Abstract We describe the marine horsehair worm Nectonema shimadai sp. nov. , which was parasitic in a snapping shrimp ( Alpheus sp.) collected at 4 m depth from the coastal water of Kikaijima Island, Japan, northwestern Pacific. The nematomorph was a female, 49 mm in length and 340 μm in width, salmon-pink when alive, with a translucent anterior body region and a distinct anterior chamber. The 18S sequence from N. shimadai sp. nov. was 12.8%, 9.8%, 9.6%, and 9.3% divergent in Kimura 2-parametar (K2P) distance from Nectonema sp. parasitic in the isopod Natatolana japonensis , Nectonema sp. parasitic in the crab Chionoecetes bairdi , N. munidae , and N. agile , respectively. In an 18S tree, N. shimadai sp. nov. was the sister taxon to a clade comprising three decapod-parasitic species, and species sharing the same host group or ocean were not monophyletic. The tree may reflect genetically distant warm- and cold-water lineages within decapod-parasitic nematomorphs.
Kakui et al. (Thu,) studied this question.