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Parasitic infestation in marine fish requires urgent attention, especially those that infect economically important fishes, which affect their aesthetic quality and palatability. Ectoparasites in Indian Mackerel, Rastrelliger kanagurta (Cuvier, 1817), have not been studied well. Morphological and seasonal study of ectoparasites in R. kanagurta from the Thiruvananthapuram coast was conducted during March-August 2018. The study investigated three parasitic groups: Trichodinids, Digenean cysts (Centrocestus Looss, 1899), and Cymothoids (Norileca indica Milne Edwards, 1840 and Nerocila phaiopleura Bleeker, 1857) from R. kanagurta during the present study. Of the 240 fishes examined, the Trichodinids and digeneans showed 100% prevalence on the gill samples. Seasons had no significant effect on trichodinids and digeneans prevalence. However, parasitic Cmothoids fluctuated significantly according to the season. They showed greater prevalence during the pre-monsoon (45%) and least in monsoon (25%) due to environmental parameters like rainfall, salinity, and temperature. Trichodinids parasitized gills of R. kanagurta showed increased mucus production, paleness in the gills, and multifocal whitish areas. The Cymothoid infested fish showed lesions with the erosion of the epidermis and underlying dermis at the site of attachment. The noticeable changes were observed in the gill epithelium due to the encystment of digeneans. The Trichodinid ciliates and Heterophyid digenean cysts (Centrocestus Looss, 1899) are reported for the first time in R. kanagurta.

Cymothoid isopods are parasitic crustaceans that cause serious impact on marine fish and might lead to fish mortality and consequently, economic losses. Histopathological alterations caused by Anilocra spp. have not been studied well. This study aims to report the histopathological changes caused by Cymothoid, Anilocra leptosoma Bleeker, 1857 in the skin of Bloch's gizzard shad, Nematalosa nasus (Bloch, 1795). Histopathological examination of processed skin tissues showed changes caused by A. leptosoma, such as hyperplasia and erosions of the epidermis associated dermal oedema and muscle degeneration. The host response also included an aggregation of subepithelial dense sheets of hemosiderin-laden macrophages within the dense mixed inflammatory cells. The cymothoid, A. leptosoma are serious parasites of marine fish that can cause severe economic loss in the commercially important fish species. The present study represents the first record of the parasitic cymothoid, A. leptosoma on N. nasus from India.