Isopod Parasite, Nerocila Depressa (Edwards, 1840) Infestation in Thryssa Stenosoma (Wongratana, 1983) as New Host, and First Record from Narmada Estuary, West Coast of India

The study reported Thryssa stenosoma, as new host for isopod Nerocila depressa, infestation. Furthermore, the report is of rst to record Nerocila depressa from the Narmada, the largest estuary of west coast of India. Thryssa stenosoma forms a commercially important sh species at Bhadbhut, a part of the Narmada estuary. The prevalence of isopod infestation was 17.39% with mean intensity 2.8 in T. stenosoma, indicating the low, at the present ecological conditions. Looking at the future proposed developmental river valley projects at Bhadbhut estuarine zone, our present information would form baseline to the parasitic diseases in estuarine shes in Narmada estuary, India.


Introduction
Fishes are considered to be a global source of animal protein, contributing about 17 % of the total animal protein intake and 7 % of all proteins (Fish site, 2020). Currently the Global sh production is estimated to have reached about 179 million tonnes (FAO, 2020). Generally, ectoparasitic isopods in ict serious threats to shes and crustaceans both in the wild and aquaculture system (Thatcher & Blumenfeldt 2001;Chu et al. 2010). Isopod parasites are common in many commercially important sh species, causing various levels of damages (Sethi 2012). Among the parasitic isopods in sh, crustacean ectoparasite are dominant groups and are around 450 parasitic species known from fresh and marine waters and about 25 % of crustacean parasites belonging to copepod, Brachiura and isopods, are found in the marine environment (Miiller and Anders 1986). Cymothoidae, a family of crustacean isopods is found in the marine, freshwater and brackish water environment and widely distributed all over the world. The present study was a part of regular sh catch composition study in the estuarine zone of the Narmada river, Gujarat with a major focus at Bhadbhut. Narmada estuary is the largest estuary in the west coast of India with an area about 30,000 ha supporting livelihood to the millions shers. The Ministry of Water Resources and Ganga Rejuvenation, Government of India, India has proposed a mega multipurpose barrage project at Bhadbhut, which will create a freshwater reservoir to cater the water need. In this background, during our regular sh sample collection, Nerocila depressa was recorded for the rst time in Thryssa stenosoma.
Thryssa stenosoma, was collected from a bag net (10 mm cod end mesh size) catch composition during June,2019 at estuarine zone of Narmada river, at Bhadbhut (21°40'51.89" N., 72°50'42.29" E, Fig. 1) Fish identi cation was done based on taxonomic key characters (Talwar and Kacker, 1984). A total of 22 sh T. stenosoma were collected and examined for presence of ectoparasites on different parts including skin, gill and buccal cavity. On site, ectoparasites were isolated from the skin of the infested sh specimen and xed in alcohol formalin acetic acid (AFA) as per Woodland (2006) Parasite was about three times longer than width, widest at pereonites 5 and 6, terminal margin of cephalon broadly rounded; pereonites 2-4 subequal, perionite 1 and 5-7 are longest, posterolateral angle of perionites1-6 not produced, perionite 7 produced backward; coxae of pereonites 1-7 produced in a rounded process, last three reaching beyond posterior of pereonites; pleonites 1and 2longest, and their ventrolateral margins are acutely directed posteriorly; pleonites 3-5 laterally produced and acute; pleotelson slightly wider than long, with a median point; pereiopods 1-7 gradually longer than each other, without any spines; uropodal exopod 2 times longer than endopod ( Fig. 3A-P).
Based on the morphological characteristics, the sh was identi ed as Thryssa stenosoma belonging to genus Thryssa, family Engraulidae and order Clupeiformes. The sh was found infested with isopod parasite in the caudal peduncle region; just below the lateral line and facing towards the head of the host in parallel (Fig. 2A). A total of 46 n sh species with length ranged from 41-196 mm were collected from the bag net catch composition and has been observed for parasitic presence. The prevalence of the ectoparasite was 17.39% with mean intensity of 2.8 (Table 1). Interestingly, sh of average 165mm were infested with this ectoparasite. No parasites were reported in the gill or in the buccal cavity.

Discussion
Nerocila depressa was found attached to the skin of Thryssa stenosoma from the estuarine zone of the Narmada river, India. This is the rst report on N. depressa infestation in Thryssa stenosoma as a new host and rst record from the Narmada estuary, the largest estuary in the west coast of India. Till date, this species is recorded in many shes species viz. in Opisthopterus turdoore ( Table 2, justifying the marine and estuarine sh species are the major host of infestation and the target tissue. Furthermore, in all these reported sh 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 n 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 sh 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 scienti c 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 sh species to species indicating host suitability depending upon the several factors including the body shape, movement, and the scale pattern in shes also signi cant 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 sh. 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.

Conclusions
This is a record of new sh host for Nerocila depressa in Thryssa stenosoma. Furthermore, the report of isopod Nerocila depressa is also for the rst time from Narmada estuary, India. The prevalence of infestation was 17.39% with a mean intensity 2.8 indicating low prevalence as compared to other sh Sardinela albella as reported. As there is strong proposal for development of river valley project at Bhadbhut, a part of Narmada estuary, this report could serve as base line information for post development changes that might happen to the Narmada estuary.  Figure 1 GIS map showing the sampling area Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.