Apomatus nishii, Kupriyanova & Flaxman, 2024

Apomatus nishii View in CoL n. sp.

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Fig. 2A–G View Figure 2

Material examined. Holotype: W.54373 (extraction number LK287), Cocos (Keeling) Islands Territory , Muirfield Seamount (13°14'32"S, 96°17'31"E), depth 932–965 m, 21/10/2022. GoogleMaps

Paratypes: W.53423 (LK259), Christmas Island SW (10°32'59"S, 105°31'59"E), depth 1388–1533 m, 09/07/2021 (1 spec., radiolar crown missing, prepared for SEM); W.54510 (LK318), Cocos (Keeling) Islands Territory , Muirfield Seamount (13°16'41"S, 96°04'06"E), depth 1395–1459 m, 20/10/2022 (1 spec) GoogleMaps .

Description. Tube: white opaque or slightly reddish, circular in cross section, attached to substrate throughout its length, with smooth surface, without either collar-like rings or flaring peristomes. Median keels absent ( Fig. 2A View Figure 2 ).

Radioles: flat ribbon-like, wide ( Fig. 2A View Figure 2 ); each lobe with up to 12 pairs of radioles, arranged in semicircles, fused basally (connected by short inter-radiolar membrane) for about 1/20 of their length. Pinnules short and thin; each radiole ending into thin short filamentous tip ( Fig. 2A View Figure 2 , not well visible here). Radiolar eyes not observed in the preserved material. Stylodes absent.

Operculum: absent or lost ( Fig. 2A View Figure 2 ), no distinct peduncle observed. Pseudoperculum absent.

Collar and thoracic membranes: collar unlobed, with entire edge, short, barely covering radiolar lobes ( Fig. 2A, B View Figure 2 ); continuous with wide thoracic membranes ( Fig. 2C, D View Figure 2 ) forming an apron across anterior abdominal chaetigers 1 or 2. Pairs of small, wart-like protuberances of collar chaetiger absent, calcium-secreting glands visible on collar.

Thorax: with collar chaetiger and 6 uncinigerous chaetigers ( Fig. 2A, B View Figure 2 ). Uncinigerous tori positioned close to each other, ventral thoracic triangular depression absent. Collar chaetae ( Fig. 2B, D View Figure 2 ) simple capillary (limbate) of two sizes, Apomatus chaetae absent ( Fig. 2D View Figure 2 ). Subsequent chaetal bundles with Apomatus chaetae ( Fig. 2E View Figure 2 ) and simple capillary and limbate chaetae ( Fig. 2D View Figure 2 ). Uncini predominantly rasp-shaped ( Fig. 2C View Figure 2 ), with approximately 20 teeth in profile, 2 teeth in posterior-most row and up to 5 teeth in a row above and continuing onto peg; anterior peg long, blunt, almost rectangular ( Fig. 2C View Figure 2 ). Pair of prostomial eyes not observed.

Abdomen: up to 40 abdominal chaetigers. Short achaetous anterior abdominal zone present. Uncini rasp-shaped with 3–6 rows of teeth and up to 25 teeth in profile view, long blunt almost rectangular peg ( Fig. 2F View Figure 2 ). Chaetae flat sickle-shaped with finely denticulate blades ( Fig. 2G View Figure 2 ). Long capillary chaetae in posterior chaetigers absent. Posterior glandular pad not observed.

Size: body length (without radioles) up to 20 mm, width of thorax up to 0.8 mm. Radioles and operculum accounting for one third of entire length. Tube up to 1.2 mm wide with lumen of up to 1.0 mm diameter.

Diagnostic remarks. The new species resembles Apomatus voightae Kupriyanova & Nishi, 2010 from the Patton-Murray seamounts (Gulf of Alaska) that differs from all other serpulid species by its very characteristic flat ribbon-like radioles. An unusual feature of A. voightae is the presence of Apomatus chaetae in the collar chaetae bundle. Normally in the genus Apomatus chaetae are present in abundance throughout most thoracic segments but are absent in the collar bundle (Kupriyanova & Nishi, 2010). Apomatus chaetae are absent in the collar bundle of Apomatus nishii n. sp. and an operculum is also lacking in this species, but soft vesicular opercula in this genus are easily lost, so this character is unreliable.

Molecular data support monophyly of the genus Apomatus and placement of the new species in the genus. However, the relationship between the morphologically distinct species Apomatus voightae and A. nishii n. sp. need to be examined in further studies.

Etymology. The species is named in honour of Professor Eijiroh Nishi (Yokohama National University, Japan) in recognition of his numerous important contributions to taxonomic and reproductive studies of Serpulidae .

Distribution. Only known from seamounts off Christmas and Cocos (Keeling) Islands, 932–1533 m.