A new deep-sea species of Flabelligena from off the South Orkney Islands, the Southern Ocean

Abstract Background A new acrocirrid species, Flabelligena hakuhoae sp. nov., is described from off the South Orkney Islands, the Southern Ocean. Individuals of the new species were collected by rock dredging, 2036–2479 m in depth. New information The new species can be distinguished from its congeners by the number of branchiae, position and length of paired ventral large papillae and length of body papillae.


Introduction
Acrocirridae Banse, 1969 (clade Cirratuliformia) consists of 43 species in nine genera distributed from the intertidal zone to the deep seafloor (Magalhães andBailey-Brock 2011, Martínez et al. 2019). They are found in sandy areas and under rocks, but some genera are pelagic (Osborn et al. 2009).
Flabelligena Gillet, 2001 belongs to the Acrocirridae. It has a minute body, body papillae, spinulose or serrated (not cross-barred) notochaetae, 1-3 pairs of branchiae and a pair of frontal palps (Aguirrezabalaga and Ceberio 2006). They live in sandy areas, mainly in bathyal to abyssal depths. The six described Flabelligena species are known mainly from the North Atlantic Ocean, three of which are known from the Southwest Atlantic, Mediterranean and South Indian Oceans (Aguirrezabalaga and Ceberio 2006). Several polychaetes faunal surveys have been carried out around the Southern Ocean (e.g. Hartman 1953, Hartman 1964, Hartman 1966, Hartman 1967, Monro 1939 During the research cruise KH19-06-Leg4 by R/V Hakuho maru, the first author found individuals of Flabelligena from off the South Orkney Islands. In this paper, we describe the specimens as a new species.

Materials and methods
Specimens were collected from off the South Orkney Islands (Fig. 1A), the Southern Ocean (60°33.54'S, 35°24.43'W-60°34.07'S, 35°23.40'W), 2036 -2479 m in depth by a rock dredge (Fig. 1B) and extracted from the rock and silt sediments using a 32 μm sieve with seawater and fixed in 70% ethanol. After preservation, these specimens were observed with Nikon SMZ18 and Nikon ECLIPSE 80i microscopes and photographed with a Nikon D5200 digital camera. The paratype specimen was washed in a phosphatebuffered saline solution and dehydrated in a graded ethanol series, dried in a critical-point dryer (HITACHI HCP-1) using liquid CO and coated with gold in an ion sputter (HITACHI E-1045) for SEM observations. Observations were conducted using an SEM instrument (HITACHI S-3000N). Type material is deposited in the National Museum of Nature and Science, Tsukuba (NSMT) and Invertebrate Collection of the Hokkaido University Museum (ICHUM).
We followed the morphological terminology of Aguirrezabalaga and Ceberio (2006) and Martínez et al. (2019) in the taxonomic description below.

Etymology
The species is named after the R/V Hakuho-maru, a gear of the ship which collected the type specimens from the Southern Ocean. The specific name is a noun in the genitive case.

Taxon discussion
Flabelligena hakuhoae sp. nov. belongs to Flabelligena because it has the following features: retractile anterior region, absence of distinct body region, presence of various pairs of branchiae, composite falcigerous neurochaetae, simple spinulose or serrated notochaetae (Aguirrezabalaga and Ceberio 2006). This species is different from the other known species by having the following features (see Table 1): i) paired ventral large papillae present on the ventral side of parapodium in chaetiger 6; ii) three pairs of branchiae; iii) short body papillae between noto-and neurochaetae. Flabelligena erratica (Orensanz, 1974) and F. gascognensis Aguirrezabalaga and Ceberio, 2006 have paired ventral papillae between chaetigers 6-7, but do not have them at the ventral side of a parapodium in chaetiger 6 (Orensanz 1974, Aguirrezabalaga and Ceberio 2006 Table 1).