Taxonomy Of Free-living Nematodes From The South China Sea And Molecular Phylogenetic Analysis Of The Order Monhysterida

Free-living nematodes are the dominant benthic groups with high species diversity in marine benthic environment.Due to their smaller size,the difficulty in taxonomy and the lack of qualified taxonomists,the taxonomy and systematics of nematodes are relatively scant and a large number of species remains unknown.The South China Sea has the highest marine biodiversity in the sea areas of China,but it is also an area where the taxonomy of marine nematodes is very deficient.The species composition and diversity of nematodes in the South China Sea,particularly those in the deep-sea area,are not well understood.In addition,the systematic positions and phylogenetic relationships of many nematode taxa,which were established on the basis of classic taxonomy,are still in doubt or in confusion.Thus,there is a need to acquire new understanding through the introduction of molecular techniques.The present study investigated the taxonomy and phylogeny of nematodes based on a total of 39 sediment samples collected from intertidal zone through continental shelf to deep-sea basin in the South China Sea.The sampling areas comprised the intertidal zone(7 stations)in Hainan and Guangdong provinces,the contiental shelf in the northeast South China Sea and the central deep-sea basin(32 stations),with water depths up to 4000 m.Through morphological taxonomy,the basic species composition of nematodes in the South China Sea was examined and new species were described herein.Moreover,the molecular phylogenetic relationships of some groups within the order Monhystera were revealed on the basis of 18S r DNA sequencing.The aim of this study is to provide reliable data for the cataloging and the understanding of species diversity of nematodes in the sea areas of China,and to clear the phylogenetic relationships within the order Monhystera.The results of quantitative and qualitative analyses on sediments from 32 stations in the continentlal shelf and deep-sea basin in the South China Sea showed that the average abundance of nematodes was 192.1±38.7 inds/10 cm~2,occupying about 85.5%of the total meiofaunal abundance.The average biomass was 76.8±15.5μg dwt/10 cm~2.A total of 366 species(or taxonomic entities)of nematodes belonging to 8 orders,39families and 187 genera were identified.The dominant nematode genera in the South China Sea were composed of Acantholaimus,Cervonema,Chromadorina,Dorylaimopsis and Halalaimus.The genera each comprising more than ten species included Acantholaimus,Cervonema,Daptonema,Diplopeltoides,Halalaimus,Leptolaimus and Sabatieria.There were 128 common nematode genera found in the South China Sea and East China Sea,46 common genera in the South China Sea and Yellow Sea,and 19 common genera in all the three seas.Sixty-five species were found only in the South China Sea.In this study,32 new species of nematodes and 12 first records of China were found and described from the South China Sea,including 7 species from the intertidal zone,16 species from the continental shelf and 21 species from the deep sea.Of the 32new species,4 species were found from the intertidal zone,15 from the continental shelf area,and 13 from the deep-sea area,including five new species that have been published(Cervonema longispicula Huang,Jia&Huang,2018;Diplopeltoides conoicaudatus Sun,Huang&Huang,2021;Minolaimus multisupplementus Sun,Huang&Huang,2020;Setosabatieria minor Huang,Xu&Huang,2019;Thalassironus filiformis Huang,Huang&Xu,2019).The other 27 new species are to be published as follows:Anticoma filiformis sp.nov.;Axonolaimus longiamphidatus sp.nov.;Bathylaimus nanhaiensis sp.nov.;Cervonema sinica sp.nov.;Desmodorella filiformis sp.nov.;Desmoscolex longispiculatus sp.nov.;Diplopeltoides microamphidatus sp.nov.;Draconema longiseta sp.nov.;Gomphionema setiforme sp.nov.;Linhystera hamatus sp.nov.;Linhystera longispicula sp.nov.;Linhystera minor sp.nov.;Linhystera nanhaiensis sp.nov.;Nannolaimoides conicaudatus sp nov.;Paracomesoma longissimum sp.nov.;Parasphaerolaimus minor sp.nov.;Phanodermopsis nanhaiensis sp.nov.;Promonhystera longicorporis sp.nov.;Promonhystera curvispicula sp.nov.;Promonhystera seta sp.nov.;Pselionema longispicula sp.nov.;Robbea praelonga sp.nov.;Solenolaimus stenocephatus sp.nov.;Southerniella longicauda sp.nov.;Stylotheristus longicaudatus sp.nov.;Terschellingia macramphida sp.nov.;and Wieseria brevicaudata sp.nov.In addition,12 are first records in the sea areas of China.These include Ceramonema carinatum Wieser,1959;Corononema parvum Warwick,1995;Cricohalalaimus vagabundus Bussau,1993;Desmoscolex americanus Chitwood,1936;Diplopeltoides pumilus Holovachov,&Bostr(?)m,2017;Draconema andamanense Kito&Chatterjee,2012;Manganonema microcephalum Bussau,1933;Marylynnia dentikulata Kim,Tchesunov&Lee,2015;Paracyatholaimus intermedius De Man,1880;Paramphimonhystrella barbula Leduc,2014;Pselionema dissimile Vitiello,1974;and Quadricoma scanica Filipjev,1922.All these new species and first records are described in detail,the establishment of the new species is discussed,and the comparison of the first records with related populations and species are provided.Based on 16 new sequences of 18S r DNA obtained from 11 species of the order Monhystera and 64 related sequences from Gen Bank,the ML and BI phylogenetic trees were constructed for Monhystera.Phylogenetic analysis was performed on the families Linhomoeidae Filipjev,1922;Sphaerolaimidae Filipjev,1918;Xyalidae Chitwood,1951;and Monhysteridae De Man,1876.The results showed that Daptonema,Geomonhystera,Terschellingia did not clustered with their related taxa.In combination with morphological taxonomy,Daptonema longiapophisis Huang&Zhang,2010 was transferred to the genus Theristus Bastian,1865,the genus Terschellingia de Man,1888was suggested to stay in the family Linhomoeidae,and the genus Geomonhystera Andrássy,1981 was transferred from the family Monhysterae to Xyalidae.