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First report of marine horsehair worms (Nematomorpha: Nectonema) parasitic in isopod crustaceans


Nectonema, the only horsehair worm (Nematomorpha) genus found in marine environments, was previously known to be parasitic only in decapod crustaceans. We report Nectonema sp. as the first record of a marine nematomorph parasitic in isopod crustaceans. This is also the third record of marine nematomorphs from the North Pacific. Six infected isopods (Natatolana japonensis) collected from 1425 m of depth in the Sea of Japan each contained one to seven (mean 2.33) nematomorphs in the body cavity in the pereon. There was no correlation between the host body length and number of parasites. For Nectonema sp., we describe and illustrate morphological features of the parasitic juvenile stage and present nucleotide sequences for the cytochrome c oxidase subunit I gene (COI or cox1; 451 nt), 18S rRNA gene (1777 nt), and region spanning the internal transcribed spacer 1 (ITS1) and the 28S rRNA gene including the 5.8S rRNA gene and ITS2 (1218 nt in total). In an 18S maximum-likelihood tree that included 24 nematomorph species, Nectonema sp. grouped with N. agile from the northwestern Atlantic; the 18S gene from these two taxa was divergent by 11.8% K2P distance, suggesting that they are different species. Nectonema species may have a broader range of host groups than previously suspected, but may have been previously misidentified as nematode parasites.

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We thank Ken Fujimoto, Shiro Sawadaishi, and the crew of R/V Soyo-maru for collecting specimens and providing facilities; Matthew H. Dick for reviewing the manuscript and editing our English; and two anonymous reviewers for improving this manuscript with their comments.


This research did not receive any specific grant support from public, commercial, or non-profit funding agencies.

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KK conceived and designed the study, collected samples, and conducted the molecular analysis. JF identified the host isopods. KK and DS made morphological observations of the nematomorphs. DS made drawings. KK wrote the first draft of the manuscript, and all authors commented on the first draft and read and approved the final draft.

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Correspondence to Keiichi Kakui.

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Kakui, K., Fukuchi, J. & Shimada, D. First report of marine horsehair worms (Nematomorpha: Nectonema) parasitic in isopod crustaceans. Parasitol Res 120, 2357–2362 (2021).

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  • Cymothoida
  • Deep sea
  • Endoparasite
  • Hairworm
  • Nectonematida
  • Peracarida