Redescription and Molecular Characterization of the External Attaching Fish Parasitic Cymothoid, Nerocila phaiopleura Bleeker, 1857 (Crustacea: Isopoda) off the Southwest Coast of India.

Purpose: The identification of the external attaching fish parasitic cymothoid, Nerocila phaiopleura Bleeker 1857, is still based on the brief description of Australian specimens provided by Bruce (1987). The present study aimed to provide a redescription and molecular characterisation of Indian specimens of N. phaiopleura.

Materials and methods: Morphological identification was carried out based on microscopic examinations and taxonomic drawings. mitochondrial DNA cox1 was selected as the target gene for sequencing and molecular identification. Nucleotide genetic divergence (p-distance) and base-pair differences among the different species were determined using MEGA11.

Results: Nerocila phaiopleura can be well separated from its congeners by the following combination of characteristics: Body about 2.4 times as long as wide, cephalon broadly rounded anteriorly; coxae posteriorly directed, acute and extending beyond their corresponding pereonite; pereonite 7 posterior angle produced, extending to the pleonite 1; pleonites 1 and 2 ventrolateral process posteriorly directed; uropod exopod straight and elongate, 1.7-2.0 times longer than endopod; uropod endopod lateral margin not serrate, no notch on medial margin; pereopods with short ischium; pleotelson triangular. The p-distance among N. phaiopleura and other available Nerocila spp. ranged from 21 to 19%.

Conclusion: This study represents the first detailed taxonomic redescription of Indian specimens of N. phaiopleura. Key taxonomic features of the life stages and molecular data are provided here to identify the species properly. Interspecific genetic divergence between N. phaiopleura and other Nerocila spp. is assessed for the first time. Studies in cymothoid life histories, genetics, and morphology are necessary to understand one of the least understood parasite families.