Description of Sychnotylenchus golestanicus sp. n. (Nematoda: Anguinoidea) from bark beetle-associated decaying wood in northern Iran.

A new species of the genus Sychnotylenchus, S. golestanicus sp. n., is described and illustrated from northern Iran. The new species is characterised by a body length of 595-831 μm, a continuous lip region, a very slender stylet 7-10 μm long with small but distinct knobs, a lateral field with six to eight incisures, a heart-shaped median bulb with a refractive valve, and an excretory pore located at or anterior to the median bulb. The crustaformeria (eggshell-forming zone) consists of 16 cells arranged in 4 longitudinal rows of 4 large, rounded cells each, with the anterior part expanded and functioning as a spermatheca. Females have the vulva situated at 88.5-90.3% of body length, a post-vulval uterine sac measuring 1.4-2.2 times the vulval body diameter, and a short, broad tail with a truncate to slightly flattened terminus, usually showing a shallow terminal depression and lacking a mucro. Males possess smoothly curved spicules 16-21 μm long, a simple gubernaculum approximately one-third of the spicule length, and broad peloderan bursal alae. The new species is morphologically most similar to S. hibisci, but differs from the mentioned species by a combination of characters including the shape of the female tail, the number of lateral field incisures, the structure of the crustaformeria, the length of the post-vulval uterine sac, and the position of the excretory pore. Phylogenetic analysis based on 18S rDNA sequences placed S. golestanicus sp. n. within a well-supported clade (posterior probability = 1.00) comprising S. hibisci and other Sychnotylenchus species, supporting its placement within the genus Sychnotylenchus. The congruence between morphological and molecular evidence supports the recognition of S. golestanicus sp. n. as a distinct species. The present study also highlights the need for additional molecular markers and further integrative taxonomic studies, potentially including TEM investigations, to clarify the taxonomic significance of the crustaformeria and better resolve phylogenetic relationships within the Sychnotylenchinae.