Mastobranchus braziliensis Silva & Amaral 2022, sp. nov.

Mastobranchus braziliensis sp. nov.

(Fig. 3A–J; Table 1)

http://zoobank.org/ EB6FBB29-D9B8-473F-B25C-25AFD4FE23CC

Material examined. Campos Basin: Holotype (ZUEC POL 20222) 21°42'37''S 40°8'59''W, 09 Mar 2009, subtidal, 147m, Stn. HAB13/H5, 1 spec. Paratype 1 (ZUEC POL 20223) 22°17'42''S 40°27'00''W, 04 Jul 2009, subtidal, 103m, Stn. HAB16/E4, 1 spec. Paratype 2 (ZUEC POL 20215) 21°28'21''S 40°48'34''W, 20 Jul 2009, subtidal, 22m, Stn. HAB17/ FOZ13, 1 spec. Paratype 3 (ZUEC POL 20211) 21°34'12''S 40°25'29''W, 23 Jul 2009, subtidal, 28m, Stn. HAB17/FOZ30, 1 spec. Espírito Santo Basin: Paratype 4 (ZUEC POL 20203) 19°35'2''S 39°38'40"W, 18 Jul 2011, subtidal, 30m, Stn. AMB2/FOZ10, 1 spec. Paratype 5 (ZUEC POL 20202) 18°40'56''S 38°55'41"W, 17 Jul 2013, subtidal, 44m, Stn. AMB13/G3, 1 spec. Paratypes 6 and 7 (ZUEC POL 20200) 18°53'31''S 39°6'20"W, 02 Feb 2012, subtidal, 43m, Stn. AMB7/F3, 2 specs. Paratypes 8–12 (ZUEC POL 20220) 18°53'30''S 39°6'21"W, 02 Feb 2012, subtidal, 43m, Stn. AMB7/F3, 5 specs. Paratype 13 (ZUEC POL 20193) 18°53'30''S 39°6'21"W, 02 Feb 2012, subtidal, 34m, Stn. AMB7/F2, 1 spec. Paratype 14 (ZUEC POL 20205) 18°40'54''S 38°55'42"W, 17 Jul 2013, subtidal, 28m, Stn. AMB13/G2, 1 spec. Paratype 15 (ZUEC POL 20801) 21°33'52''S 40°42'53''W, 10 Mar 2009, subtidal, 22m, Stn. HAB13/FOZ18, 1 spec.

Additional material examined. Campos Basin: (ZUEC POL 20210) 22°39'11''S 40°48'49''W, 11 Mar 2009, subtidal, 22m, Stn. HAB13/FOZ12, 1 spec. (ZUEC POL 20212) 21°33'52''S 40°42'53''W, 10 Mar 2009, subtidal, 22m, Stn. HAB13/FOZ13, 2 specs. (ZUEC POL 20214) 21°33'52''S 40°42'53''W, 10 Mar 2009, subtidal, 22m, Stn. HAB13/FOZ18, 6 specs. (ZUEC POL 20216) 21°33'52''S 40°42'53''W, 10 Mar 2009, subtidal, 22m, Stn. HAB13/FOZ13, 1 spec. (ZUEC POL 20217) 21°39'31''S 40°31'25''W, 13 Mar 2009, subtidal, 28m, Stn. HAB13/ FOZ25, 1 spec. (ZUEC POL 20219) 22°12'31''S 40°14'7''W, 24 Jul 2009, subtidal, 97m, Stn. HAB17/FOZ43, 1 spec. (ZUEC POL 20225) 21°44'39''S 40°43'8''W, 19 Jul 2009, subtidal, 21m, Stn. HAB17/FOZ19, 1 spec. (ZUEC POL 20226) 21°39'32''S 40°31'24''W, 23 Jul 2009, subtidal, 28m, Stn. HAB17/FOZ25, 1 spec. (ZUEC POL 20227) 21°24'45''S 40°25'17''W, 20 Jul 2009, subtidal, 32m, Stn. HAB17/FOZ29, 1 spec. Espírito Santo Basin: (ZUEC POL 20195) 18°53'30''S 39°6'21"W, 17 Jul 2013, subtidal, 43m, Stn. AMB14/F3, 1 spec. (ZUEC POL 20196) 18°53'30''S 39°6'21"W, 18 Jul 2011, subtidal, 26m, Stn. AMB2/FOZ9, 1 spec. (ZUEC POL 20197) 19°47'22''S 39°43'19"W, 19 Dec 2010, subtidal, 37m, Stn. AMB1/FOZ13, 1 spec. (ZUEC POL 20198) 20°1'2''S 39°50'18"W, 18 Jul 2011, subtidal, 49m, Stn. AMB2/FOZ16, 1 spec. (ZUEC POL 20199) 19°09'48''S 39°29'20"W, 02 Feb 2012, subtidal, 20m, Stn. AMB7/E1, 2 specs. (ZUEC POL 20204) 18°40'54''S 38°55'42"W, 02 Feb 2012, subtidal, 44m, Stn. AMB7/G3, 1 spec. (ZUEC POL 20206) 18°40'54''S 38°55'42"W, 17 Jul 2013, subtidal, 44m, Stn. AMB13/G3, 1 spec. (ZUEC POL 20207) 20°34'33''S 40°20'52"W, 17 Jul 2013, subtidal, 21m, Stn. AMB14/B1, 1 spec. (ZUEC POL 20208) 18°40'54''S 38°55'42"W, 02 Feb 2012, subtidal, 44m, Stn. AMB7/G3, 1 spec.

Comparative material examined. Holotype Mastobranchus loii Gallardo, 1968 (LACM-AHF POLY 317), 12°13'24''N 109°14'13''E, 14m, 11 Jan 1960, Bay of Nha-Trang, South Vietnam, North Pacific. Holotype Mastobranchus variabilis Ewing, 1984 (USNM 81993) and paratype 1 (USNM 81994), 27°36'29''N 82°55'48''W, Jun 1980, Tampa Bay, Gulf of Mexico, North Atlantic Ocean; and paratype 2 (LACM-AHF POLY 1369), 314°06'30''N 114°28'15''W, Feb 1980, Gulf of Mexico, North Atlantic Ocean.

Description. Incomplete holotype 17 mm long, 1.1 and 0.9 mm wide (anterior and posterior thoracic region, respectively) for 28 chaetigers. Body rounded; anterior thorax 1.5x wider than posterior thorax; posterior thorax almost of same width as abdomen. Yellowish color in alcohol.

Prostomium triangular, small, 1/3 of peristomium width; small palpode, 1/2 of prostomium length. Light brown stain on dorso-lateral prostomium may be eyespots. Peristomium 2x wider than long and 1.5x longer than the first thoracic chaetiger. Nuchal organs not everted.

Thorax with 11 chaetigers.Chaetiger 1 uniramous. Capillaries on notopodia of chaetigers 1–11 and on neuropodia of chaetigers 2–11. Chaetigers 1–4 wider and more inflated than remaining ones, with a shallow lateral groove, faintly tessellated, with shallow intersegmental division (Fig. 3A–B, D). Chaetigers 5–11 square-like, straight, 1.1x wider than long, strongly biannulated, with deep intra- and intersegmental grooves, and with mid-lateral lobes (MLL) (Fig. 3B). Right and left dorsal chaetal bundles widely separated on chaetiger 1 (dorso-lateral), distance reduced up to chaetiger 11 (dorsal). Dorsal and ventral bundles with 11 capillary chaetae. Thorax rounded (Fig. 3B). Lateral organs very small, visible only after staining. A pair of genital pores between chaetigers 7/8, 8/9, 9/10, 10/11, and 11/12 (Fig. 3B–C). Transition between thorax and abdomen subtle, marked by chaetal change, distance of chaetal bundles, and shape of chaetigers (Fig. 3B, D).

All abdominal chaetigers with hooded hooks on neuropodia and mixed hooded hooks and capillaries on notopodia (Fig. 3C). First six abdominal chaetigers rectangular (1.5x wider than long), following chaetigers square-like (as wide as long). Chaetae and hooded hooks emerge from last quarter of chaetiger, near division between chaetigers. Both notopodial bundles of hooded hooks (~12 each fascicle) and capillaries (~15 each fascicle) emerging dorsally, almost together (Fig. 3C, F); neuropodial bundles of hooded hooks (~ 50 each fascicle) long, extending laterally and ventrally (Fig. 3D). Dorso-lateral lateral organs throughout abdomen. Noto- and neuropodia with poorly developed lobes (Fig. 3C–D). Notopodial hooded hooks with three rows of teeth (3–2–3) above main fang, large main fang and slightly curved; long shaft and hood, without node (Fig. 3G–H). Neuropodial hooded hooks with three rows of teeth (3–2–3) above main fang, large main fang and slightly curved; short shaft and hood, with node (Fig. 3I–J).

One complete specimen (paratype 15) with retractile branchiae after chaetiger 70; dorsal, bifurcated branchiae, posterior to notopodial lobe (Fig. 3E); and with mixed hooded hooks and capillaries on abdominal notopodia up to chaetiger 56. Pygidium bearing a long ring after last chaetiger and four small cirri (3x longer than wide), forming a circular arrangement on the dorsal region; large anal aperture ventrally (Fig. 3E).

Variation. Specimens ranging from 3.7–38 mm long, 0.25–1.2 mm wide for 27–100 chaetigers. Juveniles (<0.6 mm wide) present hooded hooks on the last 3–4 thoracic neuropodia, transition between thorax and abdomen not distinguishable, mixed hooded hooks and capillaries throughout abdominal notopodia, and lack areolation and lateral groove on anterior thorax.

Methyl Green staining pattern (MGSP). Thorax with intersegmental division staining very dark. Abdomen with a dark stain zone around noto- and neuropodial lobes (Fig. 3F). Dorsal region of abdomen staining darker than ventral. Ventral region of abdomen with a dark median line throughout (Fig. 3F).

Remarks. Mastobranchus braziliensis sp. nov. is characterized by having thoracic chaetigers 1–4 wider and more inflated than the remaining ones, with a shallow lateral groove, and faintly tessellated; dorsal, bifurcated branchiae on posterior abdomen; and mixed capillaries and hooded hooks on notopodia of almost all abdominal chaetigers. Mixed capillaries and hooded hooks on notopodia of abdominal chaetigers is one of the most noticeable characteristic of the genus Mastobranchus, but the number of chaetiger bearing capillaries and hooded hooks varies among known species, and Mastobranchus braziliensis sp. nov. presents the highest number of chaetigers with mixed chaetae. M. braziliensis sp. nov. also differs from the known species by the morphology of branchiae, which is bifurcated, in contrast with species with multifilament branchiae (M. trinchesii, five; M. loii, 3–7; M. variabilis, 5–8; M. uru, 3–8). The number of teeth above the main fang in hooded hooks is also a useful character to differentiate Mastobranchus species. Although some species were poorly described lacking photographs, it is possible to affirm that Mastobranchus is a very homogeneous and well-defined genus. Except for the first chaetigers, which can be uniramous or biramous (M. trinchesii), all species seem to share specific characteristics, such as presence of a palpode, faintly tessellated thoracic epithelium, notopodial bundles of abdomen emerging dorsally, very close to each other, a long line of neuropodial hooks, notopodial hooks longer and straighter (without node) than neuropodial hooks, post-notopodial branchiae, pygidium with four cirri, and Methyl Green staining pattern distinct (M. braziliensis sp. nov., M. loii, and M. uru).

Etymology. This species was named after its locality, since it is the first species of the genus recorded to Brazil.

Habitat. Shallow subtidal regions (20–147m), in soft-bottoms.

Type locality. Campos Basin, Southwest Atlantic Ocean (21°42'37''S 40°8'59''W).

Distribution. Southwest Atlantic Ocean, Campos and Espírito Santo Basins (Brazil).