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Corresponding author: Yu Zhang ( biozy@szu.edu.cn ) Academic editor: Pavel Stoev
© 2024 Hai-Long Liu, Da-Hao Lin, An-Tai Wang, Zhang-Li Hu, Yu Zhang.
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A new species of the polyclad genus Prosthiostomum is described from the intertidal zone of the South China Sea, Huidong, China, based on morphological and molecular analyses. Prosthiostomum huidongense sp. nov. is characterized by i) few marginal eyes scattered between the marginal band and the cerebral eyes; ii) sucker located at two-thirds of the body length, being removed from the female gonopore by twice the distance between the male and female gonopores; iii) shallow male atrium with slightly ruffled inner wall, positioned approximately perpendicular to the body wall. Molecular phylogenetic analyses based on 28S rDNA sequence showed that the new species was nested in a clade composed of Prosthiostomum species. The uncorrected p-distance of COI between P. huidongense sp. nov. and other Prosthiostomum species ranged from 20.3 to 24.3%, and the high genetic divergence further supports P. huidongense as a new species.
Cotylea, genetic distance, molecular phylogeny, morphology, taxonomy
Polyclads are free-living, almost exclusively marine flatworms with an extremely ramified intestine. They inhabit a variety of environments ranging from the intertidal zone to the deep-sea, such as rocky shores, sand/mud flats, coral reefs, and deep-sea hydrothermal vents (
About 1000 species of Polycladida have been described worldwide; they are classified into two suborders, namely Cotylea and Acotylea, on the basis of the presence or absence of a ventral sucker (
Three specimens were collected under rocks at the intertidal zone in Huidong, Guangdong Province, China (Fig.
Total DNA was extracted using a DNeasy Blood & Tissue Kit (Qiagen, Germany). Three markers (partial nuclear 28S rDNA, mitochondrial 16S rRNA, and cytochrome c oxidase subunit I (COI) sequences) were amplified by polymerase chain reaction (PCR). The PCR was carried out using the primers: Acotylea_COI_F and Acotylea_COI_R (
A total of 27 sequences of 28S rDNA of prosthiostomid species were used for molecular analyses; Pseudobiceros stellae Newman & Cannon, 1994 was selected as outgroup (Suppl. material
Family Prosthiostomidae Lang, 1884
Genus Prosthiostomum Quatrefages, 1845
Holotype
: MBM287880, Huidong, 22°44.95'N, 114°45.05'E, Guangdong Province, China; under rocks at the intertidal zone, 21 April 2023, coll. Hai-Long Liu; sagittal sections on 10 slides, deposited at
Body oval-elongated; dorsal surface cream-colored with numerous yellowish-brown maculae; a pair of cerebral eyes clusters forming an approximately inverted “V” shape; band of marginal eyes extending backwards to behind brain, few eyes scattered between the marginal band and the cerebral eyes; a pair of prostatic vesicles distinctly separated; male atrium shallow, inner wall slightly ruffled; sucker located at two-thirds of body length, removed from the female gonopore by twice the distance between the male and female gonopores.
Body oval-elongated with rounded ends, 10.8–12.1 mm long and 5.6–7.0 mm wide at its widest in living state (n = 3) (Fig.
Prosthiostomum huidongense sp. nov. A. Photograph of entire body, 20230421A1 (left, living specimen in dorsal view), MBM287881 (middle, preserved specimen in dorsal view, paratype), MBM287880 (right, preserved specimen in ventral view, holotype); B. Schematic diagram of head, showing arrangement of the eyes; C. Magnification of head, dorsal view (holotype); D. Magnification of genital pore and sucker (paratype). Abbreviations: fg – female gonopore; mg – male gonopore; ph – pharynx; su – sucker. Scale bars: 1 mm.
Prosthiostomum huidongense sp. nov., MBM287880 (holotype), schematic diagram (A) and photomicrographs of sagittal sections (B–E), anterior to the left. A. Copulatory complex and sucker; B. Anterior end of the body; C. Penis stylet; D. Male copulatory apparatus; E. Copulatory complex and sucker. Abbreviations: ab – anterior branch of main intestine; br – brain; cg – cement glands; cp – cement pouch; fg – female gonopore; it – intestine; m – mouths; ma – male atrium; mg – male gonopore; ph – pharynx; pv – prostatic vesicle; spv – spermiducal vesicle; st – stylet; su – sucker; sv – seminal vesicle; va – vagina; ut – uterus. Scale bars: 0.5 mm (A, E); 200 μm (B); 50 μm (C, D).
The name of the new species is originated from the name of Huidong City, Guangdong Province, China.
So far only known from Huidong, Guangdong, China.
Intertidal, under rocks.
The BI and ML trees based on the partial 28S sequences (904 bp) are almost identical in their general topology, and we show only the ML tree (Fig.
Our specimens are characterized by i) a median frontal branch of the main intestine, ii) a pair of prostatic vesicles that are distinctly separated, and iii) a penis armed with a pointed tubular stylet, features consistent with the generic diagnosis of the genus Prosthiostomum as proposed by
The new species has few eyes scattered between the marginal band and the cerebral eyes and thus can be easily distinguished from most congeners. Only five known species have been described with the above characteristics: P. awaensa Yeri & Kaburaki, 1918, P. griseum Hyman, 1959, P. latocelis Hyman, 1953, P. milcum Marcus & Marcus, 1968, and P. notoensis Kato, 1944. However, these species can be easily distinguished from the new species by the dorsal coloration or pigmentation pattern (except P. milcum) and the number of the marginal eyes (see Table
Comparison of characters between nine Prosthiostomum species that have similar arrangement of marginal eyes or color pattern.
Species | P. awaensa | P. griseum | P. latocelis | P. milcum | P. huidongense sp. nov. | P. dohrni | P. grande | P. nozakensis | P. sonorum |
---|---|---|---|---|---|---|---|---|---|
Body size | 22 mm long; 5 mm wide | 4 mm long | 17 mm long; 4.5 mm wide | about 7 mm long | 10.8–12.1 mm long; 5.6–7.0 mm wide | 25 mm long; 6 mm in wide | 22 mm long; 5 mm wide | 15 mm long | 20 mm long; 2 mm wide |
Dorsal coloration | rather deep buffy, with a brownish longitudinal band in the median line | uniform dark gray | Yellow | whitish body with brown mottles | cream-colored surface uniformly covered with numerous yellowish-brown maculae | soft bright orange yellow, with darker orange yellow spots scattered over the body especially denser along midline | buffy ground color, with numerous small spots of ochraceous color distributed all over body | light brown, with numerous dark brown spots over the whole dorsal surface | translucent white covered with brown mottles, a number of which aggregate along the median line to form a brown stripe |
Cerebral eyes | two oblong clusters, each composed of 28 eyes | each cluster composed of 6–8 eyes | two wedge-shaped clusters, each composed of about 50 eyes | oblong clusters, each composed of 22–23 eyes | each cluster consisting of 15–20 eyes | two oval clusters, composed of numerous eyes | two wedge clusters, each composed of about 25 eyes | two oblong clusters, each composed of about 10 eyes | two cluster, each composed of about 30 eyes |
Marginal eyes | about 60, distributed anterior to brain; few eyes scattered between the marginal band and the cerebral eyes | about 30, distributed anterior to brain; few eyes scattered between the marginal band and the cerebral eyes | about 80, distributed anterior to brain; some eyes scattered between the marginal band and the cerebral eyes | 95, elongated to the half position of the brain; 25 small scattered eyes between the marginal band and the cerebral eyes | about 140, elongated to the level behind brain; few eyes scattered between the marginal band and the cerebral eyes | numerous; elongated to the level behind brain | two irregular rows along the anterior margin | about 60, elongated to the half position of the brain | about 50, distributed anterior to brain |
Male atrium | NA | NA | deep; anterior wall strongly folded | deep; inner wall slightly ruffled | shallow; inner wall slightly ruffled | NA | NA | NA | NA |
Seminal vesicle | NA | NA | oval; lumen oval | elongated oval; lumen narrow | spherical or oval; lumen spherical or oval | NA | spherical; lumen shape unknown | oval; lumen oval | oval; lumen oval |
Sucker | nearly central | located slightly behind body center | nearly central; distance between the female gonopore and the sucker similar to the distance between the female gonopore and the male gonopore | nearly central; distance between the female gonopore and the sucker similar to the distance between the female gonopore and the male gonopore | located at the two-thirds of body; distance between the female gonopore and the sucker twice the distance between the female gonopore and the male gonopore | present; details not described | located slightly behind body center | nearly central | nearly central; distance between the female gonopore and the sucker similar to the distance between the female gonopore and the male gonopore |
Distribution | Shirahama, Amakusa, Japan | Parry Island, USA | California, USA | Bonaire, Netherlands; Florida, USA | Huidong, China | Naples, Italy | Noto, Misaki, Shimoda, Shirahama, Amakusa, Amami Oushima, Japan | Nozaki, Noto, Japan | Tomoe-zaki, Amakusa, Japan |
Reference |
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this study |
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Kato 1944 | Kato 1938 |
Our molecular phylogenetic analysis of Prosthiostomidae based on partial 28S rDNA is largely consistent with that of
Prosthiostomum huidongense sp. nov. is nested in the clade Prosthiostomum, thus corroborates our generic assignment based on the morphology. Uncorrected p-distance of COI among 11 species/molecular entities of Prosthiostomum are 5.7–24.3% (Suppl. material
This study was supported by grants from the Scientific and Technical Innovation Council of Shenzhen Government (grant nos. jcyj20210324093412035 and kcxfz20201221173404012), Innovation Team Project of Universities in Guangdong Province (No. 2023KCXTD028) and China Undergraduate Training Program for Innovation and Entrepreneurship (S202310590068). We sincerely thank Prof. Ronald Sluys (Naturalis Biodiversity Center, P. O. Box 9517, 2300 RA Leiden, The Netherlands) for providing valuable comments on this work.
Supplementary information
Data type: docx
Explanation note: table S1. GenBank accession numbers of sequences for species taxa used in the phylogenetic analyses or genetic distance calculation; table S2. Interspecific uncorrected p-distances for the COI gene fragments; figure S1. The tree is reconstructed by Bayesian inference analyses based on the 28S rDNA.