Research Article |
Corresponding author: Jin Sun ( jin_sun@ouc.edu.cn ) Academic editor: Thomas von Rintelen
© 2024 Chong Chen, Xu Liu, Xinyu Gu, Jian-Wen Qiu, Jin Sun.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Chen C, Liu X, Gu X, Qiu J-W, Sun J (2024) Integrative taxonomy of a new giant deep-sea caudofoveate from South China Sea cold seeps. Zoosystematics and Evolution 100(3): 841-850. https://doi.org/10.3897/zse.100.125409
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Caudofoveata is a class of worm-like molluscs (aplacophorans) that typically have an infaunal lifestyle, burrowing in soft bottoms in a wide range of marine habitats from shallow to deep waters. Here, we describe a very large new species of caudofoveate from South China Sea methane seeps growing up to 154 mm in length: Chaetoderma shenloong sp. nov. It is the first caudofoveate to be named from a chemosynthetic ecosystem and the first aplacophoran mollusc associated with seeps. Our new species stands out from other Pacific Chaetoderma species by its large size, a wide body relative to its length, a barely sclerotised radula, and the presence of isosceles-triangular sclerites. Phylogenetic reconstruction using the mitochondrial cytochrome c oxidase subunit I (COI) gene placed it within a paraphyletic clade comprising Chaetodermatidae and Limifossoridae, in line with a previous phylogenetic analysis. This also revealed that C. shenloong sp. nov. is conspecific with a Chaetoderma sp. whose whole genome was recently sequenced and assembled but remained undescribed until now. The most closely related species with an available COI sequence was C. felderi, the largest caudofoveate species recorded. Our discovery suggests caudofoveates may be present in other seeps globally but so far neglected; a potential example is C. felderi from the Gulf of Mexico, where seeps are abundant but whose exact habitat remains unclear.
Aplacophora, Caudofoveata, chemosynthetic, F site, Haima, hydrocarbon seep, Jiaolong Ridge, new species
Collectively known as the aplacophorans, shell-less worm-molluscs characterised by dense calcareous sclerites covering the body surface comprise two distinct lineages currently considered separate classes, including Solenogastres, which retains a foot groove, and the footless Caudofoveata (
Caudofoveata comprises about 140 species in three currently recognised families, including Chaetodermatidae Théel, 1875; Limifossoridae Salvini-Plawen, 1970; and Prochaetodermatidae Salvini-Plawen, 1972 (
Within Chaetodermatidae, the three genera are also separated primarily by radula features (
Though both classes range widely from shallow to deep waters, the majority of the described species inhabit the upper continental shelf. Only Solenogastres has been recorded from deep-sea chemosynthetic ecosystems, with six described species in Simrothiellidae Salvini-Plawen, 1978, from east Pacific hydrothermal vents; these include four species of Helicoradomenia Scheltema & Kuzirian, 1991, plus Sensilloherpia pholidota Salvini-Plawen, 2008, and Diptyaloherpia insolita Salvini-Plawen, 2008 (
Caudofoveate molluscs were collected by the remotely operated vehicle (ROV) Pioneer using a push-corer equipped with a 60-cm-long tube from dark-coloured sediments around a population of the vesicomyid clam Archivesica marissinica (Chen, Okutani, Liang & Qiu, 2018) (originally described as “Calyptogena” marissinica) (
Specimens were photographed using a Canon EOS-5Ds R digital single-reflex lens camera equipped with an EF100mm f/2.8L Macro IS USM lens. For radula examinations, the radula was dissected out with the tissue around it and slowly dissolved using a 20% household bleach solution. Upon dissolution, the radula was washed in MilliQ water and then photographed using a Nikon Eclipse Ti2 slide microscope, where multiple photographs were stacked using Adobe Photoshop CC. To examine the spicules, the cuticle was dissected using fine tweezers and forceps under a dissecting binocular (Olympus SZX16). Sclerites were examined in six regions of the body (see Fig.
Chaetoderma shenloong sp. nov., photographs of preserved-type specimens. A. Holotype (TMBC031015), 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 (TMBC031015), enlarged dorsal view of the prepallial and pallial regions; C. Paratype 1 (TMBC031016); D. Paratype 2 (TMBC031017). Arrowheads indicate the anterior end of the animal.
We attempted to amplify the barcoding region of the mitochondrial cytochrome c oxidase subunit I gene (COI) using invertebrate universal primers (
To check the similarity of COI sequences among the studied individuals, a custom primer pair was specifically designed using the NCBI primer designing tool from available COI sequences of Caudofoveata. Our primer pairs Caudo_COI_F: TTAAGAGTATAGTGATTGCTCCTGC and Caudo_COI_R: AGGATTTGGAAACTGACTACTCCC, designed to amplify a 408-bp fragment, were used to sequence the COI gene in all study individuals from Haima. This primer pair lies within the barcoding region and can also be useful for use on other caudofoveates, with the primers aligning reasonably well with most available chaetodermatid and limifossorid sequences. The PCR amplification was done using the following protocol: 94 °C for 1 min for initial denaturation, followed by 94 °C for 45 s, 53 °C for 45 s, and 72 °C for 45 s. After 35 cycles, the reaction was held at 72 °C for 7 min. Successful PCR products confirmed using gel electrophoresis were sent to the Beijing Genomics Institute (Qingdao, China) for Sanger sequencing. Sequences were checked by eye before downstream analyses. Newly generated COI sequences were deposited in NCBI GenBank under the accession numbers PP664117–PP664119.
As the region amplified using our new primer set is significantly shorter than the
Type specimens are deposited in the Tropical Marine Biodiversity Collections of the South China Sea, Chinese Academy of Sciences, Guangzhou, China (TMBC).
Order Chaetodermatida Simroth, 1893
Family Chaetodermatidae Théel, 1875
Chaetoderma nitidulum Lovén, 1844 (type by monotypy)
‘Caudofoveata Indet. 1’ –
‘Chaetoderma sp.’ –
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 XYH01-2022-06, September 20th, 2022.
Holotype
(Fig.
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.
Animal (Fig.
Radula (Fig.
Sclerites (Figs
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).
The midgut region (Figs
The prepallial region (Figs
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.
Haima and Jiaolong Ridge methane seep sites in the South China Sea (see molecular phylogeny section below). For a map of these sites, see
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 (
Our maximum likelihood phylogenetic reconstruction using the mitochondrial COI gene (Fig.
The K2P distance between the two COI sequences of C. shenloong sp. nov. used for phylogenetic reconstruction was 0.5% across 660 bp, supporting their conspecificity. From a 393-bp alignment of all five C. shenloong sp. nov. COI sequences (the holotype and three paratypes from Haima plus the Jiaolong Ridge sequence) obtained using our newly designed primers, the K2P distance was 0.3–0.8%. The K2P distance calculated across the 660-bp COI gene fragment between C. shenloong sp. nov. and C. felderi was 15.4–16.0%.
Our phylogenetic reconstruction revealed a sister relationship between Chaetoderma shenloong sp. nov. and C. felderi, the two largest-bodied species thus far known across Caudofoveata, in a derived position within the class. Although the taxon sampling of our tree across the caudofoveate diversity is still limited, this plus the two species sharing the isosceles-triangular sclerite type are suggestive that they represent a lineage of deep-sea caudofoveates characterised by large body sizes. The distant geographic distribution between these two taxa (South China Sea vs. Gulf of Mexico) may mean many more deep-sea giant caudofoveates remain to be discovered in all oceans around the globe but have simply been overlooked, which is not surprising given the caudofoveates are severely understudied (
Chaetoderma shenloong sp. nov. is the first caudofoveate reported from chemosynthetic ecosystems (
Specimens studied in the present study have been deposited in the Tropical Marine Biodiversity Collections of the South China Sea, Chinese Academy of Sciences, Guangzhou, China, under numbers TMBC031015–TMBC031018. New sequences generated in this study have been deposited in NCBI GenBank under accession numbers PP664117–PP664119. For phylogenetic analyses, the alignment file before and after trimming as well as the consensus tree output from IQ-TREE2 are available on Figshare (
CC, J-WQ, and JS conceived and designed the study. JS collected the specimens at sea. CC, XL, and XG carried out dissections and light microscopy. XL carried out molecular work and scanning electron microscopy. CC drafted the original manuscript, which was critically revised and contributed to by all other authors.
We thank the captain and crew of R/V Xiangyanghong 01 and the pilots of ROV Pioneer for their efforts to ensure successful sampling during the cruise XYH01-2022-06. Yixuan Li (Hong Kong Baptist University) is gratefully acknowledged for her help in specimen photography, and Yunlong Li (Ocean University of China) for his help in bioinformatics. This study was supported by the Natural Science Foundation of Shandong Province (ZR2023IQ014) and the Fundamental Research Funds for the Central Universities (202172002 and 202241002). Constructive comments from Franziska S. Bergmeier (The University of Alabama), Kevin M. Kocot (The University of Alabama), and Hiroshi Saito (National Science and Nature Museum, Tsukuba) improved an earlier version of this paper. We take this chance to express our deepest condolences for the recent passing of artist Akira Toriyama (1955–2024), whose characters Shenron (= Shenlong or Shenloong) in Dragon Ball and the dragon family in the Dragon Quest series in part inspired the specific epithet of our new species.