Chaetoderma shenloong, Chen & Liu & Gu & Qiu & Sun, 2024

Chen, Chong, Liu, Xu, Gu, Xinyu, Qiu, Jian-Wen & Sun, Jin, 2024, Integrative taxonomy of a new giant deep-sea caudofoveate from South China Sea cold seeps, Zoosystematics and Evolution 100 (3), pp. 841-850 : 841-850

Chaetoderma shenloong sp. nov.

Figs 1 , 2 , 3 , 4

‘ Caudofoveata Indet. 1 ’ – He et al. (2023): 4 , table 1, fig. 2 i.

‘ Chaetoderma sp. ’ – Wang et al. (2024): 1, fig. 1.

Type locality.

Inside dark-coloured mud around a vesicomyid clam colony , Haima methane seep (16 ° 43.937 ' N, 110 ° 27.681 ' E, depth 1385 m), South China Sea, taken using a push-corer by ROV Pioneer, R / V Xiangyanghong 01 cruise XYH 01-2022 - 06, September 20 th, 2022.

Type specimens.

Holotype (Fig. 1 A, B ), total length 154 mm, maximum width 20 mm, 99 % ethanol, specimen with different parts of the body dissected and mounted on glass slides that are deposited together with the body, COI sequence PP 664117 ( TMBC 031015 ) . Paratype 1 (Fig. 1 C ), total length 106 mm, maximum width 11 mm, 99 % ethanol, COI sequence PP 664118 ( TMBC 031016 ) . Paratype 2 (Figs 1 D , 2 A ), total length 58 mm, maximum width 6 mm, 99 % ethanol, COI sequence PP 664119 ( TMBC 031017 ) . Paratype 3 (Fig. 5 inset), total length 115 mm, maximum width 8 mm, 99 % ethanol ( TMBC 031018 ); a photograph of this specimen was published as fig. 2 i of ( He et al. 2023), COI sequence OQ 836653 . All type specimens were from a single sampling event at the type locality.

Diagnosis.

A very large Chaetoderma reaching over 150 mm in body length, with a thick body up to 20 mm in width. Radula translucent with irregular sclerotisation in the median cone, a single pair of barely sclerotised teeth, and a dome-shaped membrane with circular lateral projections. Sclerites shaped like isosceles-triangles present between the foregut region and the midgut sac region.

Description.

Animal (Fig. 1 ) up to 154 mm in length, rather chunky and broad cylindrical, up to 20 mm in width. Foregut region and midgut region separated by a deep groove (‘ neck’), situated approximately one-fourth to one-fifth from anterior of the body. Anterior of the neck typically slightly thicker than the posterior. Posterium (the posterior-most part of the body from prepallial to pallial regions) short at about the hindmost one-tenth of the body. The bulbous, dorsoterminal sense organ takes the form of an elongated dorsal-median groove (Fig. 1 B ). Oral shield (Fig. 2 A ) small, about one-third as wide as the body, wider than high, with mouth opening at the centre. Colouration at different shades of yellowish white, with some variability across body regions and individuals. Dark mud accumulates between sclerites, resulting in a blackish appearance in fresh specimens before cleaning.

Radula (Fig. 2 B, C ) very small compared to the size of the body (190 by 125 μm in size), translucent, and barely sclerotised overall. Consisting of a single pair of barely sclerotised and sickle-shaped denticles about 35 μm in length, each denticle individually connected to the median cone by lateral connections. Median cone about 130 μm in length, irregularly and very weakly sclerotised in the distal 80 % along the length and about one-third to half of the width. The dome-shaped membrane is extensive and surrounds the cone, with a circular lateral projection on either side near the base of each tooth.

Sclerites (Figs 3 , 4 ) in the peribuccal region are of two types: the dominant type, small, lead-shaped or narrow teardrop-shaped cylindrical forms (Figs 3 A , 4 ) without waist, basal margin flat in the middle and slightly curved to the side, narrowing to a sharp tip distally along the blade, blade 5–6 times as long as the base, lacking obvious sculpture, up to 107 by 18 μm in size. The second type lanceolate, flat, waist indistinct, basal margin almost flat, blade 5 times as long as base, the side facing outside ornamented by a median keel that weakens towards the base, sided by weak longitudinal grooves with thickened lateral margins, the side facing the body with only very weak longitudinal lines; up to 145 by 28 μm in size. The same two types also present in the foregut region (Figs 3 B , 4 ), but larger, with the teardrop-shaped ones up to 146 by 23 μm and the lanceolate type up to 171 by 33 μm in size. In this region, a rare third type present, overall similar in morphology to the lanceolate type but with a flatter basal margin and a much wider base, with the blade being 2.5–3 times as long as the base, leading to an isosceles-triangle shape up to 149 by 53 μm in size.

The midgut region (Figs 3 C , 4 ) has both lanceolate and isosceles-triangle types, but larger at up to 287 by 40 μm and up to 257 by 51 μm in size, respectively. Here, the lanceolate type more common than the isosceles-triangle type. The isosceles-triangle sclerites here narrower, with the blades being 4–5 times as long as the base. These two types also present in the midgut sac region (Figs 3 D , 4 ) but larger, with the lanceolate type being up to 391 by 50 μm and the isosceles-triangle type up to 330 by 70 μm in size. Furthermore, an additional needle-type sclerite present from this region, cylindrical at the slanted base, lacking waist, blade 12–13 times as long as the base, straight-sided until the distal one-fourth, where it narrows and flattens to a sharp tip, the side facing outwards ornamented by weak to strong median keel sided by several weak longitudinal grooves, lateral margin slightly thickened, the side facing the body smooth except weak longitudinal grooves; up to 655 by 51 μm in size. In this region, needle-type sclerites rather rare.

The prepallial region (Figs 3 E , 4 ) has lanceolate-type and needle-type sclerites, but even larger at up to 404 by 53 μm and 850 by 55 μm in size. The needle-type becoming more common in this region than the midgut sac region. In the pallial region, only the needle-type sclerite present, where it reaches the longest dimensions across the whole animal at 1023 by 56 μm in size and also more slender, with the blade being 16–20 times as long as the base.

Etymology.

From Mandarin Chinese, " Shén " (divine, deity) + " Loong " (dragon), referring to a group of mysterious and mystic dragons in Chinese mythology. Named in allusion to the long and giant body form of the new Chaetoderma , which carries many ‘ scales’ on its body like dragons. A well-known Chinese saying is ‘ You shall never see the head and tail of " Shén Loong " at the same time, ’ used to refer to something or someone being highly elusive, like caudofoveates living deep inside sediments. Used as a noun in apposition.

Distribution.

Haima and Jiaolong Ridge methane seep sites in the South China Sea (see molecular phylogeny section below). For a map of these sites, see He et al. (2023).

Remarks.

The placement of this new species in Chaetoderma is supported by the overall body form, the oral shield morphology, and the radula. Chaetoderma shenloong sp. nov. is among the largest species known in the genus; the only species larger in size is C. felderi ( Ivanov & Scheltema, 2007) , trawled from between 610 and 850 m in the Gulf of Mexico, reaching a body length of 365 mm ( Ivanov and Scheltema 2007; Mikkelsen et al. 2019). The body shape of C. felderi is much more elongated and only reaches 9 mm in width despite being much longer ( Ivanov and Scheltema 2007), making it easy to separate from C. shenloong sp. nov. morphologically. These two species are also the only species possessing isosceles-triangular sclerites, but their radulae are completely different, with those of C. felderi being much larger at about 690 μm in length with much more reduced teeth ( Ivanov and Scheltema 2007). Chaetoderma productum Wirén, 1892 , from the northeastern Atlantic is another species that may reach a similar length as C. shenloong sp. nov., but even at 150 mm in length, the width of C. productum is only about 3 mm ( Wirén 1892), making it very distinctly different from C. shenloong sp. nov. No other described Chaetoderma species from the western Pacific are known to reach comparable sizes ( Saito and Salvini-Plawen 2014), and even small specimens of C. shenloong sp. nov. would be distinct from them by the barely sclerotised radula and the presence of isoceles-triangular sclerites.

Figures

Figure 1. Chaetoderma shenloong sp. nov., photographs of preserved-type specimens. A. Holotype (TMBC 031015), lowercase Roman numerals indicate the six body regions from where sclerites were examined: i) peribuccal region, ii) foregut region, iii) midgut region, iv) midgut sac region, v) prepallial region, and vi) pallial region; B. Holotype (TMBC 031015), enlarged dorsal view of the prepallial and pallial regions; C. Paratype 1 (TMBC 031016); D. Paratype 2 (TMBC 031017). Arrowheads indicate the anterior end of the animal.

Figure 2. Chaetoderma shenloong sp. nov. A. Oral shield of paratype 2 (TMBC 031017); radula: B. Dorsal view; C. Ventral view. Abbreviations: clp, circular lateral projection; d, denticles; dm, dome-shaped membrane; lc, lateral connections; mc, median cone.

Figure 3. Chaetoderma shenloong sp. nov., light micrographs of representative sclerites from each body region. A. Peribuccal region (i); B. Foregut region (ii); C. Midgut region (iii); D. Midgut sac region (iv); E. Prepallial region (v); F. Pallial region (vi).

Figure 4. Chaetoderma shenloong sp. nov., scanning electron micrographs of representative sclerites from each section of the body. From left to right: the peribuccal region (i), the neck represented by the foregut region (ii), the trunk represented by the midgut region (iii), and the pallial region (vi).

Figure 5. Maximum likelihood phylogenetic reconstruction of Caudofoveata carried out using a 594 bp alignment of the mitochondrial COI gene, with a photograph of paratype 3 (TMBC 031018) when alive as an inset. Node values indicate bootstrap support; only those over 60 are shown.