Foliaceous coastal worm (Thoracophelia foliformis) found in intertidal zone of Yellow Sea, South Korea

NEWS - Foliaceous coastal worm (Thoracophelia foliformis sp. nov.) was discovered in the intertidal zone as the first species of the genus from the Yellow Sea, Korea. The new species is closely related to T. dillonensis (Hartman, 1938) from California and T. ezoensis Okuda, 1936 from Japan because it has pectinate branchiae.

The new specific epithet is derived from the leafy ventral midplate shape of the pygidium. The name is a combination of the Latin words folium (meaning ‘leaf’) and formis (meaning ‘form’). The Korean name is Yip-sa-gwi-Keun-yo-jeong-get-ji-reong-yi (잎사귀큰요정갯지렁이). The specimen was collected from the upper intertidal zone consisting of sand or muddy sand in the Yellow Sea of South Korea.

Although the number of branchial filaments in well-developed branchiae varies (12-15), there is no variation in the number of pairs of branchiae (15 pairs) or the first chaetiger containing branchiae (chaetiger 14) among specimens. Oocytes are contained in a coelomic cavity about 0.1 mm in diameter, visible through the transparent body wall.

The annulation of each segment has a weak lineal staining and is thicker in the mid-dorsal abdominal region. The most posterior segments and the mid-ventral plate of the pygidium are intensely stained. A deep, large circular spot is present at the base of each branchiae anteriorly. The prostomium is weakly stained on the lateral side.

To date, 18 species have been described in Thoracophelia. The new species differs from the others in the shape of the branchiae and pygidium. By having pectinate branchiae, T. foliformis resembles T. dillonensis and T. ezoensis.

However, T. foliformis differs from T. dillonensis in having wrinkled branchiae with fewer branchial filaments (12-15 vs. 15-20) at full development and a mid-ventral plate on the pygidium. T. foliformis differs from T. ezoensis in the number of branchiae pairs (15 vs. 19) and the presence of a mid-ventral plate on the pygidium.

The new species also differs from T. minuta (Jimi, 2021) in having a small body and simple hemispherical branchiae without ciliated areas. The new species differs from T. japonica in having palmatifid branchiae with 3-7 filaments and a pair of lateral transverse rows of cone-shaped cirri on chaetiger 10 instead of a lateral ridge.

The new species also differs from T. yasudai and T. arctica in that both species have dichotomously divided branches. The new species has bifid or trifid branchiae, which is different from the other 12 Thoracophelia species. The new species differs from T. williamsi in having pectinate branchiae (vs. bifid and trifid branchiae with pinnate dorsal filaments).

In COI, the minimum interspecific variation between the new species and related species was 14.2% for T. minuta. However, the new species differs from T. minuta in the shape of the branchiae (pectinate vs. hemispherical). The new species is most closely related to T. mucronata with 0.2% and 3.6% differences in 18S and 28S rDNA, respectively, but the new species differs from T. mucronata in having pectinate branchiae rather than bifid.

In the tree based on 18S and 28S rDNA sequences, the Opheliidae are grouped into two clades of the subfamilies Ophelininae and Opheliinae based on the sequences of several gene regions assembled. Thoracophelia species form a monophyletic group including the new species with high support values (ML = 81%, BI = 0.95).

The new species is sister to two clades of Japanese and American Thoracophelia species, although morphological features reflecting this phylogenetic relationship have not been identified. The 18S and 28S rDNA sequence trees showed similar results to the assembled nuclear gene results.

Original research

Lee H-E, Lee GH, Min G-S (2024). A new species of Thoracophelia (Annelida, Opheliidae) from the Yellow Sea of South Korea. Biodiversity Data Journal 12: e129526, DOI:10.3897/BDJ.12.e129526

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