Tretopleura weijicus sp. nov.

(Figs. 1–6, Table 2)

Material examined. Holotype: SIO-POR-090, SRSIO, Haima ROV DY41B, dive MCROV02, collected by Dong-sheng Zhang, Sep. 22, 2017, Weijia Seamount in the northwestern Pacific Ocean (12.3772°N, 156.2871°E), depth 1995 m, frozen at -20°C.

Description. The holotype has a blade-like body form, bifurcating into mainly three branches at its top, with a slender peduncle attached to hard substrate and widening gradually towards the body (Fig. 1A). Four segments were sampled, including one piece of the blade-like body and three segments of the lower peduncle (Fig. 1B). Color of the specimen is light beige to white.

The body segment is 695 mm long and 45–116 mm wide (Fig. 1B1). Both surfaces longitudinally bulge in the central axes of branches to 13–20 mm thick. The bulge is gradual to 7 mm high in the dermal surface while semicylindrical to 11 mm high in the atrial surface. Thus the transverse cross-section of the body is similar to the lateral view of a bowler hat (Fig. 1C). The edges of the blade-like body are moderately thick-walled lamellae to about 4 mm thick, usually rolling up to the dermal side and pitted by non-overlapping channels penetrating vertically into the body wall (Fig. 2A,B). Epirhyses and aporhyses in the edges of both surfaces are ovoid and in no regular arrangement (Fig. 2C 1,D1). There are evident transition areas between axial and marginal areas in the dermal surface where epirhyses elongate longitudinally and occur in closed-spaced longitudinal series (Fig. 2C2). Epirhyses, 233.1–(847.8)–1865.0 µm in maximum diameter of the cleaned framework, are covered by a lattice of loose dermal pentactins and hexactins (Fig. 2C 4; Table 2). Aporhyses, 267.5–(1065.6)– 2036.8 µm in maximum diameter of the cleaned framework, are more widely spaced and mostly have no overlying of atrial spicules (Fig. 2D 3; Table 2). The bulging areas of both sides are usually smooth and without channels (Fig. 2C 3,D 2).

Three segments of the lower peduncle are 37–50 mm wide and totally 881 mm long (Fig. 1B 2). The basal part of the peduncle is cylindrical, and its bottom is relatively round and flat with a diameter of 57–70 mm (Fig. 1D). It should be noted that the basal part of the peduncle was densely covered by anchorate spicules (Fig. 1A,B). Further investigation indicated that these two-toothed anchors were extraneous and probably sourced from a lophophytous Pheronematidae sponge which once grew on the base of this specimen and may have been dead for a long time to only leave its anchorate basalia implanting into and enwinding around the base.

Skeleton. The choanosomal skeleton is a three-layered dictyonal framework consisting of an unchannelized primary middle layer and two channelized peripheral layers on both dermal and atrial sides (Fig. 3A).

The dermal and atrial peripheral layers consist of hexactine dictyonalia joined irregularly by tip-to-ray fusion at any angle, with rays slightly curved (Fig. 3B,D). The meshes of both cortices are irregularly triangular to polygonal (Fig. 4A,C). The dermal cortex is thicker than the atrial cortex and has a more elongate form of channel apertures in rows (Fig. 3A).

The primary middle layer is constructed of longitudinal strands consisting of a series of hexactin centers (Fig. 3C, 4E). Longitudinal stands are joined and supported by two types of connecting elements. One is irregularly appended hexactine dictyonalia, which are joined by ray tips fused to strands at any angle (Fig. 4F 1). Another is irregular-spaced transverse rays of the constituent dictyonalia of longitudinal stands, forming ladder-like connections (Fig. 4F 2). Synapticula are absent. The longitudinal strands, 24.0–(41.5)–58.0 µm thick, are about 1.5 times thicker than the radial or transverse beams (Table 2). Strands and beams are smooth. Beams in the dermal cortex, 20.0–(45.2)– 66.7 µm thick, are mostly thicker than those in the atrial cortex and primary layer (Table 2), but they are similar in average length (Table 2).

There are rough and digitate spurs attached thoughout the skeleton (Fig. 4B,D). The dermal and atrial spurs in the edges of the blade-like body are abundant, curving to project candelabra-like outwards on the surfaces (Fig. 4B 1,B 2,D 1). Most of the spurs are conical with sharply or bluntly pointed ends. Besides, there is another form of spurs rarely in the dermal surface, which is cylindrical with swollen and rounded ends (Fig. 4B 2). The atrial spurs, 96.0–(184.0)–312.0 µm long, are slightly longer and much denser than the dermal ones (Fig. 4D; Table 2). Rough microhexactins are attached profusely thoughout the choanosomal skeleton (Fig. 4G). Sometimes microhexactins are fused tip-to-tip.

The ectosomal skeleton is composed of a lattice of loose pentactins and hexactins with the blunt sixth distal ray outwards, overlying all choanosomal framework surfaces and entrances to epirhyses but not most of the openings of aporhyses (Fig. 2C 4,D 3). Uncinates are largely distributed horizontally on both surfaces and also project with the anterior end outwards. Sceptrules project vertically or obliquely on the surfaces with head outwards. Additionally, abundant sceptrules along with uncinates can be seen projecting vertically from the inwall of aporhyses with head outwards (Fig. 2D 3).

Megascleres. Megascleres are dermal and atrial pentactins and hexactins, discoscopules, and uncinates (Fig. 5; Table 2). The dermalia and atrialia are classified into two categories, respectively. One is pentactins or subhexactins (Fig. 5A,B) referring to the triaxon spicules with a swelling (pentactine form) or a sixth distal ray very short, rounded-tipped and distinct from the other five rays rendering the spicule hexactine. The latter is named as subhexactin, as presented by Reiswig and Kelly (2011), to distinguish from the another category of dermilia and atrilia which includes general hexactins with six rays in similar shape (Fig. 5C,D). Herein, the pentactine and subhexactine forms are placed into one category due to their similarities in shape and continuous change in length of the sixth ray.

Dermal pentactins or subhexactins (Fig. 5A) often have a bullet-like or spherical swelling (pentactine form) but may extend up to 53.6 µm long with clavate tip (subhexactine form); tangential rays are 87.0–(139.3)– 263.5 µm long and have conically pointed or rarely rounded tips, sometimes slightly inflated near the pointed tips; the proximal ray is 142.0–(307.9)–430.0 µm in length with conically pointed tips and 1.3–(2.3)–3.6 times longer than the tangential ones; the distal ray is 4.2–(8.2)–11.0 µm in diameter, slightly thicker than the proximal and tangential ones. Dermal pentactins or subhexactins are entirely microspined and rougher in the distal part of rays.

Atrial pentactins or subhexactins (Fig. 5B) have a sixth distal ray which is a short cylindrical ray extending from 6.7 to 39.2 µm long with rounded tip (subhexactine form) or rarely only a spherical swelling (pentactine form); tangential rays are usually long and gently taper to sharply pointed tips, 148.6–(431.3)– 698.3 µm in length, sometimes having one tangential ray very short but the paired one long and obviously thickened with bluntly pointed tip; the proximal ray is relatively short, 14.4–(118.6)–200.0 µm in length, with tapering and sharply pointed tip; all rays are 3.0–(5.6)– 8.3 µm in diameter. The tangential and proximal rays are microspined but the distal ray and the axial cross are smooth or sparsely spined.

Dermal hexactins (Fig. 5C) are usually sword-like with short distal ray and very long proximal ray similar to dermal pentactins/subhexactins, while some are irregular and have all the rays unequally long. The distal rays, 28.0– (114.4)–195.0 µm long, are conically pointed or sometimes slightly inflated near the pointed tips; tangential rays are 87.8–(133.8)– 254.5 µm long; the proximal ray is 100.0–(317.4)–420.0 µm long; the ray width is 4.7–(6.9)–10.0 µm.

Atrial hexactins (Fig. 5D) have an obviously short distal ray 18.0–(64.0)–122.0 µm in length; tangential rays are 107.8–(356.4)– 568.3 µm long with sharply pointed tips; the proximal ray is 211.1–(364.8)–501.0 µm in length, nearly equally long to tangential rays or more than 1.5 times longer than tangential ones rendering the hexactins sword-like; the proximal ray is 4.4–(10.4)–60.0 µm in width slightly thicker and rougher than the distal and tangential rays with sharply or rarely bluntly pointed tips.

Atrialia are generally larger and more irregular than dermalia with considerable variation occurring in the relative lengths of each ray, leading to a lot of broken-rayed triaxon spicules when observed. Additionally, no regular pentactins without any swelling are observed.

Uncinates (Fig. 5E) are basically straight, 853.9–(1709.9)– 15833.3 µm long and 5.0–(6.2)– 8.3 µm thick, sometimes slightly curved, with barbs curving out from the spicule surface. One end is full of well-developed barbs which are oriented towards the other end. The barbs of the other end are undeveloped and similar to forward-curved spines. Both tips are sharply pointed.

Sceptrules (Fig. 5F) are discoscopules, 450.0–(632.3)–1192.0 µm long, usually have 4–7 thin, basically straight or slightly curved, and rough tines capped with small marginally toothed discs. The tapered shaft is rough and straight, occupying about 88% of the total length. The middle of the shaft is sparsely microspined, 4.8–(6.5)– 12.8 µm thick, while its terminal part bears procurved spines and ends in a sharply pointed tip. Sceptrules vary in the form of the head with the head length 59.2–(76.0)– 156.8 µm and neck width 6.7–(9.6)– 16.4 µm. Overall, one form often has a neck tapering to the shaft with 4–6 nearly parallel straight tines; another form has a short neck consisting of four swellings with 4–7 relatively divergent tines.

Microscleres. Microscleres are only stellate discohexasters (Fig. 5G; Table 2). Their primary rays bear 4–11 secondary rays ending in small discs with 7–9 marginal spines. All rays are entirely ornamented with short recurved spines, but the spherical centrum remains smooth. Considerable variation occurs in the relative lengths of primary and secondary rays, the form of the terminal group, and the number of secondary rays. There are mainly two types of stellate discohexasters. Type I (Fig. 5G 1) has 6–11 relatively sigmoid secondary rays per primary ray, arranged with a central one in line with the primary rays and the others in a single whorl around it. The discohexasters of type I are 26.5–(31.8)– 36.3 µm in diameter, with their ratio of secondary to primary length being 1.2–(2.0)–2.8. Type II (Fig. 5G 2) has short primary rays bearing 4–6 relatively long, slightly curved, and tightly bunched terminal rays. The discohexasters of type II are 30.6–(40.8)–49.0 µm in diameter and their ratio of secondary to primary length is 1.7–(2.8)–4.0. Both are scattered in the whole specimen, but the former is more common.

Ecology. The Weijia Seamount in the northwestern Pacific Ocean, depth 1995 m. The flat bottom of the peduncle is attached to a hard substrate beneath a thin sediment.

Etymology. The species name, weijicus, refers to the location, the Weijia Seamount.

Remarks. The new specimen described here is a sceptrule-bearing sponge, of which the primary choanosomal framework consists of multiaxial longitudinal strands joined and supported by irregularly-appended hexactine dictyonalia and irregularly-spaced transverse rays, without synapticula. Therefore it is attributed to the family Uncinateridae, rather than Aulocalycidae . Due to its blade-like and branching body form, the presence of epirhyses and aporhyses and the absence of diarhyses, the specimen belongs to the genus Tretopleura .

The two currently valid species of Tretopleura occur in the tropical western Pacific Ocean at depths of 1500- 2080 m. Also, the new specimen was collected from the Weijia Seamount in the northwestern Pacific Ocean at a depth of 1995 m. T. candelabrum was collected as curved plate-form fragments, and loose spicules were unavailable. It is named after its candelabra-like atrial spurs, which also characterize the new specimen. The dermal spurs of T. candelabrum are thickened and often fused as compound clubs while such swollen dermal spurs are not found in the new specimen.

T. styloformis was collected integrally as a blade-like body, of which the spiculation was described but the skeleton was not mentioned. Pentactins with a spherical swelling were observed in both specimens [shown as Fig. 3.10B in Tabachnick (1988) and Fig. 5A,B in this paper]. Besides, regular pentactins without a swollen distal ray [Fig. 3.10A in Tabachnick (1988)] were described in the T. styloformis, but not in the new specimen. Moreover, there are two types of discohexasters in the new specimen, of which the type II (Fig.5G 2 in this paper) is similar to those of T. styloformis [Fig. 3.11 in Tabachnick (1988)]. The type I (Fig.5G 1 in this paper) in the new specimen is smaller and more common with relatively sigmoid secondary rays, but not shown in the T. styloformis . In addition, although uncinates which are abundant on both surfaces of the new specimen were not shown by Tabachnick (1988), their presence was included in the diagnosis by Reiswig (2002) after further review.