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Journal of Parasitic Diseases: Official Organ of the Indian Society for Parasitology logoLink to Journal of Parasitic Diseases: Official Organ of the Indian Society for Parasitology
. 2021 Sep 12;46(1):280–284. doi: 10.1007/s12639-021-01451-5

Isopod parasite, Nerocila depressa Milne Edwards, 1840 infestation in Thryssa stenosoma Wongratana, 1983, as a first record from Narmada estuary, West Coast of India

Amiya K Sahoo 1, Dibakar Bhakta 1, Dharmendra K Meena 1, Debalina Sadhukhan 1, Tanusri Das 1, Subhas P Kamble 2, Debarghya Majhi 1, Srikanta K Samanta 1, Basanta K Das 1,✉
PMCID: PMC8901821  PMID: 35299927

Abstract

The study reported Thryssa stenosoma, as a new host of isopod Nerocila depressa infestation. Furthermore, the record of Nerocila depressa from the Narmada estuary is of first kind in the West Coast of India. Thryssa stenosoma, commonly known as slender thryssa forms an important commercial fish species at Bhadbhut, a part of the Narmada estuary. The prevalence of N. depressa infestation was 17.39% with a mean intensity of 2.8. in T. stenosoma, showing a low rate of infestation. The present information would form a new addition of isopod infestation to the list of parasitic diseases of estuarine fishes in India.

Keywords: Estuarine fish, Isopod infestation, West Coast, India

Introduction

Fish and fisheries products are recognized as the healthiest animal source of protein on the planet and are being considered in national, regional and global food security towards eliminating hunger and malnutrition. The current global fish production is 179 million ton (mt) of worth 401 billion US$ (FAO 2020). However, fish production from both open waters and aquaculture, face serious threats due to pathogenic diseases. It has been recorded that ectoparasitic isopods inflict serious threats to fishes and crustaceans both in the wild and aquaculture system (Thatcher and Blumenfeldt 2001; Chu et al. 2010). Isopod parasites are common in many commercially important fish species, causing various levels of damages. Among the parasitic isopods in fish, crustacean ectoparasites 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 (Moller and Anders 1986). Cymothoidae, a family of crustacean isopods is found in the marine, freshwater, and brackish water environment and is widely distributed all over the world. Cymothoidae consists of 43 genera that are associated with infestation in many commercially important fishes (Bruce et al. 2019; Aneesh et al. 2020, 2021). Cymothoids are ectoparasites having a long slender body and are easy to identify (Smit et al. 2014). These isopods are generally found in shallow waters because they are less resistant to the water flow (Smit et al. 2014). Nerocila phaeopleura is highly host specific and also lives on a highly specific region of the fish body (Rameshkumar et al. 2013; Trilles et al. 2013; Aneesh et al. 2013). The first record of cymothoids in Indian marine fishes was reported in the year 1783, but still, records of cymothoids infestation in Indian fishes are limited (Trilles et al. 2011). Nerocila is a large genus, including 55 species, (WoRMS 2021) and once infest to a suitable host, starts their parasitic life for feeding on blood and tissues (Pillai 1964; Ravichandran et al. 2010; Rameshkumar et al. 2011; Trilles et al. 2011).

The present study is a part of a regular fish catch composition and diversity study in the estuarine zone of the Narmada river, Gujarat with a major focus at Bhadbhut. The Ministry of Water Resources and Ganga Rejuvenation, Government of India has proposed a mega multipurpose barrage project at Bhadbhut, which will create a freshwater reservoir to cater the water for multiple stakeholders. Narmada estuary is the largest estuary on the West Coast of India with an area of about 30,000 ha supporting livelihood to the millions of fishers. In this background, during our regular fish sample collection, Nerocila depressa infestation was recorded for the first time in Thryssa stenosoma.

Materials and methods

Thryssa stenosoma, was collected from a bag net (10 mm cod-end mesh size) catch composition operated in the estuarine zone of Narmada river, at Bhadbhut (21° 40′ 51.89″ N., 72° 50′ 42.29″ E, Fig. 1) during June 2019. Fish identification was done based on taxonomic key characters (Talwar and Kacker 1984). A total of 46 fish T. stenosoma were collected and examined for the presence of ectoparasites on different parts including skin, gill, and buccal cavity. Ectoparasites were isolated from the skin of the infested fish specimen and fixed in alcohol formalin acetic acid (AFA) as per standard method by Woodland (2006). The total body length and width of the parasite were measured by vernier scale reading. The parasite was dissected for drawing of the mouthparts and appendages using camera lucida attached to the microscope. The parasite morphological identification was made based on the well-reported descriptions (Bowman and Tareen 1983; Bruce and Harrison-Nelson 1988; Trilles et al. 2011; Ravichandran et al. 2019) and the image was captured by using a stereo zoom microscope (Zeiss, Stemi 2000C, Germany). The specimen was submitted to the Zoological Survey of India (ZSI), Kolkata for further confirmation and specimen deposit.

Fig. 1.

Fig. 1

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GIS map showing the sampling area

Results

The morphological character of the isopod was prominent, with two distinct dark strips along the lateral side of the body, which is continued up to the exopods of uropods. The detailed structure is indicated as in Fig. 2A–C. The isopod was about three times longer than width, widest at pereonites 5 and 6, terminal margin of cephalon broadly rounded, pereonites 2–4 are subequal, pereonite 5–7 are longest, posterolateral angle of pereonites 1–6 are not produced, pereonite 7 is produced backward, coxae of pereonites 1–7 are produced in a rounded process, last three are reaching beyond posterior of pereonites, pleonites 1 and 2 are longest, and their ventro-lateral margins are acutely directed posteriorly, pleonites 3–5 are laterally produced and acute, pleotelson is slightly wider than long with a median point, pereiopods 1–7 are gradually longer than each other without any spines, uropodal exopod is 2 times longer than endopod (Fig. 3A–P). Based on these above morphological characters and as described by Bowman and Tareen (1983), Bruce and Harrison-Nelson (1988), Trilles et al. (2011) and Ravichandran et al. (2019), the isopod was identified as Nerocila depressa. Furthermore, the isopod was confirmed by the Crustacea Division, Zoological Survey of India (ZSI), Kolkata, India. One ovigerous female specimen of N. depressa with 23–27 mm in length and 17–19 mm in width was submitted to the National Repository Centre, ZSI, Kolkata. The specimen Registration Number is C 8036/2.

Fig. 2.

Fig. 2

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(A) dorsal, (B) ventral, (C) lateral, view of Nerocila depressa (D) N. depressa attached to caudal peduncle of Thryssa stenosoma

Fig. 3.

Fig. 3

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Line drawing of N. depressa (A) dorsal view (B) pleon (dorsal view) (C) antenna (D) uropods and pleotelson (E–K) pereopods 1–7 (L–P) pleopods 1–5

Based on the different morphological characteristics, the fish was identified as Thryssa stenosoma belonging to family Engraulidae. The fish was found infested with the N. depressa at the base of caudal peduncle and below the lateral line (Fig. 2D). No parasites were reported in the gill or the buccal cavity or in any other parts of the body. A total of 46 fish T. stenosoma, with lengths ranged from 41–196 mm were collected from the bag net catch composition and has been observed for the parasitic infestation. The prevalence of isopod N. depressa was 17.39% with a mean intensity of 2.8 (Table 1).

Table 1.

Prevalence and mean intensity of Nerocila depressa infestation in Thryssa stenosoma

Fish sp. No. of fish examined No of parasitic infested fish Prevalence% No. of parasites collected Mean intensity
Thryssa stenosoma 46 8 17.39 23 2.8
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Discussion

Nerocila depressa was found infested to the Thryssa stenosoma from the estuarine zone of the Narmada river, India indicating as the first report of N. depressa infestation in T. stenosoma as a new host. To date, this isopod is being 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) justifying the marine and estuarine fish species are the major host of infestation with their target tissues. Morphologically N. depressa and N. lovenii are almost the same by many characters. But the key identifying characters of N. depressa are the coxae and the postero-lateral comers of the pleonites which are posteriorly directed and not bent dorsally. Apart from this, the structure of the ventrolateral margins of pleonites 1 and 2 are acute and directed posteriorly, pleotelson margins are curving to a median point.

In all these reported fish species, external body surface particularly at the base of caudal peduncle region; just below the lateral line is the most preffered site of infestation. This preferred site of infestation could be due to better mode of attachment to the muscle and easy feeding. Printrakoon and Purivirojkul (2011) reported that N. depressa was most intensively infested in the upper pectoral fin area of S. albella. 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 infestation site i.e. at base of the caudal peduncle and just below lateral line rticularly as the target site for the attachment. We could not record the isopod infestation in any other body parts i.e. the gill chamber or in the buccal chamber as site of infestation. This could be due to the larger size of the isopod N. depressa as one of the reasons. Furthermore, we could observe these parasites majorly infest fish of average size of 165 mm or more from the collected samples. Though we could not get much sample above the 165 mm size 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 infestation was 17.39% with a mean intensity 2.8. While Printrakoon and Purivirojkul (2011) showed 54% prevalence of N. depressa in S. albella. This underlines the prevalence percentage could vary from fish species to species indicating host suitability depending upon the several factors such as the body shape, movement, and the scale pattern in fishes (Printrakoon and Purivirojkul 2011). Ali and Aboyadak (2018) reported that the intensity of isopod's prevalence depends on the swimming speed of the fish. Therefore, it is assumed that the low rate of prevalence in T. stenosoma as recorded in our study, might be due to faster-swimming behaviour of T. stenosoma than the S. albella, though both are under the same order Clupeiformes. However, the same needs to be investigated further.

Conclusions

The present study recorded for the first time isopod Nerocila depressa infestation in Thryssa stenosoma, collected from the Narmada estuary, West Coast of India. Furthermore, record of isopod N. depressa from the Narmada estuary is also of first information. The prevalence of isopod infestation was 17.39% with a mean intensity 2.8, indicating low rate of prevalence as compared to other fish, Sardinela albella as reported. This scientific information could serve as the baseline record and could add to the new list of parasitic diseases in estuarine fishes of India.

Acknowledgements

The authors are thankful to Mr.Santanu Mitra, Assistant Zoologist, Zoological Society of India, Kolkata for isopod identification and the research scholars, technical and supporting staff of the ICAR-CIFRI, Vododara Center, Gujarat.

Authors contributions

AKS: Design, writing, and final editing, BKD: Supervision and editing DB: Fish sample collection and identification DS&TD: Parasite sample analysis and submission to ZSI DKM&SPK: Parasite collection and identification SS: Coordination.

Declarations

Conflict of interest

The authors have no conflict of interest.

Footnotes

Publisher's Note

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