Poliopogon is a genus of Pheronematidae with a fan-like body in which the concave side represents the atrial cavity. Basalia include some monaxons with clavate distal ends and two-toothed anchors. Eight species of Poliopogon are known in the world: (1) P. amadou Thomson, 1878 is divided into two subspecies: P. amadou amadou Thomson, 1878 and P. amadou pacifica Tabachnick, 1988. Poliopogon amadou amadou was found at the depth of 2 480–4 022 m in the Atlantic Ocean (Tabachnick and Menshenina, 2002) and P. amadou pacifica was found at the depth of 3 800–4 270 m in the Pacific Ocean (Tabachnick, 1988); (2) P. canaliculatus Wang, Wang, Zhang and Liu, 2016 was found at the depth of 2 897 m in the Northwest Pacific Ocean (Wang et al., 2016); (3) P. claviculus Tabachnick and Lévi, 2000 was found at the depth of 1 100–1 348 m in New Caledonia of the South Pacific Ocean (Tabachnick and Lévi, 2000); (4) P. maitai Tabachnick, 1988 was found at the depth of 1 690–2 000 m in the Pacific Ocean (Tabachnick, 1988); (5) P. mendocino Reiswig, 1999 was found at the depth of 2 332 m in California of the Northeast Pacific Ocean (Reiswig, 1999); (6) P. micropentactinus Tabachnick and Lévi, 2000 was found at the depth of 454–2 160 m in the South Pacific Ocean (Tabachnick and Lévi, 2000); (7) P. zonecus Tabachnick and Lévi, 2000 was found at the depth of 697–1 348 m in New Caledonia of the South Pacific Ocean (Tabachnick and Lévi, 2000), (8) P. microuncinata Kersken, Janussen and Martínez Arbizu, 2017 was found at the depth of 1 673.1 m in the Northeast Pacific Ocean (Kersken et al., 2017). Among the Poliopogon species, except for the subspecies of P. amadou amadou is distributed in the Atlantic Ocean, others are distributed in the Pacific Ocean (Fig. 1). Longitudinally, the genus of Poliopogon extends from 25°S (New Caledonia) to 40°N (California) in the Pacific and from 15°N to 29°N in the Atlantic. The greatest diversity of Poliopogon occurs between 20°N and 25°S in the Pacific Ocean. Outlying species occur in the Northeast Pacific and the Atlantic. The new species was found at the Northwest Pacific which located in the area with the greatest diversity scope of the Poliopogon.

Sample collection. The sponge sample was collected by the submersible remotely operated vehicle (ROV) Faxian during the cruise of the R/V Kexue at the Yap trench in the Northwest Pacific Ocean. The sample was deposited in the Marine Biological Museum of the Chinese Academy of Sciences (MBM) in the Institute of Oceanology of the Chinese Academy of Sciences, Qingdao, China (IOCAS).

Spicule analysis. The methods of preparing, observing and measuring the spicules followed those of Gong et al. (2015).

16S rDNA amplification. DNA extraction method and the amplified procedure of a 481 bp segment of the 16S rDNA gene were according to Gong et al. (2014). The partial 16S rDNA sequence of the new species was submitted to GenBank.

Diagnosis. Pheronematidae with a fan-like body in which the concave side represents the atrial cavity. Basalia are in relatively broad tufts and include some monaxons with clavate distal ends and two-toothed anchors (Tabachnick and Menshenina, 2002).

(Figs 2–4, Table 1)

Material examined. Holotype: MBM286037, Yap seamount (8°51.745 212 9′N, 137°44.416 014 3′E), 15 December 2014, 932 m depth.

Paratype: MBM286528, Caroline seamount (10°31′34.577″N, 140°10′02.147″E), 26 August 2017, 775.3 m depth.

Description. Sponge of holotype is columnar with body length of 344 mm (not including basalia), maximal diameter is 76 mm and minimal diameter is 49 mm (Fig. 2a). The concave atrial area (Fig. 2d) and convex dermal area (Fig. 2e) are on opposite sides of the body. Color is beige after leaving the water. Sponge fixed at the stock with its basalia (length: 10 mm) (Fig. 2f). Top of the sponge presents an asymmetrical “C” curve (Fig. 2b) which is thinner than other body part. Both of the dermal and atrial area meshes are rectangle or triangle shape. Meshes of atrial area (of 0.2–0.8 mm diameter) (Fig. 3b) are bigger and more regular than dermal area (of 0.1–0.5 mm diameter) (Fig. 3a).

Sponge specimen of paratype is bigger than holotype, it can reach 750 mm (not including basalia), maximal diameter is 320 mm and minimal diameter is 70 mm (Figs 2g, h), its basalia is 43 mm in length.

Spicules: Pentactins (Figs 3c, d) with smooth rays make up choanosomal skeleton, the four tangential rays are usually unequal with 1 018.5–5 087.8 μm long, the proximal rays are always very short, making them difficult to measure under a light microscopy. Dermalia and atrialia are pinular pentactins. The pinular rays of dermalia (Fig. 4d) are longer and spinier than atrialia (Fig. 4e), with length of 91.8–261.6 μm and 121.9–172.2 μm respectively. Both the tangential rays of atrialia (length: 37.4–76.6 μm) and dermalia (length: 43.1–72.1 μm) have small spines on their terminals, but the tangential rays of atrialia are straighter than that of dermalia. Prostalia are represented by basalia, scepters and clavate monaxons. Basalia (Fig. 4m) have a smooth shaft and terminal anchor bearing two teeth. Scepters (Fig. 3e), with smooth shaft and spiny terminals, are 534.5–1 112.7 μm long. Clavate monaxons mainly exist among the basalia and dermal area. Their clavate parts (26.0–58.2 μm) are smooth or have one to three spines (Figs 4a–c), their middle shafts are covered by spines, their distal ends have pointed terminations. There are two types of uncinates: macrouncinates and microuncinates. Macrouncinates are easily broken and can reach several millimeters in length (length: 2 490.2–4 876.2 μm). Microuncinates (Fig. 4l), covered with short spines, are 324.8–883.2 μm in length.

Microscleres contain three kinds of amphidiscs and micropentactins. Macramphidiscs are oval (Figs 4g, h), and the diameter of its umbel is bigger than length. Total length of macramphidiscs is 178.0–269.6 μm, umbel length is 80.2–129.5 μm, umbel diameter is 101.3–145.3 μm. The umbel of macramphidiscs has eight teeth, the terminals of the teeth are smooth (Fig. 4g) or contain small teeth (Fig. 4h). Mesamphidiscs (Fig. 4i) with shafts covered by spines, total length is 49.4–106.4 µm, umbel length is 15.7–35.1 µm, umbel diameter is 11.2–24.7 μm. Micramphdiscs (Fig. 4j) have the same shape with mesamphdiscs, total length is 33.2–49.0 µm, umbel length is 9.8–16.5 µm, umbel diameter is 7.5–12.8 μm. The rays of micropentactins (Fig. 4f) are covered by very short and minute spines, with length 37.9–87.6 µm. Measurement results of holotype spicules are provided in Table 1.

Etymology. The specific name distortus is named after the Latin word “distortus”, meaning distorted, referring to the notably distorted body shape.

Genbank accession number: MF098799 (16S rDNA).

Remarks. Eight valid species of Poliopogon are reported in the world. Most species of Poliopogon are elongate columnar body shape and possess convex dermal areas and concave atrial cavities. By presenting the oval macramphidiscs, our new species and P. claviculus can be distinguished from other Poliopogon species easily. Both species possess three categories of amphidiscs and clavate monaxons, but the new species has some notable differences: (1) the new species contains macrouncinates and microuncinates while P. claviculus only contains microuncinates; (2) the new species possesses scepters which are absent in P. claviculus; (3) the clavate monaxons of P. distortus sp. nov. are bigger than those of P. claviculus. Poliopogon mendocino has an abroad flaring funnel body shape which differs from P. distortus sp. nov. drastically in the external morphology. Poliopogon maitai, P. micropentactinus and P. zonecus only contain one type of amphidiscs, P. mendocino and P. amadou pacifica contain two types of amphidiscs, this makes them different from P. distortus sp. nov. Our new species contains clavate monaxons, both P. canaliculatus and P. microuncinata do not contain clavate monaxons. Poliopogon amadou amadou has longer clavate monaxons (40 mm) than the new species (average 1.7 mm). Based on these differences, we consider it is a new species.

Poliopogon are mainly distributed in the Pacific Ocean, but most studies did not describe its original substrate. The original growing substrate was only known in a few Poliopogon species (P. amadou, P. microuncinata and P. canaliculatus), and all of the three species grow on hard substrate. Poliopogon amadou were found have an aggregation at the depth of 2 700 m on the Great Meteor seamount, they were always found attaching to hard rocky substrate and never to the surrounding sandy substrate (Xavier et al., 2015). It is interesting that P. distortus sp. nov. is also attached to the surface of a rock. It is the third Poliopogn species found at a seamount. More work is needed to decide whether Poliopogon species are typical species grow on hard substrate with their basal tuft or whether if they have specific relation with seamount system.

The authors are grateful to Xu Kuidong (Institute of Oceanology, Chinese Academy of Sciences) for sponge samples collection during the cruise. Special thanks to Liu Wei (Institute of Oceanology, Chinese Academy of Sciences) for his helping on the SEM pictures of sponge spicules.