Two Species of the Family Cyatholaimidae (Nematoda: Chromadorida) from Korea

Abstract

During a survey of the Wando-gun rockpool area in Korea, two species belonging to the family Cyatholaimidae were discovered within a sponge, Hymeniacidon sinapium, and are hereby reported. Despite the monophyletic nature of the family Cyatholaimidae, it has been a topic of discussion among nematologists due to its lack of synapomorphic characteristics and overlapping generic diagnoses. Many genus-defining features consist of a combination of non-unique characters, which may not hold significant taxonomic importance. Consequently, most genera within the family are non-monophyletic according to phylogenetic findings. The absence of molecular data complicates the identification of taxonomically important characteristics that may have been previously overlooked. While the pore complex and the lateral pore-like structure have been suggested as potentially important traits, these insights are lacking in most recorded species, along with molecular data to confirm their significance in topology. Given the considerable amount of work required from the ground up, this study provides both morphological information (including taxonomic description, morphometric measurements, tabular key comparing key features to all valid congeners, illustration, and microscope photographs) and molecular data (partial sequences of three molecular regions, K2P distance comparison, and phylogenetic trees) for the two species discovered in Korea. The study also offers insights on potentially significant traits, such as the pore complex and other pore structures. The Korean specimen of Acanthonchus (Seuratiella) tridentatus resembles the original description regarding aspects such as the number and type of precloacal supplements, as well as the shape of the spicules and gubernaculum. However, variations are observed in the following characteristics: body ratio, nerve ring location, pharynx length, and the presence or absence of the oscelli. Our partial molecular sequences of the specimens also match identically with a single existing entry on NCBI identified as A. (S.) tridentatus. The new species, Paracanthonchus spongius sp. nov., is most similar to P. kamui in terms of the general shape of the body and gubernaculum but can be distinguished by differences in the number of precloacal supplements, amphid position, and gubernaculum ratio. The phylogenetic tree constructed based on the 28S rRNA region shows both species grouped within the monophyletic clade of Cyatholaimidae with high bootstrap support. However, achieving generic resolution of these species will require more molecular sequences of congeners to accumulate in the future.