Nerocila depressa was found attached to the skin of Thryssa stenosoma from the estuarine zone of the Narmada river, India. This is the first report on N. depressa infestation in Thryssa stenosoma as a new host and first record from the Narmada estuary, the largest estuary in the west coast of India. Till date, this species is recorded in many fishes species viz. in Opisthopterus turdoore (Bal and Joshi 1959), Sardinella fimbriata (Bruce and Harrison-Nelson 1988)), Sardinella albella (Printrakoon and Purivirojkul 2011), Coilia dussumieri (Aneesh et al. 2013), Sardinella gibbosa (Trilles et al. 2013), Selaroides leptolepis, Carangoides malabaricus (Rameshkumar et al. 2013), Setipinna tenuifilis, Alectic indicus, Netuma bilineata (Kumar et al. 2017), Lepturacanthus pantului, Lagocephalus lunaris (Balakrishnan and Tudu, 2020) as indicated in the Table 2, justifying the marine and estuarine fish species are the major host of infestation and the target tissue. Furthermore, in all these reported fish species external body surface i.e skin was found to be the infestation site indicating the target tissue as skin/muscle, particularly in the caudal peduncle region; just below the lateral line which provide suitable site for attachment and feeding. Printrakoon and Purivirojkul (2011) reported that N. depressa was most intensively infested by the parasite, especially in the upper pectoral fin area of Sardinella albella. Further the authors also reported that the hooks of the pereopods penetrates into the skin and supports for attachment, while the mouthpart helps in exposing the underlaying tissue for feeding. Our study also showed similar result of skin, a target site for the attachment. We could not record any isopods in the gill chamber or in the buccal chamber could be due to the larger size of the parasite. Furthermore, we could observe these parasites majorly infest fish of average size of 165mm or more from the collected samples. Though we could not get much sample above the 165mm to validate statistically, the host size and parasitic infestation, we could observe that larger size group are mostly infested. However, this needs further scientific validation.
In the present study, the prevalence of N. depressa was 17.39% with the mean intensity 2.8. While, Printrakoon and Purivirojkul (2011) showed 54% prevalence of N. depressa in Sardinela albella. This underlines the prevalence percentage could vary from fish species to species indicating host suitability depending upon the several factors including the body shape, movement, and the scale pattern in fishes also significant role in parasitic infestation (Printrakoon & Purivirojkul, 2011). Ali & Aboyadak, (2018) reported that the intensity of isopod's prevalence depends on the swimming speed of the fish. Therefore, it is assumed that the Thryssa stenosoma might have faster swimming behaviour than the Sardinela albella though both are under the same order Clupeiformes.