Paucibranchia gilberti n. sp.

Figures 42–45, Tables 1, 4

Marphysa sp. B Gathof 1984:40 –15, Figs. 40–9, 10a–i.

Material examined. Type material: Holotype USNM 90018, West Delta Lease Area, Louisiana , USA, Gulf of Mexico, 28°44'04'' N 89°44'07'' W, Aug 1978, 85 m, coll. SWRI for BLM. Paratypes: USNM 90021 (1), sta. HR1, Hospital Rock, Texas, USA, 27°32'05'' N 96°28'19'' W, Jul 1976, 75 m, coll. BLM . USNM 75453 (1), Mississippi Sound, Alabama , USA, Gulf of Mexico, 2 Nov 1980, 28, 5 m. USNM 90019 (2), off Port Isabel, Texas , USA, Gulf of Mexico, 26°10'12.22'' N 96°31'12.18'' W, fall 1976, 65 m. Additional material: USNM 90020 (2), off Port Isabel, Texas , USA, Gulf of Mexico, 26°10'12.22'' N 96°31'12.18'' W, spring 1977, 65 m. USNM 90022 (2), off Port Isabel, Texas , USA, Gulf of Mexico, 26°10'12.22'' N 96°31'12.18'' W, 65 m. USNM 90023 (1), off Panama City, Florida, USA, Gulf of Mexico, 29°30'02.42'' N 86°24'59.02'' W, Sep 1975, 189 m.

Description. Holotype incomplete, dissected laterally, with 80 chaetigers, L10= 3.5 mm, W10= 1.5 mm, TL= 24 mm. Anterior region of the body with dorsum convex and flat ventrum, without groove; body depressed from chaetiger 6, widest at chaetiger 18, tapering after chaetiger 57.

Prostomium entire, 0.7 mm long, 0.8 mm wide, frontally rounded, without median sulcus (Fig. 42A–C), ventral sulcus deep (Fig. 42B). Prostomial appendages in semicircle, equidistant. Palps reaching first chaetiger; lateral antennae reaching third chaetiger; median antenna reaching four chaetiger. Palpophores and ceratophores ring-shaped, short, slender; palpostyles and ceratostyles tapering slender, without articulation. Eyes absent.

Peristomium wider than prostomium (0.8 mm long, 1.3 mm wide), first ring slightly the same size than second ring; separation between rings distinct on all sides (Fig. 42A–C). Inferior lip smooth (Fig. 42B).

Maxillary apparatus with MF= 1+1, 7+7, 8+0, 4+6, 1+1 (Fig. 43A). Maxillary carriers 2.4 times shorter than length of MI. MI forceps-like; closing system 8.5 times shorter than length of MI; ligament between MI and MII slightly sclerotized (Fig. 43A–B). MII wide; teeth recurved, distal teeth pronounced longer, cavity opening oval, 5 times shorter than length of MII; ligament between MII and MIII, and right MIV, not sclerotized (Fig. 43A–B). MIII short; with triangular teeth; with attachment lamella not sclerotized. Left MIV with smaller basal teeth; attachment lamella semicircular, slender, situated in a short portion of the posterior edge of maxilla, sclerotized. Right MIV with teeth triangular; attachment lamella slender, semicircular, better developed in the middle, situated in a short portion of posterior edge of maxilla (Fig. 43A–B). MV square, with a short rounded tooth. Mandibles slightly dark; cutting plates translucent, with 10 growth rings (Fig. 43C).

Branchiae pectinate with up to 14 filaments, in chaetigers 11R–13L–23 (Figs. 42C; 44C). Number of branchial filaments per chaetiger in order anterior-posterior: 7, 10, 10, 13, 11, 12, 14, 14, 14, 14, 14, 12, 10. Branchial filaments longer than dorsal cirri.

First three parapodia smallest; most developed in chaetigers 4–16, following ones becoming gradually smaller. Notopodial cirri conical, increasing in size from chaetiger 4 (Ldc3: 0.38 mm; Ldc20: 0.65 mm), from chaetiger 25, gradually decreasing in width and increasing in length, in posterior region filiform, 2.1 times longer than prebranchial region ones (Ldc79: 0.8 mm) (Fig. 44A–E). Prechaetal lobes as a transverse fold in all chaetigers (Fig. 44A–E). Chaetal lobes in chaetigers 1–26, rounded, shorter than postchaetal lobes, with aciculae emerging dorsal to midline; from chaetiger 27, triangular, longer than other lobes, with aciculae emerging dorsally to midline (Fig. 44A–E). Postchaetal lobes developed in chaetigers 1–48, bluntly conical; thinner, elongated in branchial region; decreasing in size in chaetigers 26–48, following ones inconspicuous (Fig. 44A–E). Ventral cirri digitiform in chaetigers 1–5; in chaetigers 7–34 with oval swollen base and digitiform tip; from chaetiger 35, digitiform, gradually reducing in size posteriorly (Fig. 44A–E).

Aciculae blunt, with reddish basal end, distally translucent (Fig. 44A–E). First 13 chaetiger with 3 aciculae; in chaetigers 14–20 with 2 aciculae; from chaetiger 21, with only one aciculae.

Limbate supracicular of two sizes in the same chaetiger, larger in anterior region, reduced in number around chaetiger 15. Two types of pectinate chaetae in anterior region isodont narrow with long and slender teeth, with 1– 2 pectinate, with up to 5–6 teeth, with transverse distal edge (Fig. 45A); in posterior region isodont narrow with short, and slender teeth, with 3–4 pectinate with up to 7–8 teeth, with oblique distal edge (Fig. 45B). Compound spinigers present in all chaetigers; in anterior region with blades of three sizes blade (longer 200 µm, Fig. 45E; median 128 µm, Fig. 45D; smaller 100 µm, Fig. 45C); in median-posterior region with blades of two sizes (longer 200 µm, smaller 130 µm). Subacicular hooks unidentate, with reddish basal end, distally translucent, starting in chaetiger 27; with one hook per chaetiger (Fig. 45F).

Variation. Material examined varied in the following features: L10= 2.4–4.5 mm, W10= 0.8–2.1 mm. Palps reaching second peristomial ring or second chaetiger. Lateral antennae reaching first or third chaetiger. Median antenna reaching second or fourth chaetiger. The maxillary formula varies as follows: MII 5–7+6–7, MIII 6–8, MIV 3–4+6–8. The proportion of maxillary apparatus varies as follows: maxillary carriers shorter with respect to the MI varies 2–2.4 times; closing system shorter with respect to the MI varies 7–8.5 times; cavity opening shorter with respect to MII varies 2.5–3 times. Branchiae from chaetigers 1 0–13 to 19–27. Maximum number of branchial filaments varied from 10 to 19. Well developed postchaetal lobe in first 34–52 chaetigers. Ventral cirri with swollen base from chaetigers 4–7 to 24–34. Start of subacicular hooks in chaetigers 22–33.

Type locality. Off Louisiana, Gulf of Mexico.

Distribution. Northern Gulf of Mexico.

Etymology. This species is named in honor of Dr. Gilbert Rowe in recognition of his kindness in lending material from deep water collection of the Texas A&M University.

Remarks. Paucibranchia gilberti n. sp. is the first species of the genus described from the Gulf of Mexico that has only compound spinigers, unlike the species with both types of compound chaetae which are the most typical of this region. Gathof (1984) characterized Marphysa sp. B related to P. disjuncta n. comb. due to the presence of only compound spinigers; however, he did not describe it as new, although he found some differences such as the absence of eyes and the branchial distribution.

Paucibranchia gilberti n. sp. is similar to P. cinari n. comb., P. disjuncta n. comb., and P. kinbergi n. comb. based on the presence of compound spinigers along the body and unidentate subacicular hooks, reddish color basally and translucent distally. However, P. disjuncta n. comb. has eyes, and the dorsal cirri of postbranchial chaetigers are almost three times longer than pre-branchial chaetigers; whereas P. gilberti n. sp. lacks eyes, and dorsal cirri in postbranchial chaetigers are only 2.1 times longer. Moreover, P. cinari n. sp. has spinigers with blades of two sizes in anterior region, lower number of branchial filaments (ESFM-POL/2013–1024, L10: 4.1 mm, 12 filaments), and all pectinate chaetae with oblique distal edge along the body; whereas P. gilberti n. sp. has spinigers with blades of three sizes in the same region, a higher number of branchial filaments (USNM 0 75453, L10: 4.5 mm, 19 filaments), and the pectinate chaetae have transverse distal edge in anterior chaetigers and oblique distal edge in posterior ones.

Paucibranchia gilberti n. sp. also differs from P. kinbergi n. comb. because in the former the dorsal cirri in postbranchial chaetigers are 2.7 times longer than pre-branchial chaetigers, the postchaetal lobes are tongue-shaped in the pre-branchial chaetigers and conical in the branchial regions, the ventral cirri with a swollen base start from chaetiger 4, and compound spinigers have blades of similar size in all chaetigers; whereas, in P. gilberti n. sp. dorsal cirri are 1.6 times longer in postbranchial chaetigers, postchaetal lobes are always bluntly conical, ventral cirri with a swollen base start from chaetiger 7, and spinigers have blades of three sizes in anterior region and of two sizes the in posterior chaetigers. The comparison with the others features in the last two species is provided in Table 4.