https://treatment.plazi.org/id/B2610B1B-1874-4E73-9D40-C4B509CC913A
taxon LSID	lsid:zoobank.org:act:B2610B1B-1874-4E73-9D40-C4B509CC913A
treatment provided by	ZooKeys by Pensoft
scientific name	Promastobranchus variabilis
status	sp. nov.

Treatment

Promastobranchus variabilis sp. nov.

Figs 2A-F View Figure 2 , 3A-K View Figure 3 , 4A-G View Figure 4

Material examined.

Holotype: TIO-BTS-Poly139, complete, Beibu Gulf, sta. S31 (19.65°N, 107.70°E), 65 m depth, coll. Jun-Hui Lin, 23 May 2021. Paratypes: TIO-BTS-Poly140 • 1 spec., incomplete, same information as holotype. TIO-BTS-Poly141 • 4 specs, all incomplete, Beibu Gulf , sta. S42 (19.25°N, 108.10°E), 59 m depth, coll. Jun-Hui Lin, 24 May 2021 GoogleMaps . TIO-BTS-Poly142 • 3 specs, TIO-BTS-Poly143 • 5 specs and TIO-BTS-Poly144 • 6 specs, all incomplete, Beibu Gulf , sta. S44 (19.25°N, 108.50°E), 36 m depth, coll. Jun-Hui Lin, 24 May 2021 GoogleMaps . TIO-BTS-Poly145 • 4 specs, TIO-BTS-Poly146 • 3 specs and TIO-BTS-Poly147 • 1 spec., all incomplete, Beibu Gulf , sta. S51 (18.85°N, 108.50°E), 15 m depth, coll. Jun-Hui Lin, 26 May 2021 GoogleMaps . TIO-BTS-Poly148 • 1 spec., incomplete (posterior fragment with pygidium), Beibu Gulf , sta. S14 (20.456°N, 108.50°E), 55 m depth, coll. You-Ling Ye, 27 Nov 2022 GoogleMaps .

Description.

Holotype complete, but broken into two fragments. Anterior fragment heavily coiled (Fig. 3A View Figure 3 ), measuring 35.99 mm long by 1.27 mm wide (at chaetiger 15) for 71 chaetigers; posterior fragment measuring 10.69 mm long for 35 chaetigers. Paratypes incomplete, ranging from 2.68-19.12 mm long, 0.45-1.52 mm wide for 15-34 chaetigers. Body nearly cylindrical, widest in anterior abdomen. Color in alcohol tan (Fig. 3C View Figure 3 ). Nuchal organ not observed.

Prostomium rounded without palpode, partially concealed by peristomium (Figs 2A, B View Figure 2 , 3A-C View Figure 3 ); eyespots present on lateral sides of prostomium when dissected (obviously seen in some specimens as shown in Fig. 3D View Figure 3 ). Proboscis retracted in holotype (Fig. 3A-C View Figure 3 ), with numerous minute papillae. Peristomium achaetous, wider than long, longer than chaetiger 1 (Figs 2A View Figure 2 , 3A View Figure 3 ). Intersegmental groove distinct between peristomium and chaetiger 1 (Figs 2A, B View Figure 2 , 3B View Figure 3 , 4A, B View Figure 4 ).

Thorax with 11 chaetigers (Fig. 2A View Figure 2 ) in holotype, first chaetiger biramous (Figs 2A, B View Figure 2 , 3B View Figure 3 ). Thoracic segments bi-annulated, wider than long, with epithelium areolated from peristomium to chaetiger 7, faintly areolated on chaetiger 8, and smooth on following segments (Figs 2A, B View Figure 2 , 3B View Figure 3 ). Chaetigers 1-11 with only capillaries in both rami (Figs 2A View Figure 2 , 3A View Figure 3 ), 18-30 per fascicle in notopodia and 20-30 per fascicle in neuropodia. Notopodia inserted dorso-laterally, and neuropodia ventro-lateral. Chaetal fascicles inserted just near midline of thoracic segments (Figs 2A-C View Figure 2 , 3B View Figure 3 ). Lateral organs located between noto- and neuropodia at intrasegmental grooves, closer to notopodia in thorax and anterior abdomen, as small pores (Figs 2A View Figure 2 , 3B View Figure 3 ). Genital pores present on intersegmental grooves of between chaetigers 9/10, 10/11, 11/12, and 12/13 in holotype.

Transition between thorax and abdomen marked by chaetal change (Figs 2A, C View Figure 2 , 3E View Figure 3 , 4C, D View Figure 4 ). Abdominal segments longer and wider than posterior thoracic chaetigers in anterior abdomen (Fig. 3A, E View Figure 3 ), tapering gradually towards pygidium (Fig. 3G, H View Figure 3 ). Parapodial lobes reduced in anterior abdomen (Fig. 3A, E View Figure 3 ), well separated. Neuropodial tori pad protruded above body surface from middle-posterior abdomen (Fig. 3H View Figure 3 ). Abdominal chaetigers transitional with notopodial capillaries and neuropodial hooks throughout (Figs 2C-E View Figure 2 , 3G, H View Figure 3 ), with 15-20 notopodial capillaries and 60-70 neuropodial hooks in anterior abdomen, decreasing to 8-10 capillaries and 30-40 hooks in posterior end. In anterior abdomen, notopodial lobes located dorso-laterally and neuropodial lobes ventro-lateral (Figs 2C View Figure 2 , 3E View Figure 3 ). From middle abdomen, notopodial lobes lateral and neuropodial lobes ventral (Figs 2D, E View Figure 2 , 3A, H View Figure 3 ). Chaetal fascicles positioned posterior to midsegment in anterior abdomen (Fig. 3E View Figure 3 ), and near posterior edge of segment toward the pygidium (Fig. 3F View Figure 3 ). In the far posterior abdomen, neuropodial lobes close to each other with a small gap (Figs 2E View Figure 2 , 3H View Figure 3 , 4E View Figure 4 ).

Hooded hooks with angled node, evident constriction, developed shoulder, posterior shaft longer than anterior one, attenuated to terminal end (Figs 2F View Figure 2 , 3J View Figure 3 ). Hood with dentate distal edge (Figs 3K View Figure 3 , 4F, G View Figure 4 ), slightly longer than wide (Figs 2F View Figure 2 , 3J View Figure 3 ). Hooded hooks (Fig. 4G View Figure 4 ) with three rows of teeth above main fang: four teeth in basal row, three teeth in middle row, two smaller teeth in apical row. Main fang subtriangular, longer than wide.

No branchiae observed in abdomen. Pygidium with two digitate anal cirri on ventral side (Figs 2E View Figure 2 , 3I View Figure 3 ).

Methyl green staining pattern (Figs 2A, C, D View Figure 2 , 3A, E-H View Figure 3 ). Body stained with pale green and small dark spots of stain scattered from peristomium to prechaetal part of chaetiger 9. Body stained dark blue in dorsum of from postchaetal part of chaetiger 9 to chaetiger 20, excluding intra- and intersegmental grooves and parapodial lobes, extending to dorsal sides of neuropodia. Four pairs of genital pores (between chaetigers 9-13) stained dark blue in holotype. From chaetiger 21, each segment has a dorsal transverse band of dark stain located on between notopodial tori, as well as stain around noto- and neuropodial tori. In far posterior abdomen, dorsum stained dark blue, reaching dorsal side of neuropodial tori, interrupted by intersegmental groove.

Sequences.

The amplification of the COI gene failed for all specimens. In total, 11 partial sequences of 16S, 14 partial sequences of 18S, 13 partial sequences of 28S, and 14 partial sequences of H3 were successfully obtained from 14 specimens with 9-13 thoracic chaetigers (Table 1 View Table 1 ). There is no genetic divergence (mean K2P <0.003, with up to two base changes) among specimens from the type locality, and between the type locality and off the Fujian coast (Table 3 View Table 3 ).

Distribution.

Currently known from Beibu Gulf (South China Sea) and off the Fujian coast.

Ecology.

The new species inhabits shallow-sea (10-65 m) sediments characterized by mud, muddy sand, or sandy mud with shell fragments.

Etymology.

The specific name was derived from its variable number of thoracic chaetigers (9-13) during ontogeny.

Variation.

The majority (79%) of the type specimens possess 10 or 11 thoracic chaetigers with capillaries in both rami. Larger specimens (>1.0 mm wide) have 11-13 chaetigers with only capillaries. Areolated epithelium was clearly seen in large specimens, while obscured in the small ones. The holotype stained dark blue on four pairs of genital pores (between chaetigers 9-13), whereas some specimens have blue stain on two pairs of genital pores (on chaetigers 9-11).

Remarks.

Among the type specimens included in this study, the holotype is the only complete one. It has more discernable morphological characters than the others, such as the form of the thoracic epithelium and genital pores and, although it was not the largest specimen, it was considered the best for the holotype. Judging from its body size (body width>1 mm), it should be a mature specimen.

The new species shares a few morphological features with the two previously known species. They all possess a rounded prostomium without a palpode, eyespots on the lateral sides of the prostomium, four pairs of genital pores on the anterior body, reduced parapodia in the anterior abdomen, two anal cirri on the ventral side of the pygidium, and a variable number of chaetigers with capillaries in both rami. Despite the highly similar body appearance, Promastobranchus variabilis sp. nov. differs from P. huloti mainly in the dental formula of hooks and neuropodia in the preanal region, as shown in Table 4 View Table 4 . The hooks of the new species have nine teeth in three rows (2+3+4), instead of four teeth in a single row as in P. huloti . The number of neuropodial hooks in the preanal region is much higher in Promastobranchus variabilis sp. nov. (30-40 hooks) than in P. huloti (2 or 3 hooks). Promastobranchus variabilis sp. nov. can be also distinguished from P. orbiculatus mainly by the dental formula of hooks and the position of the genital pores. The former species has nine teeth while the latter has six teeth (2+4). Besides, the new species has four genital pores present on the intersegmental grooves of chaetigers 9-13, instead of those of chaetigers 10-14 as in P. orbiculatus . As for MGSP, the new species has a single dark transverse band per segment located on notopodia from chaetiger 21, which was absent in the other two species. Moreover, the new species bears an areolated epithelium from the peristomium to chaetiger 8 and protruded neuropodial tori in the posterior abdomen, which are not observed in its congeners.