Norileca indica (Crustacea: Isopoda, Cymothoidae) Infects Rastrelliger kanagurta Along the Malabar Coast of India - Seasonal Variation in the Prevalence and Aspects of Host-parasite Interactions.

This paper reports seasonal variations in the prevalence of host-parasite associations between Norileca indica, a cymothoid, and the Indian mackerel Rastrelliger kanagurta along the Malabar Coast of India. Eighty-eight marine fish species belonging to diverse families were examined, and only R. kanagurta was shown to be parasitized by N. indica, indicating a narrow host specificity. The prevalence, mean intensity, and abundance were 30.70, 1.71, and 0.52%, respectively. Different life cycle of this parasite, from larva to adult, infect the host fish. In most instances, a pair of N. indica infected the host, and in these instances a male-female combination was the most common (89.17%). The monthly occurrence of N. indica was charted for a period of 38 months (July 2012 to July 2014; March 2017 to March 2018), and statistical comparison of the data showed a significant difference (p < 0.001) among seasons. A positive correlation (r = 0.40) was observed between the size of female parasites and that of their respective host fish. There was a positive correlation (r = 0.78) between the size of female parasites and their fecundity. In all instances, adult N. indica individuals were found to specifically attach to the mucus membrane of branchial operculum, causing visible physical damage, including atrophy (reduced length) of the gill filaments and overall loss of gill normalcy. Furthermore, permanent occupancy by female N. indica resulted in the formation of a deep pit in the gill chamber floor and also caused atrophy of gill filaments. Overall, our findings yielded a greater understanding of the occurrence, season-wise prevalence, and potential host- parasite associations of N. indica.