Abstract
The coastal region of West Bengal is bestowed with a wide range of natural forest and aquatic resources. The estuarine complex is a culmination of the interaction of land, sea and freshwater. It offers diverse specialised habitats such as mangroves, non vegetated mudflats, inter tidal zones and reclaimed areas. Such areas are home to a number of terrestrial, freshwater and marine communities. Edible oyster resources in these regions are Crassostrea gryphoides and Saccostrea cucullata, which are the keystone species found in the intertidal zone and can tolerate huge variation of salinity. These are used as food by local people and marketed to earn cash. The population of this species has been declined due to parasitic infection and pollution. There are many protozoan parasites which infect these oysters causing diseases. During survey period, a protozoan parasite of the genus Cristigera have been observed only from the edible oyster Crassostrea gryphoides collected from Kaikhali and Frasergunj among three selected sites namely Kaikhali, Frasergunj and Digha of West Bengal mainly during monsoon and post-monsoon season. Considering such rare and specific prevalence of Cristigera sp, it may be considered as potential bio-indicator.
Keywords: Edible oyster, Cristigera sp., Protozoan parasites, Bio-indicator
Introduction
In Protozoa, comparatively highly complicated organisms are represented by numerous ciliates. Ciliates are a diverse and successful group of protozoan parasites. The genus Cristigera (Roux 1901) belonging to the phylum Ciliophora, class Ciliatea, order Hymenostomatida, family Cyclidiidae are common in invertebrates of marine and fresh water organisms. Most of the species of the genus Cristigera are free living ciliates excepting Cristigera susmai (Jamadar and Choudhury 1988) and Cristigera crassostrae (Biswas and Bandyopadhyay 2011), which are leading parasitic life. In India, the Cristigera parasites have so far been reported from Crassostrea cuculata collected from the meeting place of the Hooghly river with the Bay of Bengal, Sagar Island, West Bengal, India (Jamadar and Choudhury 1988) and from Crassostrea gryphoides from Kaikhali near Jaynagar-Majilpur block, 24 Parganas (South) West Bengal, India (Biswas and Bandyopadhyay 2011). Species of ciliates are capable of facultative parasitism (Corliss 1961). This paper deals with the incidence of the genus Cristigera from edible oyster Crassostrea gryphoides and Saccostrea cucullata of coastal region, West Bengal which have been bestowed with a wide range of natural forest and aquatic resources. The estuarine complex which is a culmination of the interaction of land, sea and freshwater offers diverse specialised habitats such as mangroves, non-vegetated mudflats, intertidal zones, reclaimed areas which are the home of a number of terrestrial, freshwater and marine communities. A number of rare and endangered species have been recorded in this region. Edible oyster recorded from these region are Crassostrea gryphoides and Saccostrea cucullata which are considered as one of the keystone species, found in the intertidal zone of this region and can tolerate huge variation of salinity. These are used as food by local people and marketed to earn cash. Indeed, there are many protozoan parasites which infect these oysters for causing diseases. Thus population of this species has been declined due to parasitic infection and pollution.
Materials and methods
Host specimens, Crassostrea gryphoides and Saccostrea cucullata were randomly sampled from the Kaikhali, Frasergunj of South 24 Parganas and Digha of East Midnapore district along the coastal region of West Bengal and brought alive or in moribund condition to the laboratory. Oysters were collected seasonally, directly from the reefs of the specified sites during the study period.
In the laboratory the shell of the oyster were opened with fine knife and smears from the body of oyster were scrapped out on glass slides in 0.6 % saline and observed under microscope for examination of parasites. For permanent preparations the smeared slides are semi air-dried, fixed in Schaudinn’s fluid and stained in Heidenhain’s iron alum haematoxylin for general staining. Silver impregnation techniques after Corliss (1953); Klein (1958) were employed to study the infra ciliature and kineties of ciliates. The prevalence was calculated as number of infested oysters divided by number of observed oysters multiplied by hundred.
The temperature of the surface water was determined in situ during sampling using a mercury thermometer. pH is also measured in the field using a portable pH-meter. The bottle filled completely with the seawater sample was brought to the laboratory for measurement of salinity by electrical conductivity meter.
The prevalence of Cristigera infection to the edible oyster of each selected sites was compared to the hydrological parameters like temperature, salinity and pH by Pearson correlation test. Probability less than 0.05 (P < 0.05) was considered as statistically significant.
Results and discussion
During survey period, the protozoan parasite of the genus, Cristigera have been found from the edible oyster (Crassostreagryphoides) collected from Kaikhali and Frasergunj of South 24 Parganas among three selected sites, namely, Kaikhali, Frasergunj and Digha along the coastal region of West Bengal mainly during monsoon and post-monsoon seasons. The genus Cristigera is characterized by a longitudinal, ventral groove in its body. Macronucleus spherical, usually located at the posterior half of the body accompanied by a small micronucleus. Cristigera sp. was isolated from the mantle, gill, and labial palp of the edible oysters, Crassostrea gryphoides only, which was collected from Kaikhali and Frasergunj. No Cristigera sp. was found in Saccostrea cucullata collected from any of the three sites and in any seasons which reflects the species specific nature of Cristigera sp.
The percentages of infection by Cristigera sp. in edible oyster, in three seasons have been incorporated in Table 1. It is evident that prevalence of Cristigera sp. is dependent on season. It has been observed in monsoon and post-monsoon season (June to February). It has also been noticed that Cristigera sp. are totally absent in particular pre-monsoon season, which leads to the conclusion that physiological factors of the host as well as the water quality are responsible for infestation. Though the parasitic infestation was noticed only in the adult oysters, the young oysters are more susceptible to infection; since they are more delicate. The water temperature, pH and salinity are the three important water quality parameters that are responsible for parasitic infestation as they fluctuate more rapidly. The data of these three parameters have been incorporated in Table 2. The correlation between surface water temperature (°C) of three different seasons with the parasitic infestation in Kaikhali and Frasergunj has been depicted in Figs. 1 and 2 respectively. The correlation between mean pH value of three different seasons with the parasitic infestation in Kaikhali and Frasergunj has been presented in Figs. 3 and 4 respectively. The correlation between the salinity (PSU) in three different seasons and the parasitic infestation in the host at Kaikhali and Frasergunj has been incorporated in Figs. 5 and 6 respectively. The Cristigera infection is highest during monsoon when the water quality deteriorates due to decrease of the temperature and Salinity level.
Table 1.
Data showing the prevalence of Cristigera infections of the edible oysters, Crassostrea gryphoides and Saccostrea cucullata
| Host species | Years | Season | Prevalence of infection (%) of Cristigera sp. in Kaikhali | Prevalence of infection (%) of Cristigera sp. in Frasergunj | Prevalence of infection (%) of Cristigera sp. in Digha |
|---|---|---|---|---|---|
| Crassostrea gryphoides | 2009 | Post-monsoon | 0 | 0 | 0 |
| 2010 | Pre-monsoon | 0 | 0 | 0 | |
| 2010 | Monsoon | 8.696 | 0 | 0 | |
| 2010 | Post-monsoon | 10 | 7.692 | 0 | |
| 2011 | Pre-monsoon | 0 | 0 | 0 | |
| 2011 | Monsoon | 11.111 | 11.538 | 0 | |
| 2011 | Post-monsoon | 6.667 | 8.333 | 0 | |
| 2012 | Pre-monsoon | 0 | 0 | 0 | |
| 2012 | Monsoon | 12.5 | 12 | 0 | |
| Saccostrea cucullata | 2009 | Post-monsoon | 0 | 0 | 0 |
| 2010 | Pre-monsoon | 0 | 0 | 0 | |
| 2010 | Monsoon | 0 | 0 | 0 | |
| 2010 | Post-monsoon | 0 | 0 | 0 | |
| 2011 | Pre-monsoon | 0 | 0 | 0 | |
| 2011 | Monsoon | 0 | 0 | 0 | |
| 2011 | Post-monsoon | 0 | 0 | 0 | |
| 2012 | Pre-monsoon | 0 | 0 | 0 | |
| 2012 | Monsoon | 0 | 0 | 0 |
Table 2.
Seasonal variation in water temperature, pH and salinity values of water during November 2009–August 2012
| Sites | Years | Season | Temperature (°C) | pH | Salinity (PSU) |
|---|---|---|---|---|---|
| Kaikhali | 2009 | Post-monsoon | 24.5 | 8.21 | 32 |
| 2010 | Pre-monsoon | 35.2 | 8.3 | 34 | |
| 2010 | Monsoon | 32 | 7.96 | 27.5 | |
| 2010 | Post-monsoon | 23 | 8.14 | 29 | |
| 2011 | Pre-monsoon | 34.8 | 8.28 | 35.56 | |
| 2011 | Monsoon | 34.1 | 7.52 | 28.7 | |
| 2011 | Post-monsoon | 26 | 8.29 | 31.3 | |
| 2012 | Pre-monsoon | 36.6 | 8.34 | 34.8 | |
| 2012 | Monsoon | 33.2 | 8.1 | 27.6 | |
| Average ± SD | 31.044 ± 5.125 | 8.127 ± 0.257 | 31.162 ± 3.126 | ||
| Frasergunj | 2009 | Post-monsoon | 25 | 8. 27 | 34 |
| 2010 | Pre-monsoon | 34.7 | 8. 35 | 35 | |
| 2010 | Monsoon | 31.5 | 7.92 | 32.2 | |
| 2010 | Post-monsoon | 22.8 | 8.3 | 33.23 | |
| 2011 | Pre-monsoon | 35 | 8.32 | 34.97 | |
| 2011 | Monsoon | 33.8 | 8.16 | 32.14 | |
| 2011 | Post-monsoon | 25.2 | 8.25 | 33.32 | |
| 2012 | Pre-monsoon | 35.7 | 8.36 | 35.6 | |
| 2012 | Monsoon | 32.9 | 8.17 | 32.35 | |
| Average ± SD | 30.733 ± 4.996 | 8.211 ± 0.137 | 33.645 ± 1.316 | ||
| Digha | 2009 | Post-monsoon | 27 | 8. 25 | 29.44 |
| 2010 | Pre-monsoon | 30.7 | 8. 3 | 32 | |
| 2010 | Monsoon | 29.5 | 7.85 | 30.2 | |
| 2010 | Post-monsoon | 26 | 8.3 | 32.7 | |
| 2011 | Pre-monsoon | 32 | 8.32 | 32.19 | |
| 2011 | Monsoon | 31.8 | 8.1 | 30.7 | |
| 2011 | Post-monsoon | 27.8 | 8.25 | 32.5 | |
| 2012 | Pre-monsoon | 33.7 | 8.36 | 31.8 | |
| 2012 | Monsoon | 31.4 | 8.17 | 28.15 | |
| Average ± SD | 29.989 ± 2.585 | 8.193 ± 0.157 | 31.075 ± 1.558 | ||
Fig. 1.
Relationship between prevalence of infection (%) of Cristigera sp. and temperature (°C) in Kaikhali
Fig. 2.
Relationship between prevalence of infection (%) of Cristigera sp. and temperature (°C) in Frasergunj
Fig. 3.
Relationship between prevalence of infection (%) of Cristigera sp. and pH in Kaikhali
Fig. 4.
Relationship between prevalence of infection (%) of Cristigera sp. and pH in Frasergunj
Fig. 5.
Relationships between prevalence of infection (%) of Cristigera sp. and salinity (psu) in Kaikhali
Fig. 6.
Relationship between prevalence of infection (%) of Cristigera sp. and salinity (psu) in Frasergunj
The graphs show that the prevalence of Cristigera sp. infection is more dependent on salinity than temperature and pH. Significant negative correlation is obtained between salinity of water and prevalence of Cristigera sp. in the edible oyster Crassostrea gryphoides collected from Kaikhali and Frasergunj (P < 0.05). The prevalence of Cristigera sp. infection shows no such significant correlation with temperature and pH (P > 0.05). Evans (1960) reported Cristigera sp. which was believed to be bacterial feeders from the Great Salt Lake, Utah. Cristigera sp. exhibited optimum growth in salt concentrations of 1–18 %; its growth completely declined at 23 % salt concentration that is higher salinity. Cysts of Cristigera, however, could survive long periods in a saturated salt solution. Evans (1960) also concluded that Cristigera were specialized halophilic protozoa and that certain of the other species of protozoa may be salt-tolerant freshwater forms.
Among three selected sites namely, Kaikhali, Frasergunj and Digha the genus Cristigera have been found only from the edible oyster, Crassostrea gryphoides of Kaikhali and Frasergunj. No parasite infestation was recorded from the oyster such as Crassostrea gryphoides and Saccostrea cucullata collected from Digha. It may be due to thick population of human being and more pollution as rapid alteration and development during recent times, has increased the pollutant discharge which led to the deterioration of the estuarine and related habitats quality. The waste water arising from anthropogenic and industrial activities also have been discharged into estuaries.
In the present study, a consistent general trend appeared that Cristigera infestation is not found throughout all the season. It is common in monsoon and pre-monsoon seasons. This is principally related to the salinity fluctuation of water. Prevalence of Cristigera sp. is negatively correlated with salinity of water. It can be concluded that the water quality has great impact on the abundance of pathogens and their ability to survive on host. The temporal and spatial variation trends of Cristigera sp. infestation in the body of oyster reflects the usefulness of using this species to identify temporal and spatial distribution patterns of salinity of aquatic bodies. Considering such rare and specific prevalence of the Cristigera, this species may be considered as a potential bio-indicator of environment.
Acknowledgments
The first author is thankful to the University of Kalyani for financial support in the form of a research scholarship to carry out this work. Sincere thanks are also due to Dr. Nelendu Jyoti Moitra and Avay Haldar of Ramkrishna Ashram Krishi Vigyan Kendra, Nimpith, South 24 Parganas, WB. for their active cooperation for collection of specimens during the field study.
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