LENGTH AND WEIGHT RELATIONSHIP AND GROWTH PERFORMANCE OF Meretrix casta UNDER EXPERIMENTAL CULTURE CONDITION IN THREE DIFFERENT ESTUARINE ENVIRONMENT

. The Meretrix casta is an edible clam for human beings and it is widely distributed around the aquatic ecosystem. The clam has relatively thick shells that can attain a length of three inches. The knowledge of the biology, culture and biochemical composition of any edible organism is extremely important. Though the size, shape and colour of clam are different their nutritive value is very good. A maximum average length of 27.15mm was observed in the M.casta studied during 2011 and a maximum average length of 26.51mm was observed during 2012. The average lengths of M.casta in all the three different estuaries studied were as follows: 27.15 > 25.82 > 25.04 mm in 2011 and 26.51 > 25.88 > 25.17mm in 2012. A maximum average weight of 7.824 and 7.155g was observed during 2011 and in 2012; a maximum weight of 7.528g and minimum of 6.941g was recorded uniformly in all the three different estuaries. However, in 2011and 2012 a maximum and minimum of average length of 26.86, 25.10 mm, and average weight of 7.67 and 7.05g respectively were noticed in both Vellar and Uppanar estuaries of M.casta . the present investigation, it is clearly observed that M.casta can grow a weight of 0.13g/day and length of 0.122mm/day among the three different estuaries studied in the Cuddalore district which is located in Bay of Bengal.


INTRODUCTION
The seafood is a real health food and brain food too for humans. It is also organic in nature. It is easily digestible because it has very little connective tissue. For the above reason, fin fishes and shell fishes are recommended as special diets around the world. A newer species should be recommended for human consumption only after assessing the biology of the species with regard to its nutritional merits.
The Meretrix casta is an edible clam for human beings and it is widely distributed around the aquatic ecosystem. The clam has relatively thick shells that can attain a length of three inches. The knowledge of the biology, culture and biochemical composition of any edible organism is extremely important. Though the size, shape and colour of clam are different their nutritive value is very good. The clam aquaculture is significant and most valuable of the shellfish for aquaculture industry, [1, 2, and 3]. The past two decades have seen a rapid expansion of alternative and innovative methods for the cultivation of clams. However, there is little information on the effects of these practices on the surrounding environment, [4].
The environmental parameters can affect the bivalve population densities and growth of clams. These basic aspects have been studied previously by several researchers, [5, 6, 7, and 9]. The clam meats are having high demand in locally and globally indicating the importance for culture. Increasing clam production to meat of clam and subsequent enhancing captive culture is only way to meet the demand. Aim of the present study is to evaluate the length and weight relationship and growth performance of M.casta in three different (Vellar, Uppanar and Pennaiyar) estuarine environment.

MATERIALS AND METHODS
The seeds of M.casta were collected from the natural clam beds in different sampling sites (brackish water) Vellar, Uppanar and Pennaiyar estuaries along Tamilnadu coast. The seeds were collected (Plate 1 a-d) with an average total length of 20 mm and average total weight of 1.50g in each estuary (Plate 1c-d).
The experimental trials were made for ten month between each month internal from January -October 2011 and January -October 2012. In each sampling site more than thirty individuals of M.casta seeds were collected by manual picking and placed in a plastic basket. Upon collection the seeds from each estuary, the clam seeds were stocked in pre-fabricated nylon cages of 1mm mesh size (100 x 50 x 5 cm). The bottom set cages were stocked evenly in each estuary with 30 numbers / m2. The growth and survival were studied at monthly interval by studying total length and weight of the estuarine clam M.casta.  On the other hand, in 2012 the average length and weight parameters were recorded as 37.5mm and 6.7g in both Vellar and Uppanar estuaries respectively. In 2011 and 2012, a highest average weight of 12.8, 12.3g and a least average weight of 1.02, 0.94 g were recorded with the clam M.casta studied in Vellar and Uppanar estuaries.

RESULTS
The weight ranged from 0.94 and 13.42mm. The maximum length of 13.42mm was recorded in the Vellar estuary during October 2011 and a minimum of 0.94mm was recorded in the Uppanar estuary in the month of January 2012.  In Vellar estuary, the length of M.casta varied from 7.5 and 37.7mm. The maximum length of 37.7mm was recorded in October 2011 and a minimum of 7.5mm was recorded in January 2012. The weight of clam varied from 1.04 and 12.84g. A maximum weight of 12.84g was recorded October 2011 and a minimum of 1.04g was recorded in January 2012.
In Uppanar estuary, the length of M.casta varied from 6.7 and 35.3mm. A maximum length of 35.3mm was recorded in October 2011 and a minimum of 6.7mm was recorded in January 2012. The weights of M.casta were varied from 0.94 and 11.8g (Fig., 4 & Table 2). A maximum weight of 13.42g was recorded in October 2011 and a minimum of 0.94g was recorded in January 2012.  (Fig., 4 ).

DISCUSSION
The yellow clam Meretrix casta forms a significant sustenance -level fishery in all major estuaries of India, [9]. The M. casta is euryhaline and hence can tolerate wide range of salinities and prefers high sand and silt habitat, [10]. The collections of edible clams were dependent on the spat fall and subsequent recruitment of the fishery. Salinity, temperature sediment texture and organic matter contribute to the abundance of bivalves, [11].
The abundance of M. casta is positively correlated to the organic matter content in the substrate, [

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water strongly related to clam densities, [13,14]. The monthly average growths of M. casta were slightly higher in Vellar estuary than Uppanar and Penniyar estuaries. The monthly average growth of 7.5mm was recorded in the Vellar estuary and it was 6.7mm in Uppanar estuary whereas in Penniyar estuary the growth of the clam noticed 7.1mm respectively. The yellow clam M. casta attained a maximum of 37.7mm in Vellar estuary, 35.3mm in Uppanar estuary and 36.4mm in Penniyar estuary in 10 months. However, M. casta was reported to have attained 33.5 mm in the period of 9 months as reported by [15].35.4 mm length in 11 months in Cochin Barmouth estuary, West coast as accounted by [15]. However, [16] was reported slower growth of 0.79 mm per month was reported in Mandapam and this was due to high salinity in the Gulf of Mannar region. [17] reported the growth of 34mm in 13 month in M. casta transplanted to Vellar estuary. [18] Reported 23, 38.30 and 50.6mm growth in the subsequent 1, 2 and 3 rd year of study period in the yellow clam M. casta in the Vellar estuary. These variations in growth are due to the changes in the hydrological conditions in the estuaries as mentioned by [18, 19, and 17]. The weight of clam of Vellar estuary ranged from 1.04 to 13.42g and in Uppanar estuary ranged from 0.94 to 11.8g subsequently in the Penniyar estuary range varied from 0.99 to 12.6g. The weight of clam M.casta found higher in Vellar estuary (13.42g) than in other estuaries namely Uppanar and Penniyar etc., The hydrographical conditions and productivity of the three estuaries have an important bearing on the growth of the clams as pointed out by [1,20]. The condition indexes of the clams from study area were almost same. Nevertheless, the clam M. casta studied in Adyar backwater has been reported to attain a size of over 56.5 mm in a period of three years. Although, in the Adyar backwater more than 95% of the clams are fished before they reach a length of 30 mm [1]. In Korapuzha and Beypore estuaries of West coast, the maximum size of 31 mm was obtained during 1950-1951 [20]. In Bhimunipatnam estuary on the East coast also the same values were recorded. The M. casta of over 20 mm formed only 5% of the fished clams and there over 40 mm were rare, [21]. Similarly in Chaliyar and Kavvai estuaries, clams of over 40 mm were rarely seen. The growth parameters reported for M.casta from five different sites in the Dutch canal of Sri Lanka ranges from 34 to 43.1 mm, [22]. The ranges of growth were M. casta from all study areas are nearly the same and growth was isometric. Isometric growth has been reported in M. casta, [23]. The factors that have enabled this resilience are the high rate of growth, especially during early stages 1 -3 month (growth rate 6.7mm), the abbreviated larval stages, early attainment of sexual maturity, discontinuous breeding throughout the year with at least two peaks and euryhaline tolerance to salinity, [18, 13and 24]. The growth rate of M. casta were estimated in Vellar estuary was higher than that of Uppanar and Penniyar estuaries. Heavy mortality due to heavy flooding in the West coast and poor yield of clam in the Mumbai coast due to excessive rainfall have been reported by [25,26].
However, the major issue of concern is the changes in the habitat or sand bed of the clams. Large scale sand mining also affects the spat settlement due to high levels of total suspended solids, [27]. The yellow clam M. casta is intensely fished from the most estuaries for the meat (for local consumption) and maturation feed for shrimp brood stock development besides as feed in aquaculture practicing ponds. The undersized clams are exploited during the peak season, the meat is allowed to putrefy and shells are used by the lime industry, [26].This affects the livelihood of the fishers, dependent on this important fishery for their sustenance.
It is well known that in bivalves the rate of growth is rapid during early part of life and the growth decreases as the clams advance in age. [1]. Observed faster growth in the early part of the life of M. casta and the specimens attained 20.4mm within two months and 36.6mm within ten months. During the course of the present study, it was observed that the young clams grew to a size of 36.6mm in 10 months, showing a growth rate of 3.8g / month. The salinity was found to influence the growth of marine bivalves at Madras. The factors such as salinity, dissolved oxygen, phosphates, organic contents, silt and nature of the bottom are believed to affect the health and condition of shell-fish, [28, 29 and 30] have reported the age and growth of M. casta and observed a mean growth of 17mm in six months and the growth at the end of one year was found to be about 24mm. [31] observed the size of M. casta to be larger, which ranged between 44 and 55mm with a mean size of 49.44mm.Obviously, from the present investigation, it is clearly observed that M.casta can grow a weight of 0.13g/day and length of 0.122mm/day among the three different estuaries studied in the Cuddalore district which is located in Bay of Bengal.