A single specimen was collected from the abyssal plain adjacent to the Kermadec Trench in the South Pacific Ocean (maximum depth ~ 10,000 m). We used the manned submersible vehicle ‘Fendouzhe’ at a depth of 5735 m (Fig. 1) as part of the Global Trench Exploration and Diving Program (Global TREnD). The specimen was preserved in 99% high grade absolute ethanol. To perform comprehensive phylogenetic inference on the family Laetmogonidae, we supplemented molecular data from two specimens collected from the South China Sea in the West Pacific Ocean using the manned submersible vehicle ‘Shenhaiyongshi’ at depths of 3566 m and 3568 m, respectively (Fig. 1) and they were preserved at - 80 °C. The specimen of the new species was registered while we were onboard using the Specify niwainvert database of National Institute of Water and Atmospheric Research (NIWA), which was loaned by the NIWA Invertebrate Collection (NIC) to Institute of Deep-sea Science and Engineering (IDSSE). The South China Sea specimens were deposited at IDSSE, Chinese Academy of Sciences (CAS), Sanya, China.

Figure 1. 

Sampling locations of Laetmogone multiradiolus sp. nov. NIWA164015 (red dot) and Laetmogone cf. wyvillethomsoni Théel, 1879 (red diamond).

External morphological features were observed on underwater pictures in situ and images taken onboard immediately after collection, which included skin colour, width, length, body shape, the number and arrangement of dorsal papillae, tube feet and tentacles. Some of these features were examined under a dissecting stereomicroscope (OLYMPUS SZX7). A solution of 15% sodium hypochlorite was used to remove the tissues and isolate the ossicles. The general types of ossicles were observed under an optical microscope (OM) and photographs were taken with a scanning electron microscope (Phenom ProX). The ossicles were rinsed with absolute ethanol, dehydrated, bonded on double-sided carbon tapes and coated with gold before SEM observation. We followed Thandar (1998) and Rogacheva et al. (2013) for the terminology of wheel ossicles.

For genetic sequencing, we used one specimen of Laetmogone multiradiolus sp. nov. collected from the abyssal plain adjacent to the Kermadec Trench and two specimens of Laetmogone cf. wyvillethomsoni collected from the South China Sea. The genomic DNA of each individual was extracted using a TIANamp Marine Animals DNA Kit (TianGen, Beijing), following the manufacturer’s protocol. Mitochondrial cytochrome c oxidase subunit I (COI) was generated for all specimens using the primers outlined in Miller et al. (2017). The PCR conditions consisted of an initial denaturation at 95 °C for 3 min, followed by 40 cycles at 95 °C for 10 s, annealing temperature at 50 °C to 52 °C for 10 s, extension at 72 °C for 10 s and a final extension at 72 °C for 5 min. The 50 μl reaction used for the amplification contained 2 μl of template DNA, 1 μl of each primer and 46 μl of GoldenStar® T6Super PCR Mix (1.1×). PCR products were assessed on GelRed-stained 1.5% agarose gel electrophoresis and sequenced in both directions using the ABI 3730 DNA Analyzer sequencing facility from BGI Genomics, Shenzhen, Guangdong Province, China. The final sequences were deposited in the Science Data Bank of the Chinese Academy of Sciences (Xiao and Zhang 2024) and the GenBank database with the accession numbers PP817260–PP817262.

Three COI sequences of three specimens in this study and 23 COI sequences from GenBank, were used for phylogenetic analyses (Suppl. material 1). We used COI sequences of Peniagone diaphana (Théel, 1882) and Peniagone sp. (Table 1) from the family Elpidiidae as an outgroup. All sequences were aligned using MAFFT7 (Katoh and Standley 2013). Maximum Likelihood (ML) phylogeny was inferred using IQ-TREE with 20,000 ultrafast bootstraps (Minh et al. 2013; Lam-Tung et al. 2015). PartitionFinder2 (Lanfear et al. 2017) was used to choose the best-fitting model, which was GTR + I + G, with all algorithms and AICc criteria. Bayesian Inference (BI) phylogenies were carried out using MrBayes 3.2.6 (Ronquist et al. 2012) under the partition model (two parallel runs, 5,000,000 generations). The first 25% of sampled trees were discarded as burn-in and then the remaining trees were used to construct a 50% majority rule consensus tree. The results were visualised using FigTree v. 1.4.4. (Rambaut 2018). The genetic distances of COI amongst Laetmogone species were calculated using MEGA X (Kumar et al. 2018).

The existing distribution data of Laetmogonid species were obtained from the Ocean Biodiversity Information System (https://obis.org/), the Global Biodiversity Information Facility (https://www.gbif.org/zh/), from published literature and include data from this study. The distribution of six genera in the family Laetmogonidae was illustrated using Generic Mapping Tools, which is a cartographic scripting toolset developed by Wessel and Smith (1995). Using the World Register of Marine Species (WoRMS) and primary literature (Théel 1879; Ludwig, 1893; Mitsukuri 1897; Sluiter, 1901; Fisher 1907; Massin 1987; Thandar 1998; Rogacheva et al. 2013), we obtained data of all accepted species of this family to discuss the geographical distribution of different groups.

Order Elasipodida Théel, 1882

Family Laetmogonidae Ekman, 1926

 Laetmogone multiradiolus sp. nov.

https://zoobank.org/2BC0CDB1-086A-41DE-B71C-A9FD5325D02F

Figs 2, 3, 4

Material examined

Holotype South Pacific • 1 specimen; from the abyssal plain adjacent to the Kermadec Trench; 31°55.54'S, 176°57.09'W; depth 5735 m; 3 Nov 2022; preserved in 99% high grade abso­lute ethanol; NIWA164015.

Type locality

The abyssal plain adjacent to the Kermadec Trench, the South Pacific, depth 5735 m.

Diagnosis

A member of the genus Laetmogone with the following features: Colour uniformly dark violet; tentacles 17, with rounded terminal discs, slightly lobed; calcareous ring absent; papillae conspicuous, nine in each dorsal radius; tube feet 12 pairs, placed in single rows along ventrolateral radii. Body-wall ossicles in form of elasipodid wheels, circular in outline, with 4–5 (rarely 6) central rays, 8–17 spokes, nave covered by a calcareous membrane; rods and few irregular ossicles in papillae, tube feet and tentacles; cross-shaped ossicles absent.

Description

Body long, cylindrical, slightly pointed anteriorly (Fig. 2A, B), 31 cm long, 7.5 cm wide in situ (Fig. 2C, D), 11 cm long, 1.5 cm wide in preserved state. Colour uniformly dark violet in both vivo and preserved states (Fig. 2A–F). Tentacles 17, large, 0.6–1 cm in length after several days of fixation, slightly lobed (Fig. 2E, F). Dorsal papillae conspicuous and nine pairs, arranged in single rows along dorsal radii, 0.7–3 cm long after preservation. Ventrolateral tube feet large, robust and conical, up to 12 pairs, 0.4–0.9 cm in a state of preservation. Mouth ventral, anus terminal. Ventrolateral brim absent. Calcareous ring absent; gonads with numerous branched tubules, arranged in several clusters. Only wheel ossicles in dorsal and ventral body walls (Fig. 3A–G), with no sharp distinction between the two types. The nave of every wheel covered by a calcareous membrane (Fig. 3D). Wheels in body wall 0.04–0.21 mm in diameter, central rays 4–5, rarely 6, 8–17 spokes. Rods and wheels in tube feet, papillae and tentacles, wheels similar to those of the body wall. Papillae wheels, rods and few irregular ossicles (Fig. 4A), rods smooth and stout, curved or straight, occasionally possessing a few little spines on rod ends, 0.2–0.5 mm long. Wheels and curved rods in tentacles (Fig. 4B), rods up to 0.8 mm in length, with small spines on both ends, some rods terminally bifurcated (Fig. 4B). Ossicles in tube feet (Fig. 4C) wheels, rods and irregular ossicles, rods similar in size and shape to those in papillae, irregular ossicles may represent a developmental stage of wheels.

Figure 2. 

Laetmogone multiradiolus sp. nov. NIWA164015. A, B. Specimen before fixation; C, D. In situ images; E, F. Tentacles; dp – dorsal papillae, tf – tube feet. Scale bars: 4 cm (A–B); 10 cm (C–D); 2 mm (E); 500 μm (F).

Figure 3. 

SEM images of ossicles from dorsal and ventral body wall of Laetmogone multiradiolus sp. nov. NIWA164015. Scale bar: 100 μm (A–G). ‘S + number’ indicates the number of spokes.

Figure 4. 

SEM images of dorsal papillae, tentacles and tube feet ossicles from Laetmogone multiradiolus sp. nov. NIWA164015. A. Papillae; B. Tentacles; C. Tube feet. Scale bars: 100 μm (A–C).

More details of the hub, rim, central rays, spoke spaces and orientation of wheels from L. multiradiolus sp. nov. were shown in Fig. 5.

Figure 5. 

SEM images of wheel-like ossicles from Laetmogone multiradiolus sp. nov. NIWA164015. A–E. Lower side, lateral oblique view; F, G. Lower side, lateral oblique view; H. Upper side, lateral oblique view; I–K. Lower side (I. 4 rays, 8 spokes; J. 5 rays, 10 spokes; K. 6 rays, 17 spokes); L. upper side. Scale bars: 50 μm (A–L). The yellow box and line show more details of the central rays.

We counted the spokes and central rays and the diameter was measured in the specimen collected from the Kermadec Trench (Table 1). A sample of 100 wheels taken from the body wall, papillae, tube feet and tentacles (Table 1) revealed the following data:

A total of 44% of wheels had four central rays, 55% had five and only 1% had six rays. The number of spokes per wheel varied from 8–17 with the majority (84%) having 9–14 spokes (11% with 9, 23% with 10, 11% with 11, 15% with 12, 14% with 13 and 10% with 14 spokes). The largest wheel (0.21 mm) had only 11 spokes. Only wheels with a diameter of 0.06 mm had the maximum spokes (17). A higher spoke number (15–17) is typical of smaller (< 0.1 mm) wheels.

Etymology

The specific name was derived from the Latin words multi (many) and radius (ray or beam), which refers to the large number of the spokes of wheel ossicles.

Distribution and habitat

Known only from the type locality so far. In this field, the holotype was found on flat sedimentary terrains. Benthic species, no swimming behaviour was observed.

Remarks

Laetmogone multiradiolus sp. nov. clearly belongs to the genus Laetmogone and possesses 12 pairs of tube feet, which makes Laetmogone multiradiolus sp. nov. unique amongst the known Laetmogone species. The new species was characterised by a single type of wheel, with 4–5 (rarely 6) central rays and 8–17 spokes. These features place it most similar to Laetmogone wyvillethomsoni Théel, 1879. However, L. multiradiolus sp. nov. can be differentiated from L. wyvillethomsoni by the following features: (1) The number of tentacles and tube feet were different. L. multiradiolus sp. nov. had 17 tentacles and 12 tube feet on each side, whereas L. wyvillethomsoni had 15 tentacles and at least 15 tube feet on each side. (2) In the new species, larger wheels reached 0.21 mm in diameter, whereas in L. wyvillethomsoni, wheels > 0.16 mm in diameter were not found. (3) Wheels in L. wyvillethomsoni had 8–14 spokes and some wheels in L. multiradiolus sp. nov. had spokes > 14 (8–17).

The new species was also quite different from other Laetmogone species. Laetmogone multiradiolus sp. nov. differed from L. maculatus by the absence of rosettes, from L. violacea Théel, 1879 by the absence of crosses, from L. scotoeides, L. maculata, L. fimbriata, L. billetti, L. ijimai, L. biserialis and L. pervipedata by the absence of two distinct types of wheels. The number of tentacles and tube feet makes L. multiradiolus sp. nov. and L. theeli different; the latter species had numerous tube feet and 20 tentacles, whereas the new species had relatively few tube feet and 17 tentacles. The difference between L. interjacens, L. perplexa and L. multiradiolus sp. nov. is that the new species had large, conspicuous papillae and the papillae of the former two species were small or minute. The dorsal papillae were arranged in two rows along the dorsal radii (four rows along dorsal radii in L. parvipedata), which distinguished L. multiradiolus sp. nov. from L. parvipedata.

 Laetmogone cf. wyvillethomsoni Théel, 1879

Figs 5, 6

Laetmogone cf. wyvillethomsoni, Bribiesca-Contreras et al., 2022: 78–79, fig. 49.

Material examined

West Pacific • 1 specimen; South China Sea; 18°38.20'N, 114°21.29'E; depth 3568 m; 13 July 2019; preserved in - 80 °C; IDSSE-EEB-HS48. • 1 specimen; South China Sea; 18°38.22'N, 114°21.36'E; depth 3566 m; 13 July 2019; preserved in - 80 °C; IDSSE-EEB-HS49.

Description

Body cylinder-shaped and slender. 15.6–24 cm long and 5.2–7 cm wide before preservation (Fig. 6A–E), with the length exceeding the width by more than three times. Mouth anterior, subventral (Fig. 6A–D). Anus terminal, slightly dorsal. Colour dark violet in both vivo and fixed states. Tentacles 15, of similar size. Odd ambulacrum naked. Conical tube feet 21–28, arranged in single rows on ventrolateral radii. Each dorsal radius with a single row of 12–17 long papillae. Ossicle morphology unavailable due to poor condition of the South China Sea specimens.

Figure 6. 

Laetmogone cf. wyvillethomsoni Théel, 1879 IDSSE-EEB-HS48 and IDSSE-EEB-HS49. A, B. Dorsal and ventral view (IDSSE-EEB-HS48); C, D. Dorsal and ventral view (IDSSE-EEB-HS49); E. In situ image. Scale bars: 5 cm (A–D); 10 cm (E).

Remarks

The South China Sea specimens in this study (Fig. 6) belong to the same species (see Table 2 below, 0–0.8% K2P genetic distance) of the Clarion-Clipperton Zone (CCZ) specimen described by Bribiesca-Contreras et al. (2022) and our phylogenetic analyses showed a well-supported clade (see Fig. 7 below, BS = 90, PP = 0.94). The CCZ specimen resembled L. wyvillethomsoni closely, as reported by Théel (1879) in the original publication, but no rod-shaped ossicles were found in the dorsal skin. The South China Sea specimens were in poor condition and mostly fragmented, which made it impossible to obtain ossicles from specific tissue sites. Therefore, we only provided molecular data and images and morphological studies of more specimens are needed to determine the specific taxonomic status of this species. The discovery of specimens collected from the South China Sea expanded their geographical distribution from the eastern to the western Pacific Ocean, with a maximum recorded depth of 3568 m.

Figure 7. 

Bayesian Inference (BI) and Maximum Likelihood (ML) phylogenetic analysis, based on COI among species of the family Laetmogonidae. A. ML tree, with bootstrap (BS) replications labelled; B. BI tree, with posterior probability (PP) labelled. The bold-annotated branches of the ML and BI trees represent the differences in their topologies. Red font: the new sequences provided in this study. BS values < 50 and PP values < 0.5 are not displayed.

Table 2.

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XLSX

Estimates of p-distance of the COI gene amongst Laetmogone species with available molecular data.

	1	2	3	4
1 L. wyvillethomsoni group 1	0%–0.77%
2 L. wyvillethomsoni group 2	5.41%–6.26%	0.15%–2.18%
3 L. cf. wyvillethomsoni	4.92%–5.40%	3.38%–4.73%	0%–0.80%
4 L. multiradiolus sp. nov.	5.75%–6.09%	3.39%–4.11%	4.37%–4.48%	-
Intraspecific distance in bold. – means no data.

Phylogenetic analyses of the family Laetmogonidae were performed, based solely on the COI gene (Fig. 7) because sequences of other gene fragments (16S, 12S, H3, 18S, 28S) were insufficient to analyse them. Three COI sequences of the two species (Laetmogone multiradiolus sp. nov. and L. cf. wyvillethomsoni.) were deposited into GenBank (Suppl. material 1). The ML and BI trees were reconstructed to determine the taxonomic status of L. multiradiolus sp. nov. using the available COI sequences from the present study and GenBank (Suppl. material 1). In general, the ML and BI tree topologies were similar (Fig. 7), with the exception that the nested relationship between Pannychia species was slightly different. The phylogenetic relationships of Laetmogonidae were consistent with the results of the morphological classification. The species in the genera Laetmogone (BS = 100, PP = 1), Pannychia Théel, 1882 (BS = 100, PP = 1) and Psychronaetes Pawson, 1983 (BS = 99, PP = 1) formed monophyletic clades with high support in both the BI and ML trees. Benthogone abstrusa (Sluiter, 1901) was supported as a sister taxon to Pannychia species (BS = 76, PP = 0.79) and it was the only species in the genus Benthogone Koehler, 1895. The Laetmogone species were separated into two clades, Clade 1: the new species Laetmogone multiradiolus sp. nov. formed an independent clade; Clade 2: Laetmogone wyvillethomsoni and L. cf. wyvillethomsoni formed another clade. Clade 2 was divided into two subclades: (1) Laetmogone wyvillethomsoni group 1 clustered with L. cf. wyvillethomsoni with low support (BS = 70, PP = 0.71); (2) Laetmogone wyvillethomsoni group 2 formed another independent subclade.

Both morphology and molecular phylogenetic analyses confirmed that the new species belonged to the genus Laetmogone. The COI marker has been used frequently to identify many echinoderm species (Hebert et al. 2003; Ward et al. 2008; Layton et al. 2016) and to successfully delimit species of holothuroids (Uthicke et al. 2010; Sonet et al. 2022). In the present study, the genetic distance analysis of COI also indicated its role in delimiting species of Laetmogone.

The genetic divergences of the COI gene in Laetmogone were calculated using the Kimura 2 parameter (K2P) distance method (Table 2). O’Loughlin et al. (2011) defined species complexes as clades with the same morphospecific identification of their members or with different identifications, but within a 5% K2P distance in COI. In the phylogenetic trees of this study, the Laetmogone wyvillethomsoni complex from the Ross Sea and Marie Byrd seamounts showed two clades, as reported by O’Loughlin et al. (2011), who distinguished two genetic clades by depth (from 1620–1990 m compared with 2281–3485 m) (Fig. 7). We divided the Laetmogone wyvillethomsoni complex into two groups: group 1 and group 2.

The genetic distances amongst specimens in group 1 were very low (0%–0.77%, Table 2) and the distances amongst specimens in group 2 were 0.15%–2.18%. In general, taxonomic units with sequence differences of 2%–5% constituted a ‘grey area’ where cryptic species may have been present or where specimens from different geographic locations may have represented different species. The taxonomic units with differences < 2% most likely belonged to the same species, but taxonomic units with differences > 5% almost certainly indicated different species (Uthicke et al. 2010). Therefore, all specimens in group 1 belonged to the same species due to the very low genetic distance between them (0%–0.77%, Table 2). The distances between group 1 and group 2 were 5.41%–6.26% (Table 2), which suggested that the L. wyvillethomsoni complex (group 1 and group 2) comprised at least two different species. Based on the results of the phylogenetic analyses (Fig. 7) and the genetic distances amongst all the specimens in group 2, which were 0.15%–2.18% (Table 2), we inferred that all the specimens in group 2 belonged to the same species or to several cryptic species. The exact number of species contained in group 2 requires morphological examination and species identification of these specimens. Therefore, the genetic distances between the Laetmogone wyvillethomsoni complex were not included in our genetic distance analysis of the genus Laetmogone. In our study, the intraspecific distances ranged from 0%–0.8% (Table 2) and the interspecific genetic distances in Laetmogone ranged from 3.38%–6.26%.

Laetmogone multiradiolus sp. nov. could be differentiated from other congeners by 12 pairs of tube feet, a single type of wheel, 4–5 central rays and 8–17 spokes. The separations were confirmed by the p-distance analyses, which showed that the genetic distances between Laetmogone multiradiolus sp. nov. and other available congeners were 3.39%–6.09%, which was consistent with interspecific genetic distances (3.38%–6.26%) in Laetmogone, but higher than the intraspecific variation of Laetmogone species (0%–0.8%) in this study. To elucidate the inter- and intraspecific genetic distances in Laetmogone species more accurately, it will be necessary to obtain molecular data from more specimens.

The family Laetmogonidae is widely distributed in the deep sea. The genus Laetmogone is the largest of the six genera, with a total of 13 species, including the newly-described species. Laetmogone species are distributed mainly in the Pacific and Atlantic Oceans (Fig. 8A), at depths that range from 130–4410 m (Hansen 1975; Massin 1987; Thandar 1998; Rogacheva et al. 2013). The Pacific Ocean has, by far, the highest diversity of Laetmogone species (Fig. 8). Except for L. perplexa, which is found only in the Atlantic Ocean (Cape Peninsula), all other species of Laetmogone were discovered in the Pacific Ocean (Fig. 8). Three species of Laetmogone occur in the South Pacific: L. fimbriata (Tasman Sea, New Zealand and Australia, depth 360–705 m), L. violacea (Australia and New Zealand, depth 271–1700 m) and L. wyvillethomsoni (New Zealand, depth 837–1320 m; the Kermadec Trench, depth 4410 m). The discovery of the new species is the fourth Laetmogone species in the South Pacific Ocean and the second Laetmogone species found in the Kermadec Trench region. Most Laetmogone species are distributed in the lower bathyal to abyssal zone and the only specimen distributed deeper than 3000 m was found in the Kermadec Trench sampling site (Hansen 1975), which was at a depth of 4410 m. The new species, Laetmogone multiradiolus sp. nov. was collected from the deepest record for this genus (depth 5735 m) and its discovery broadens our understanding of the distribution of laetmogonid holothuroids at various depths.

Figure 8. 

The world distribution of laetmogonid species, based on the Ocean Biodiversity Information System (OBIS) and the Global Biodiversity Information Facility (GBIF) data. A. Laetmogone Théel, 1879; B. Pannychia Théel, 1882; C. Benthogone Koehler, 1895; D. Apodogaster Walsh, 1891; E. Gebrukothuria Rogacheva & Cross, 2009; F. Psychronaetes Pawson, 1983. Species are represented by different colours.

Compared with the large number of species contained in the genus Laetmogone, there are only six species in Pannychia Théel, 1882, three species in Benthogone Koehler, 1895 and only one species in each of the following genera: Apodogaster Walsh, 1891, Gebrukothuria Rogacheva & Cross, 2009 and Psychronaetes Pawson, 1983 (Fig. 8). In the genus Pannychia, only Pannychia taylorae O’Loughlin in O’Loughlin et al. 2013 occurs in the South Indian Ocean, while the other Pannychia species are widely distributed in the Pacific Ocean and four species are found only in the North Pacific Ocean (Fig. 8B). In the genus Benthogone, B. fragilis (Koehler & Vaney, 1905) is found only in the Indian Ocean, B. rosea Koehler, 1895 is distributed widely in the North Atlantic, but also in the South Pacific and the Indian Ocean and B. abstrusa is distributed in the Indian Ocean and the South Pacific (Fig. 8C). In the other three genera, Apodogaster alcocki Walsh, 1891 and Gebrukothuria profundus Rogacheva & Cross, 2009 are distributed in the Indian Ocean and Psychronaetes hanseni Pawson, 1983 is distributed mainly in the eastern Pacific Ocean (Fig. 8D–F).

Overall, the family Laetmogonidae is very diverse and widely distributed in the Pacific Ocean. Since many areas of the Pacific, including numerous seamounts, have not yet been systematically explored, many more species remain to be discovered. Further investigations should be carried out to provide more additional supporting information (e.g. morphological descriptions, geographic distribution and water depth). In addition, molecular data for the family Laetmogonidae are scarce. Although the genus Laetmogone currently contains 13 species, molecular data are currently available for only two species (including the new species described in this study), due to the fact that around 70% of Laetmogone species were described, based on a single or few specimens collected before 1907. It is necessary to collect more molecular data of Laetmogonidae specimens from different regions and depths to conduct more comprehensive biogeographical analyses and to better understand the relationship amongst laetmogonid species. For this, molecular studies could also incorporate additional molecular markers for species delimitation.