Paucibranchia stragula Molina-Acevedo 2018, n. comb.

Paucibranchia stragula (Grube, 1878) n. comb.

Figures 74–75, Table 3

Eunice (Marphysa) stragulum Grube, 1878:v (species list)

Eunice stragulum Grube, 1878:163 –164.

Marphysa stragulum Hartman 1959:322.

Material examined. Type material: Holotype MPW 284, Talibon, Bohol, Philippines, Sep 1863, on stony ground, 14.6–18.3 m, coll. C.G. Semper.

Description. Holotype incomplete, with 57 chaetigers, L10= 3.7 mm, W10= 1.2 mm, the fragment with TL= 17 mm. Anterior region of body with convex dorsum and flat ventrum, without groove; body depressed from chaetiger 9, widest at chaetiger 32, tapering unknow.

Prostomium entire, 1 mm long, 0.6 mm wide, frontally rounded, without median sulcus (Fig. 74A), ventral sulcus deep (Fig. 74B–C). Prostomial appendages in a semicircle, median antenna isolated by a gap. Palps reaching the middle of first peristomial ring; lateral antennae reaching first chaetiger; median antenna reaching the middle of first chaetiger. Palpophores and ceratophores ring-shaped, short, slender; palpostyles and ceratostyles tapering, thick, without articulation. Eyes rounded, brown, between palp and lateral antenna.

Peristomium wider than prostomium (0.8 mm long, 1.2 mm wide), first ring two times longer than second ring, separation between rings distinct on all sides (Fig. 74A). Inferior lip with a slight central depression (Fig. 74B–C).

Maxillary apparatus with MF= 1+1, 7+7, 6+0, 3+6, 1+1 (Fig. 74D). Maxillary carriers 2.3 times shorter than length of MI. MI forceps-like; closing system 4.7 times shorter than length of MI; ligament between MI and MII not sclerotized (Fig. 74D). MII wide; teeth recurved; cavity opening oval, 2.5 times shorter than length of MII (Fig. 74D); ligament between MII and MIII, and right MIV, not sclerotized (Fig. 74D–E). MIII short; with triangular teeth, with irregular attachment lamella, slightly sclerotized, situated only in center of posterior edge of maxilla (Fig. 74D–E). Left MIV with a small basal tooth; attachment lamella semicircular, wide, situated 2/3 along length of posterior edge of maxilla. Right MIV with smaller teeth; attachment lamella wide, better developed in the middle, situated 3/4 along length of posterior edge of maxilla (Fig. 74D–E). MV square, with a short rounded tooth (Fig. 74D–E). Mandibles translucent; cutting plates whitish, with 9–10 growth rings.

Branchiae pectinate with up to 10 filaments, in chaetigers 11–22 (Fig. 75B). Number of branchial filaments per chaetiger in order anterior-posterior: 7, 9, 9, 9, 10, 9, 9, 9, 8, 7, 6, 4. Branchial filaments longer than dorsal cirri.

First two parapodia smallest; most developed in chaetigers 4–10, following ones becoming gradually smaller. Notopodial cirri conical, increasing in size from chaetigers 4 (Ldc3: 0.50 mm; Ldc15: 0.57 mm), from chaetiger 14, gradually decreasing in wide, in posterior region dorsal cirri broken, seems longer than the anterior region ones (Fig. 75A–C). Prechaetal lobes as a transverse fold in all chaetigers (Fig. 75A–C). Chaetal lobes in chaetigers 1–23, rounded, shorter than postchaetal lobes, with aciculae emerging dorsal to midline; from chaetiger 24, triangular, longer than other lobes, with acicula emerging in midline (Fig. 75A–C). Postchaetal lobes well developed in chaetigers 1–26, digitiform in pre-branchial chaetigers; conical, thinner, elongated in branchial region; decreasing in size in chaetigers 22–26, following ones inconspicuous (Fig. 75A–C). Ventral cirri digitiform in chaetigers 1–4; in chaetigers 5–29 with oval swollen base and digitiform tip; from chaetiger 30, digitiform, gradually reducing in size posteriorly (Fig. 75A–C).

Aciculae blunt, translucent (Fig. 75A–C). First 14 chaetigers with 2 or 3 aciculae; in chaetigers 15–20 with 2 aciculae; from chaetiger 21, with only one acicula.

Limbate chaetae of two sizes in same chaetiger, larger in anterior region, reduced in number around chaetiger 12. Pectinate chaetae; in anterior chaetigers with 5–6 pectinate with blade and teeth broken; in median-posterior chaetigers with 12 pectinate with blade and teeth broken. The most compound chaetae broken. Three compound spiniger present in chaetiger 42, with blade of similar size (Fig. 75F); in the same chaetiger two compound falcigers also present, with blades of similar size (40 µm, Fig. 75D–E), with triangular teeth, distal shorter than proximal, directed upward, proximal tooth directed laterally. Subacicular hooks bidentate, translucent, starting in chaetiger 24, with triangular teeth, distal tooth smaller than proximal tooth, directed upward; proximal tooth directed laterally; with 2 hooks in chaetigers 25–27, with 3 hooks in chaetiger 28, with 4 hooks in chatigers 29–33, with 5 hooks in chaetigers 34–36, with 6 hooks in chaetigers 37–40, with 5, 6 hooks in chaetigers 41–58 (Fig. 75G– H).

Distribution. Talibon, Bohol, Philippines.

Remarks. Grube (1878) did not state the etymology of his new species “ stragulum ” (from the greek, "cover or mantle"), but he possibly proposed that name to highlight the long pectinate branchiae that cover the dorsum of a short body region. However, the word “ stragulum ” (neutral) was mistakenly declined in Eunice Cuvier, 1817 (feminine). Since the species is herein transferred to Paucibranchia n. gen. (feminine), the correct declination is Paucibranchia stragula n. comb..

The holotype of P. stragula n. comb. is incomplete and in poor condition, whereby some features were not described herein. Despite the parapodia have lost partly digested musculature, looking flabby and translucent, it was possible to determine the shapes of the cirri and lobes. Nevertheless, features such as the chaetal distribution, the number of teeth in pectinate chaetae, and the size of the blades of compound chaetae were not described in detail since most chaetae were broken. It was observed few compound spinigers and falcigers in a single median parapodium (chaetiger 42), but another single later parapodium (chaetiger 50) showed falcigers chaetae only. This suggests that the spinigers extend posteriorly to approximately chaetiger 42, and that there are falcigers only in the posterior region. This condition matches the original description by Grube (1878), since the single specimen was mentioned with spinigers in the anterior chaetigers and falcigers in the posterior chaetigers, but the accurate chaetal distribution along body is still uncertain. It is necessary to revise topotype materials to determine this relevant feature.

Fauvel (1953) recorded the species from Sri Lanka (formerly Ceylon), but using specimens that have unidentate subacicular hook and only one hook per chaetiger; whereas P. stragula n. comb. has several bidentate subacicular hooks per chaetiger. It is likely that the Sri Lanka specimens may belong to an undescribed species, but further material is needed to support or refute this idea.

Paucibranchia stragula n. comb. is close to P. carrerai n. sp. by the presence of multiple subacicular hooks in the same chaetiger; however, the differences between the two species is provided in the Remarks section of P. carrerai n. sp. or in Table 3.