A new species of the genus Amphicteis Grube, 1850 (Annelida, Ampharetidae) from the Yellow Sea, China, together with a redescription of A. dalmatica Hutchings & Rainer, 1979

Abstract A new species of the ampharetid genus Amphicteis, A. hwanghaiensissp. nov., is described based on material from the Yellow Sea. The new species is characterized by the possession of long, stout, golden paleae with blunt tips, digitiform rudimentary notopodia on the abdominal uncinigers, uncini with a subrostral process, and a narrow rectangular hump separating branchial groups. Amphicteis dalmatica was redescribed from type materials at the Australian Museum, Sydney, and the differences between A. dalmatica and A. hwanghaiensissp. nov. are discussed. A key to distinguish Amphicteis species described or reported in Western Pacific waters is provided.


Introduction
Ampharetids are deposit feeders (Jumars et al. 2015) found worldwide, especially at high latitudes and in deep marine environments (Schiaparelli and Jirkov 2016). The genus Amphicteis Grube, 1850 is one of the most widely distributed and speciesrich genera of the family Ampharetidae (Parapar et al. 2011). Since Schiaparelli and Jirkov (2016) elevated the subspecies of Amphicteis gunneri (Sars 1835) (A. gunneri antarctica Hessle, 1917, A. gunneri atlantica McIntosh, 1885, and A. gunneri japonica McIntosh, 1885 to the rank of species, the genus Amphicteis encompasses a total of 33 valid species according to WoRMS (Read andFauchald 2020, as of October 22, 2020). Sui and Li (2017) recently described an additional species (A. chinensis Sui & Li, 2017) from China. Amphicteis is characterized by the presence of a prostomium with paired longitudinal glandular ridges and transverse or oblique nuchal ridges, four pairs of cirriform branchiae usually arranged in two transverse rows, the presence of paleae, smooth buccal tentacles, 17 thoracic chaetigers with tuberculate ventral cirri on the notopodia, and 13-19 abdominal segments with uncinigerous pinnules and rudimentary notopodia (Day 1964;Fauchald 1977;Holthe 1986;Parapar et al. 2011;Reuscher et al. 2015). Five species of Amphicteis were previously described or reported in Chinese waters (Sun 1990;Huang 1994;Liu 2008;Sui and Li 2017): A. gunneri; A. glabra Moore, 1905;A. scaphobranchiata Moore, 1906;A. mederi Annenkova, 1929, andA. chinensis. Sui andLi (2014) examined all of the specimens deposited in the Marine Biological Museum of the Chinese Academy of Sciences and found that the record of A. mederi in China (Sun 1990) was a misidentification of animals belonging to another genus, described therein as Paramphicteis sinensis Sui & Li, 2014. Amphicteis gunneri, the type species of the genus, has previously been considered as a cosmopolitan species until Hartley (1985) redescribed the type specimen held at the Zoological Museum, University of Oslo, and concluded that the cosmopolitan status of A. gunneri was unjustified. Parapar et al. (2011) suggested that the true distribution of that species may be restricted to the Arctic and North Atlantic European waters, with a southern boundary at the English Channel. They also proposed that until a global revision of Amphicteis was completed, the specific name A. gunneri should be used with caution.
During recent biodiversity surveys in the Yellow Sea of China, two specimens were collected and confirmed as belonging to an undescribed species of Amphicteis. We herein describe these specimens as new to science.
In order to support the "new species" status of our specimens, we also redescribe herein A. dalmatica Hutchings & Rainer, 1979, from southeastern Australia, which had not been discussed by Schiaparelli and Jirkov (2016) as belonging to the "blunt stout paleae" group of species, as defined by those authors. Finally, we include a key to the identification of all species of Amphicteis occurring in Western Pacific.

Materials and methods
The two specimens were collected using a 1.5 × 0.5 m Agassiz trawl from the southern Yellow Sea of China in November 2019. Specimens were preserved in 75% ethanol, then deposited in the Marine Biological Museum of the Chinese Academy of Sciences. The specimens were photographed with a digital camera attached to a Nikon AZ100 microscope and drawn with a camera lucida attached to a Nikon SMZ1500 microscope.
The type material of A. dalmatica was examined at the Australian Museum, Sydney. The animals were studied using a stereomicroscope, and one paratype was photographed. Notopodia and neuropodia were removed from different regions of the body, mounted on slides with polyvinyl lactophenol (PVLP), and examined and photographed using compound microscopes. For SEM examination, another paratype was dehydrated in an ethanol series, then critical-point dried, sputter coated with gold, and examined at the SEM Laboratory, AM. Photos using stereo and compound microscopes were also taken at the SEM Laboratory, AM. All photos were edited with Adobe Photoshop CC software.

Family Ampharetidae Malmgren, 1866
Genus Amphicteis Grube, 1850 Type species. Amphitrite gunneri Sars, 1835 Diagnosis. Prostomium with middle lobes as paired longitudinal glandular ridges and nuchal organs as transverse or oblique nuchal ridges. Buccal tentacles usually smooth. Four pairs of cirriform branchiae usually arranged in two transverse rows. Notochaetae in segment II present and usually developed as strong paleae. Seventeen thoracic chaetigers from segment III with notopodia bearing tuberculate ventral cirri. Modified notopodia and intermediate uncinigers absent. Fourteen thoracic uncinigers with uncini-bearing neuropodial tori (usually with small dorsal papilla) from segment VI. Usually 15 abdominal uncinigers present. Abdominal uncinigers with rudimentary notopodia and uncini-bearing pinnules with digitiform or cirriform dorsal cirri. Usually one pair of anal cirri present, inserted laterally on pygidium. Thoracic and abdominal uncini with a single row of teeth.  Description. Holotype Complete, length 27.8 mm, thoracic width 5.5 mm. Dorsum of thoracic segments and branchiae with red pigmentation (Fig. 1A, D). Thorax and abdomen well defined; thorax approximately twice as wide and long as abdomen; barely tapering towards posterior part. Prostomium with middle lobe as paired longitudinal glandular ridges, slightly diverging distally, V-shaped, gap between glandular ridges absent ( Fig. 2A); eyespots absent. Nuchal organs as paired nuchal ridges separated by a small median gap, V-shaped ( Fig. 2A). Segment I inconspicuous, barely visible laterally, in superior view. Segment II developed ventrally and laterally, bearing paleae, covered by branchiae dorsally ( . Thoracic and abdominal uncini arranged in single vertical rows with subrostral process and five or six teeth in a single row over basal prow (Figs 2G,H,3D,E). Pygidium with terminal anus and two laterally attached tapering anal cirri, approximately as long as the last five chaetigers (Fig. 1H).
Paratype complete, 31 mm long, 4.5 mm wide, with ten paleae on right side and eight on left (Fig. 4A, B). Eighteen thoracic chaetigers one side and 17 thoracic chaetigers on the other side (Fig. 4C). Dorsum of thoracic segments shows no pigmentation and only inner branchiae have several red bands.
Etymology. The species is named after its type locality in the Yellow Sea of China. The species name is an adjective in the nominative singular, derived from "hwanghai" which means "Yellow Sea" in Chinese, with the Latin suffix -ensis to denote a place.
Distribution. Yellow Sea at 79 m depth.
Remarks. The presence of stout paleae with blunt tips is characteristic for A. hwanghaiensis sp. nov.. Schiaparelli and Jirkov (2016) provided a revision of the genus Amphicteis and concluded that out of the 38 Amphicteis species (known at that time), only five species have this type of paleae: A. mederi, A. midas (Gosse, 1855), A. taurus Reuscher, Fiege & Imajima, 2015, A. ninonae Jirkov, 1985, and A. teresae Schiaparelli & Jirkov, 2016. According to Reuscher et al. (2015), A. dalmatica Hutchings &Rainer, 1979 andA. philippinarum Grube, 1878 also have short and poorly developed paleae. The latter species differs from A. hwanghaiensis sp. nov. by having foliose branchiae and uncini without a subrostral process, while A. hwanghaiensis sp. nov. only have cirriform branchiae and uncini with a subrostral process. To make clear the distinction between members of A. dalmatica and our new species, the type material of A. dalmatica was examined, redescribed, and compared with the new species (below).
All the other Amphicteis species have paleae with fine filamentous tips; the difference between fine-tipped and stout-tipped paleae is easy to distinguish. According to Schiaparelli and Jirkov (2016), the shape of blunt, stout paleae from the five known species belonging to this group are all very similar, but there are other diagnostic morphological differences, which can be used to distinguish them from the new species.The difference between A. mederi and A. hwanghaiensis sp. nov. is that A. mederi has abdominal pinnules with a cirriform dorsal cirrus, while the new species has a tuberculate dorsal cirrus; the thoracic and abdominal uncini of A. hwanghaiensis sp. nov. have five or six teeth in a single row over the basal prow while the uncini in A. mederi have six teeth (Annenkova 1929;Uschakov 1955). According to Schiaparelli and Jirkov (2016), who checked the holotype of A. mederi, the prostomial glandular ridges of A. mederi are separated by a wide median gap equal to the width of the ridge, while a gap between glandular ridges is absent in A. hwanghaiensis sp. nov. (Fig. 2A).
A comparison of A. midas and A. hwanghaiensis sp. nov. shows differences in the rounded spots on the anterior dorsum and the dark transversal pigment bands on its branchiae (Schiaparelli and Jirkov 2016); in contrast, the new species has red pigmentation on its branchiae. In addition, the area between the branchial groups is very different. Amphicteis hwanghaiensis sp. nov. has a narrow mid-dorsal rectangular hump between the inner branchiae while the area between the branchial groups of A. midas is flat and unmodified (Hartley 1985).
Amphicteis taurus is clearly distinct and differs from A. hwanghaiensis sp. nov. in the following features. The paleae of A. taurus are unique in the genus Amphicteis, being strongly enlarged, nearly straight with a uniform thickness over the entire length, and tips rounded, at about a 45-degree angle to the body. Amphicteis taurus is also different from A. hwanghaiensis sp. nov. by the smaller prostomial glandular ridges and the wide gap separating them. Other differences between them are the longer, annulated cephalic region (peristomium and possibly segment I) of A. taurus and the shorter cephalic region of the new species (Reuscher et al. 2015).
According to original description, A. teresae has a larger number of paleal chaetae (15-17 on each side). Amphicteis hwanghaiensis sp. nov. has a lower lip with a narrow, distinct, and white middle transversal band which is absent in A. teresae.
Uncini of A. hwanghaiensis sp. nov. have five or six teeth besides the subrostral tooth, while uncini of A. teresae usually have five. As for eyespots, which are absent in new species, Schiaparelli and Jirkov (2016: 541) said that "Another clear character of Amphicteis teresae sp. n. that distinguishes it from the other related ones having blunt paleal chaetae is the presence of an eyespot". Furthermore, A. teresae is found in Antarctica.
Amphicteis ninonae, recorded from Norwegian Sea and Arctic Seas, is most similar to the new species; however, members of this species are distinguished because, according to Jirkov (1985), the paleae are dark brown, while those of A. hwanghaiensis sp. nov. are golden. Amphicteis hwanghaiensis sp. nov. also has a narrow rectangular hump between the branchial groups, while the area between the branchial groups of A. ninonae is flat and unmodified. Parapar et al. (2011) also suggested that A. ninonae seems to be restricted to the north and east coasts of Iceland.
Four species of Amphicteis, A. glabra, A.gunneri, A. scaphobranchiata, and A. chinensis, have been recorded from Chinese seas, according to Sui and Li (2017). The new species differs from these species by having blunt and stout tipped paleae, as these four species belong to the group of Amphicteis species with tips of paleae sharply tapering into fine filaments.
Among the species from the Western Pacific, specimens belonging to Amphicteis malayensis Caullery, 1944 differ from those of A. hwanghaiensis sp. nov. by the possession of prostomial eyespots and a wide median gap between glandular ridges. Members of A. theeli Caullery, 1944 andA. quadridentata Caullery, 1944 have 14 and 16 abdominal uncinigers, respectively. The branchiae of individuals of A. spinosa Reuscher, Fiege & Imajima, 2015 have four rows of pointed protuberances, while branchiae are smooth among specimens belonging to our new species. Finally, specimens belonging to A. uncopalea Chamberlin, 1919, found in the North-western Pacific, have well-developed paleae with curly and fine tips, and a distinct rounded lobe behind the paleae originating from segment III.

Amphicteis dalmatica Hutchings & Rainier, 1979 Figs 5-7
Amphicteis dalmatica Hutchings and Rainer 1979: 783-786, fig. 9 A-E. Redescription. Types with dorsum of anterior segments speckled with pigmented spots, pigmentation decreasing posteriorly. Pigmentation still visible after decades of storage in alcohol, although paler than when originally described (compare our Fig. 5 with fig. 9 in the original description, especially the branchial pigmentation, most of which has been lost over time). Mucous tube with embedded sand and shell particles (Fig. 5A). Prostomium well developed, with a mid-dorsal trilobed process (Figs 5B, G, H, 6A-C), middle lobe as paired longitudinal glandular ridges, wider distally, T-shaped, gap between glandular ridges absent (Figs 5B, G, H, 6A-C); lateral lobes each with a cluster of eyespots basally. Nuchal organs as paired nuchal ridges, touching each other basally, V-shaped, lacking median gap; buccal tentacles smooth (Figs 5B-E, G-I, K, 6A-C, E, F). Segment I inconspicuous, barely visible laterally, in superior view (Fig. 6B, E); segment II developed ventrally and laterally, bearing paleae, covered by branchiae dorsally (Figs 5B-D, G-I, K, 6A-C, E, F). Four pairs of long and tapering branchiae, arising free from body wall in 2 transverse rows, on segments III and IV, each with 1 long and thick filament on each side, separated in left and right groups by a mid-dorsal triangular hump (Figs 5B-K , 6A-C). All branchial filaments about same size, originating from segments II-V, arising as free filaments from segments III and IV, 2 pairs on each segment, in transverse rows, as follows: on segment III, outer pair originating from segment II, inner pair originating from segment III; on segment IV, outer pair originating from segment IV, inner pair from segment V (Figs 5B-K, 6A-C, F). Segment II with ~10 short stout notopodial paleae on each side, arranged in shallow arcs, paleae distally pointed, with short filiform tip, frequently broken off; paleae remarkably small, about same size as notochaetae, but stouter (Figs 5B, C, G-I, K, 6B, E, F, 7A, B). Notopodia with capillary chaetae starting from segment III and extending through 17 chaetigers; notopodia each with a tuberculate ventral cirrus (Figs 5L, 6D, F, G, 7C-E); first 3 pairs aligned laterally to following pairs and increasing progressively in size, all 3 much shorter than those from segment VI onwards (Figs 5B, C, E-G, I, J, 6A-C, F). Neuropodial tori with uncini from segment VI, present in 14 thoracic uncinigers; tori as raised trapezoidal structures throughout, larger on thorax (Figs 5C, D, I, L, 6B, F). Continuous ventral shields present to approximately thoracic unciniger 12 (Fig. 5C, D, I, K). Elevated or modified notopodia absent. Intermediate uncinigers absent. Fifteen abdominal uncinigers with tuberculate rudimentary notopodia (Fig. 6H, J). Pinnules with tiny tuberculate dorsal neuropodial process (Fig. 6H, I).   Thoracic and abdominal uncini arranged in single vertical rows with barely conspicuous subrostral process and five or six teeth in a single row over basal rounded prow; thoracic uncini with teeth progressively increasing in size until fourth, fifth (distal) tooth shorter (Figs 6K, 7F); abdominal uncini with sixth tooth, when present, much shorter than other teeth (Figs 6L, 7G, H). Pygidium with one pair of long, gently tapering anal cirri.
Distribution. New South Wales, Australia. Remarks. Members of A. dalmatica are clearly distinct and differ from our new species in the following features. In A. dalmatica, the paleae are poorly developed, prostomial lateral lobes each have a cluster of eyespots basally, uncini have barely conspicuous subrostral process, and a distinct spotted pigmentation pattern is present on the dorsum of thoracic segments and branchiae. Members of Amphicteis hwanghaiensis sp. nov. have well-developed paleae that are twice as long as the prostomium. Uncini of A. hwanghaiensis sp. nov. have a much larger subrostral process. Furthermore, the type locality of A. dalmatica is New South Wales, Australia, in seagrass beds.
Key to Amphicteis species from the Western Pacific Branchiae with four rows of pointed protuberances ..