The Discovery and Delimitation of a New Cryptic Species of Spirinia (Nematoda: Desmodoridae) Using SSU and LSU rDNA Divergence

The cosmopolitan nematode Spirinia parasitifera has long been considered a single, morphologically variable species; however, mounting molecular evidence suggests that it represents a complex of cryptic taxa. In this study, we describe Spirinia koreana sp. nov., a new species collected from intertidal sediments of the Republic of Korea. The new species exhibits a high degree of morphological resemblance to both S. antipodea and S. parasitifera, with overlapping ranges in most morphological traits. While certain measurements, such as relatively shorter body length, more slender form (higher a ratio), moderately long tail length, and shorter spicule length differ from those in some described populations, no single morphological character alone reliably separates S. koreana from all previously reported specimens of S. parasitifera or S. antipodea. Nevertheless, molecular evidence from multiple genetic markers clearly supports its distinction as a separate species. Molecular data from mitochondrial COI, 18S rRNA, and 28S rRNA genes confirm the genetic distinctness of the Korean specimens from S. parasitifera and S. antipodea. Notably, S. koreana sp. nov. differs from other Spirinia species by 2.1–3.4% in 18S and up to 34.4% in 28S sequences, surpassing thresholds previously used to delimit marine nematode species. Our results emphasize the value of integrative taxonomy combining fine-scale morphology and multi-marker molecular data to uncover hidden diversity in meiofaunal nematodes.