The giant freshwater prawn Macrobrachium rosenbergii has a wide distribution throughout the Indo-Pacific region and most favoured for farming in tropical and subtropical areas of the world (New, 2005). In India, its culture was developed few decades ago as an alternative to the Penaeus monodon and to compensate for the substantial losses due to the epidemics of white spot syndrome in penaeid shrimp farming, hypothesizing that M. rosenbergii is resistance to white spot disease (Sahul et al., 2000). This freshwater palaemonid prawn is popularly known as ‘scampi’ in Indian trade, farmed chiefly in small to medium-sized earthen ponds in the states of West Bengal, Andhra Pradesh, Tamil Nadu and Kerala states in India (Nair and Salin, 2012).

In its culture and natural settings, crustaceans serve as hosts  for   a   wide  range  of   protozoan  and  metazoan parasites, some of them cause considerable pathogenicity by affecting the growth and reproductive performance of the hosts (Jayasree et al., 2001). In Asia, the viral, bacterial and parasitic infections of M. rosenbergii were found to be important causes of significant pond production losses (Chu et al., 2011). However, the study on parasites and diseases has received considerable attention on cultivable penaeid prawns and Penaeus monodon in particular, but non-penaied prawns have received less attention in this regard. Viral diseases of the M. rosenbergii were extensively reviewed by Bonami and Widada (2011). Studies on protozoans and metazoans parasites from M. rosenbergii were carried out from various parts of the world (Beck, 1980; Rohde, 1984; Schuldt and Rodrigues-Capitulo, 1985; Areerat, 1988; Nash, 1989; Johnson, 1995; Johnson and Bueno, 2000; Rodriguez et al., 2001; Montoya, 2003) but majorities were focused either on individuals or groups of parasites. In India, several authors reported parasitic occurrences in M. rosenbergii (Santhakumari and Gopalan, 1980; Sankoli et al., 1982; Saha et al., 1988; Shanvas et al., 1989, Jayasree et al., 2001; Mariappan et al., 2003).

In recent years M. rosenbergii is expanding in India due to its relatively fast growth rate, high market price and export value. The total scampi production from India in 2010 to 2011 was about 8778 metric tons and the West Bengal state was the leading producer with a production of 2906 metric tons (MPEDA, 2011). In the year 2011 to 2012, India exported 2723 metric tons M. rosenbergii with an increase of 31.61% in quantity than the previous years (MPEDA, 2011). In view of the facts, information on parasites is essential to prevent any disease outbreak in culture setting and to draw sustainable management plan to avoid any parasitic outbreak. In continuation to the fact, an investigation of the ecto and endo parasites was carried out to benchmark the parasitic occurrences of farmed M. rosenbergii from two major prawn producing districts of West Bengal state in India.

The present study was done during the period of May to July, 2014. Live M. rosenbergii (n = 75) (length: 3.95-8.25 inch; weight: 35.6-102.05 g) was randomly sampled from different freshwater as well as brackish water impoundments of South and North. 24 Parganas districts of West Bengal were brought to the laboratory in oxygenated poythene bags for detail investigations. The water quality parameters of the water bodies like temperature, salinity, dissolved oxygen and pH ranged from 28.5 to 32.5°C, 0 to 5 ppt, 3.5 to 7.5 ppm and 7.8 to 8.5 respectively. The stocking density of the prawn in this traditional farming system was generally 5 to 10 pcs/m². All prawns were dissected, and scrapings from cuticles, appendages, gills, digestive tracts, intestine and hepatopancreas were taken on clean slides with saline solution (0.75% NaCl) and examined (Mondal et al., 2014). The smeared slides were air dried, fixed in acetone free methanol and stained with Giemsa (HiMedia, Mumbai) (Chakraborti and Bandyapadhyay, 2010). Standard methods were employed for fixation and  staining  of  parasites. The prepared slides were examined under Carlzeiss stereo microscope using 10×, 40× and 100× objectives. The identification of parasites was done based on Couch (1983), Kabata (1985), Lightner (1996) and Mehlhorn (2008). After that, the slides were deposited at the Laboratory of Aquaculture Management and Technology, Vidyasagar University, India. 

During the present investigation, a total 14 species of parasites were recorded from different organs of M. rosenbergii (Table 1, Figures 1 and 2). Protozoans belonging to two groups were mainly encountered, namely the epibiontic ciliates and the gut dwelling gregarines (Chakraborti and Bandyapadhyay, 2011).  Among the 7 ciliates, peritrichous ciliates belonging to the genera Zoothamnium, Epistylis and Vorticella were found to predominant in pleopods, uropods, gills and intestine. 1 species of gregarines were found to restrict in intestine. Among the observed protozoan parasites, 4 were ecto-commensals and other 4 were endo-commensals. Among the 6 metazoan parasites, 4 species belonging to phylum nematode was predominant only in digestive tracts.  1 species of cnidarian ecto-commensal and acanthocephalan endo-commensal, respectively were also encountered during the present parasitic survey.

A wide variety of epicommensal have been reported from the gills and external surfaces in postlarval and adult fresh water prawn (Johnson and Bueno, 2000). Ciliate infections, in particular Zoothamnium, Epistylis and Vorticella were recorded earlier from penaeid as well as non-penaeid prawns and from several geographical regimes (Brock, 1983; Colorni, 1985; Overstreet, 1987). In consistent with the present findings, several authors reported peritrichous ciliates from India dealing with their occurrence, pathogenicity and epizootiology (Rajendran et al., 1982; Felix et al., 1994; Nandi and Das, 1995; Rajendran, 1997; Jayasree et al., 2001; Chakraborti and Bandyapadhyay, 2011). The heavy infections of ectosymbionts peritrichous ciliates ultimately causing death to the host was discussed by Overstreet (1973).

The seasonal incidence of Zoothamnium infections and their relation with salinity and rainfall was also discussed by Jayasree et al. (2001). Gut dwelling gregarine parasites are all pathogenic to the decapod crustaceans and may lead to reduced absorption of food or occasional intestinal blockage and possibly mortality of their host (Sprague and Couchi, 1971; Lightner, 1993). The infection rate with gregarinids is high in summer than winter as discussed by Timofeev (2001). In India, Jayasree et al. (2001) reported two species of gregarines from Metapenaeus dobsoni but those were completely absent in M. rosenbergii. Nematode infection recorded rarely from M. rosenbergii (Nash, 1989). It is contacted through foraging on infected copepods that serve as intermediate hosts for these helminths. There is thus every potential for some human helminth infections  to be carried through freshwater prawns as described by Sen-Hai and Kenneth (1994).  

The present study demonstrated that the giant freshwater prawn M. rosenbergii serve as hosts for a wide range of epibionts and parasites. Diseases are the most serious limiting factors in fishery sector and prime cause for chronic mortalities and poor growth which affects yield and marketability of aquatic animals. The floodplain wetlands in West Bengal are mostly eutrophicated and clogged with aquatic vegetation resulting sub-optimal water quality, which ultimately affected the general health condition of the aquatic animals. However, the finding of the present study will serve as a baseline for designing parasite specific extensive study in future. In West Bengal, inland culture and capture based fishery activities were mainly rural based and operated by poor farmers. Developing  better   management  practices  through  pre–stocking pond management,  stocking of specific pathagen free seeds, water quality management , better feed managemnt etc can prevent advaerse impact of diseases and assist poor farmers for sustainable production.

The authors are grateful to local fishermen for allowing them to collect specimens. Authors would like to show gratitude to the staffs of AEM department of West Bengal University of Animal & Fishery Sciences (WBUAFS) and VU for equipment supports. S. K. Dubey is grateful to Prof. R. K. Trivedi for his constant encouragement.

The authors declared that they have no conflicts of interest.