Morphological variability, ultrastructure and molecular characterisation of Hysterothylacium reliquens (Norris & Overstreet, 1975) (Nematoda: Raphidascarididae) from the oriental sole Brachirus orientalis (Bloch & Schneider) (Pleuronectiformes: Soleidae)

- Specimens of H. reliquens with the broad ranges of morphological variability represents a single species.
- H. reliquens collected from the Indian and Atlantic Oceans possibly represent different geographical populations.
- Some previously unreported morphological features of H. reliquens were revealed by SEM for the first time.

Hysterothylacium reliquens (Norris & Overstreet, 1975), a common ascaridoid nematode parasitic in the marine fishes in the Indian and Atlantic Oceans, has been reported to exhibit remarkable morphological variability in the lengths of body, oesophagus and spicules, and the number and arrangement of postcloacal papillae. In order to determine whether H. reliquens with a broad range of morphological variability in the above-mentioned respects, represents a complex of sibling species or a single species, and to evaluate if the specimens of H. reliquens collected from the Indian and Atlantic Oceans are the same species, numbers of H. reliquens collected from the oriental sole Brachirus orientalis (Bloch & Schneider) (Pleuronectiformes: Soleidae) in the Arabian Gulf (off Iraq) were characterised using molecular approaches by sequencing and analysing the ribosomal [large ribosomal DNA (28S) and internal transcribed spacer (ITS)] and mitochondrial [cytochrome c oxidase subunit 2 (cox2)] target regions, respectively. The results of molecular analysis strongly supported that: (i) the broad ranges of morphological variability in the above-mentioned respects in the newly collected specimens of H. reliquens, should be considered as intraspecific variation and the nematode material collected from B. orientalis in the Arabian Gulf represented a single species; (ii) the specimens of H. reliquens collected from the Indian and Atlantic Oceans belong to the same species, but they possibly represent different geographical populations. Moreover, the detailed morphology of the labial papillae, lateral alae, excretory pore, distal end of spicules, midventral precloacal papilla, phasmids and caudal papillae was revealed by SEM for the first time. The molecular and morphological data of H. reliquens obtained herein contribute to a more accurate diagnosis of this little known ascaridoid nematode.

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