Microrbinia hartmanae Blake & Ravara, 2025

Blake, James A., Ravara, Ascensão & Bongiorni, Lucia, 2025, Two new species of Orbiniidae (Annelida, Polychaeta) from the Condor Seamount, Azores, NE Atlantic Ocean, Zootaxa 5686 (3), pp. 393-405 : 400-403

Microrbinia hartmanae Blake & Ravara new species urn:lsid:zoobank.org:act:4880B8D3-B44F-4B45-9FD5-3E7985543F16

Figures 3–4

Questa sp. A : Bongiorni et al. 2013: 10.

Material examined. North Atlantic Ocean, Condor Seamount, off Faial Island, Azores, coll. Jul 2010, R/ V Noruega, USNEL 0.25 m 2 Box core, Site 9, Summit, 38.549°N, 29.0478°W, 206 m, Rep. 1, holotype (MCZ IZ 172991 ) , 5 paratypes (MCZ IZ 172992 ) , 3 spms ( DBUA 3539.01 ) ; 1 paratype as permanent slide mount (MCZ IZ 172994 ) ; Rep 3, 4 paratypes (MCZ IZ 172993 ) .

Description. A small threadlike species; holotype (MCZ IZ 172991) complete with 38 setigers, 4.2 mm long, 0.2 mm wide across setiger 16; incomplete paratype (MCZ IZ 172992) larger, with 27 setigers, 5.0 mm long, 0.35 mm wide across setiger 4. Body cylindrical, with 5–7 short thoracic setigers with uniannulate interramal rings, followed by larger, thicker, abdominal setigers with biannulate interramal rings along most of body ( Figs 3A–B , 4A ); a few transitional setigers with either uni- or biannulate interramal rings variously developed; thoracic setigers with parapodia lateral, shifting a little dorsally along abdominal setigers, but never completely dorsally elevated. Dorsal and ventral groove and ridges absent apart from a large flattened narrow plate on ventral surface of thoracic setigers ( Fig. 3B ). Specimens believed males with a large conical gland-like dorsal organ on individual segments ( Fig. 4B–C ); these with distinct apical pore present ( Fig. 4C ). No specimens observed with oocytes or ova. Color in alcohol light tan.

Pre-setiger region long, narrow, about as long as first 2.5 to 3 setigers, merging with setiger 1 both dorsally and ventrally ( Figs 3A–B , 4A ). Prostomium conical, tapering to narrow rounded tip ( Fig. 4A ); nuchal organs not observed; eyespots absent. Peristomium with two rings, first smaller, merged anteriorly with short prostomium ( Fig. 4A ); second larger, encompassing mouth ventrally with about ten short lobes ( Fig. 3B ); proboscis not observed.

Thoracic setigers numbering 5–7, each relatively short, about 2–3 times wider than long and with a single narrow transverse intersegmental ridge or ring both dorsally and ventrally ( Fig. 3A–B ). Transition to abdominal segments denoted by a narrowing and elongation of individual segments and with change from a single intersegmental ring to two rings ( Figs 3A , 4A ). Abdominal parapodia and setae arising from middle of each segment. Branchiae entirely absent.

Notopodia short, conical, with a distinct postsetal lobe on all setigers; this becoming longer and fingerlike along most of body ( Fig. 3A, D ); neuropodia short, weakly curved, with a short digitate postsetal lobe first present in posterior thoracic setigers continuing posteriorly, not as long as in notopodia ( Fig. 3B, D ).

Setae include camerated capillaries and noto- and neuropodial acicular spines ( Fig. 3E–F ). Noto- and neuropodia with a spreading fascicle of 6–10 camerated capillaries in thoracic setigers and anterior abdominal setigers; gradually reduced to 2–5 capillaries in abdominal setigers. Notopodia with 1–2 curved acicular spines with hooked tips from setigers 12–14, continuing posteriorly; neuropodia with 1–2 pointed acicular spines from setigers 8–9.

Pygidium divided into two lobes; with two anal cirri, one arising on each lobe ( Fig. 3C ).

Remarks. Microrbinia hartmanae sp. nov. is the second species of the genus to be described. The first, M. linea , was described by Hartman (1965) from off New England in deep water. Blake (2021) provided a detailed description of the species based on extensive collections off the U.S. Atlantic coast in 600–3015 m. Blake (1993, 2021) reported that some specimens of M. linea had unusual a conical gland-like dorsal organ on some abdominal segments. These were suggested to be on males because other specimens with eggs were obviously females and lacked these structures. In the present study, similar dorsal organs were observed on a few specimens of M. hartmanae sp. nov. However, no evidence of eggs or oocytes were found either on these specimens or those that lacked the dorsal organs.

Morphologically, M. hartmanae sp. nov. differs from M. linea in having a small conical-shaped prostomium, two peristomial rings, abdominal segments that are about as wide as long and with parapodia arising from midbody, blunt-tipped and apically curved acicular spines in the notopodia and straight, more pointed spines in the neuropodia, and a pygidium with two anal cirri. In contrast, M. linea has a larger, triangular-shaped prostomium, one peristomial ring, abdominal segments that are narrow and as much as three times longer than wide and with parapodia arising from near the posterior end of each segment. In addition, the notopodial acicular spines of M. linea are serrated along one edge before curving to a pointed tip instead of smooth shafts, neuropodial spines are absent instead of present, and the pygidium has four anal cirri instead of two.

Biology. Microrbinia hartmanae sp. nov. is known only from the summit of the Condor Seamount in 206 m. In contrast, M. linea is a deep-water species that occurs off the U.S Atlantic coast from 600 to 3015 m and off the Amazon River in 770–1500 m ( Hartman 1965; Blake 2021). The unusual dorsal glandular organs found in both species have not been reported for other orbiniids. It is likely that these are a copulatory organ because specimens of M. linea with eggs did not have these organs ( Blake 1993, 2021).

According to Bongiorni et al. (2013), a total of 1711 specimens of benthic invertebrates were identified from three replicate box cores collected at Station 9 on the summit where Microrbinia hartmanae sp. nov. was collected. From these specimens, 81 species were identified, of which 45 species were polychaetes. The onuphid polychaete, Mooreonuphis pallidula ( Hartman, 1965) , the syllid polychaete, Pionosyllis weismanni ( Langerhans, 1879) , and an amphipod, Lembos sp. , collectively comprised 76% of the total macrofaunal abundance at the site.

Microrbinia hartmanae sp. nov. is likely a subsurface deposit feeder, but owing to its small size and narrow body, probably occurs within 5 cm of the surface. Its only congener, M. linea , was found to have 96.5% of its total abundance in the upper 5 cm of sediment cores, with the majority in the 2–5 cm depth interval ( Blake 1994).

Etymology. This species is named for the late Dr. Olga Hartman, prominent annelid systematist who originally described the genus Microrbinia .

Distribution. NE Atlantic Ocean, Condor Seamount, off the Faial Island, Azores archipelago, 206 m.

Figures

FIGURE 3. Microrbinia hartmanae sp. nov. A, anterior end, dorsal view; B, anterior end, ventral view; C, posterior end ventrolateral view; D, setiger 20, anterior view of parapodia; E, notopodial acicular spines in different orientations; F, neuropodial acicular spines. A, C holotype (MCZ 172991); B, D, E, F paratype (MCZ IZ 172992). Abbreviations: anC, anal cirrus; acSp, acicular spines; mo, mouth; neL, neuropodial lobe; noL, notopodial lobe; per, peristomium; pr, prostomium.

FIGURE 4. Microrbinia hartmanae sp. nov. A, anterior end, ventral view; B, posterior segment with dorsal glandular organ; C, detail of dorsal glandular organ. A, holotype (MCZ IZ 172991); B, C paratype (MCZ IZ 172992).