A New Species of Gammanema (Nematoda: Chromadorida: Selachinematidae) from Jeju Island, South Korea

During a survey of the intertidal zone on the eastern coast of Jeju Island, Korea, a new species of free-living marine nematode belonging to the Selachinematidae (Chromadorida) family was discovered and described. Gammanema papillatum sp. nov. is morphologically most similar to G. lunatum and G. agglutinans, both recorded from New Zealand, by sharing a loop-shaped amphid in males, unlike the multispiral amphid typical of most congeners, and the presence of cuticular spines. It differs from G. lunatum in body length (1122–1366 µm vs. 754–1196 µm), a-ratio (21–23 vs. 13–15), shape of the supplementary organs (papilla-shaped vs. cup-shaped), and distance from the posterior-most supplement to the cloacal opening (58–63 µm vs. 18–32 µm). In terms of precloacal supplementary organ morphology, the new species also resembles Gammanema conicauda, as both are the only congeners with papilla-shaped precloacal supplementary organs. However, G. papillatum sp. nov. differs from G. conicauda by the number of supplementary organs (7–8 vs. 22), amphidial shape (loop-shaped vs. unispiral), and the presence of cuticular spines (absent in G. conicauda). Near full-length SSU and D2–D3 region LSU rDNA sequences were obtained for the new species. Molecular analyses revealed the lowest divergence from G. lunatum (SSU: 1.7%; LSU: 19.8%), with greater divergence from other congeners (SSU: 4.0–4.6%; LSU: 30.5–37.1%). This represents the second record of Gammanema from Korean waters and provides new insights into trait combinations that may help define a subgroup within the genus.