1 Introduction

The Ocypodidae family possess most familiar amphibious and terrestrial crabs like fiddler crabs and ghost crabs Dotilla fenestrate Hilgendorf, 1869 belonging to family Dottilidae. Dotilla crabs are commonly called solder or bubbler crabs. Dotilla crabs are an ecologically important component of the intertidal zone of tropical. Sometimes Dotilla form dense population and these crabs play an important role in ecology of the faunal community as a result of their burrowing and feeding activity. They adapted sediment condition tidal fluctuation and varying salinities found in the mangroves. The crabs have a significant role in detritus formation nutrients recycling and dynamics of the ecosystem together with many Annelids and nematodes living in the sediments. The Ocypodidae crab have the same role as Sesarmidae and fiddler crabs in term of retention of forest products, large amount of algal primary production, organic matter processing and as ecosystem engineers, change particle size distribution and enhance the soil aeration in the mangrove world (Cannicci et al., 2008; Kristensen et al., 2008). Ghost crabs are common in the world. They are larger than fiddler crabs, which are nocturnal and move to the lower beach to pray on clams and mole crabs or to scavenger for food (Barns, 1987).

The species of the genus inhabits sandy shores where their specialized mouth parts enabling them to sort sand with a high efficiency in order to extract the low proportion of organic material but exceptionally, and incidentally, Dotilla fenestrate Hilgendorf, 1869 occurs, like Mictyris, among the pneumatophores of Avicennia in mangrove swamps (Gherardi and Russo, 2001; Dray and Paula, 1998). Dotilla crabs reduced layer while digging out their burrows, thus promoting their oxidation (Hartnoll, 1973). The substratum, water level, salinity, temperature and floral distribution were possible factors influencing zonation and distribution of crabs in the mangrove environment (Dissanayake and Chandrasekara, 2014).

The number of eggs produced by per female is important for determining the reproductive potential of a species (Mantelatto and Fransozo, 1997). Fecundity and sexual maturity are commonly estimated, since they are important tools in forecasting the turnover capacity of natural populations, providing essential information for studies of environmental impact (Mantelatto and Fransozo 1997; Pinheiro et al., 2003).

The Ocypodid crabs (Dotilla Sp.) are the dominant group inhabitants of tropical sandy of Arabian Sea, Persian Gulf and Pakistan. They remain buried whilst their habitat is submerged, emerging to feed by sorting the organic detritus from the surface layer of the substrate only after the tides have receded. Hashmi (1963) reported ninety-four species of brachyurans in list of marine crabs occurring on the coast of Karachi, Pakistan. Tirmizi and Kazmi (1986) and Tirmizi and Ghani (1986) reported 105 species of crabs. Siddiqui (1989) studied feeding season and fecundity of many species of marine crabs.

Pakistan coast is rich in the anomuran and brachyuran crab. The brachyuran (or true crabs) are prized for food to some extent. Several species of the genus Charybdis and Thalamita belonging to the family Portunidae were reported by Khan (1975) and Mustaquim and Rabbani (1976). Sandy beaches particularly Clifton and Korangi Creek are getting a lot of pollutant (Abbas and Qari, 2010). In Pakistan, the coastal areas of Karachi accommodate well over 60% of the country’s industries. The unregulated industrial and municipal waste inputs into the sea through the Indus, Layari, Malir, Hub and Winder Rivers have increased sharply due to industrialization. Accumulation of heavy metals in the marine soil, sediments, fauna and flora particularly Fe, Mn, Cu, Ni, Zn, Cr, Pb, Co and Cd in the regions of Karachi coast due to pollution from different areas as well as mixing of polluted waters and the effluents discharge from industries and runoff, residential wastes, soil erosion and other factors could also contribute towards the overall metal contamination (Abbas and Qari, 2010; Qari and Siddiqui, 2004; Qari et al., 2005; Qari and Siddiqui, 2008). Pakistan is a developing country and as compared to its population (192 million) food is not much. Therefore as a part of monitoring and conserving species diversity at the community, it is a great need to investigate the population structure, ecology and reproduction of Dotilla blanfordi crab.

2 Materials and Method

2.1 Study area

2.1.1 Clifton

Clifton is one of the long (20 KM) sandy beach of Pakistan, located in between longitude 67^o00´ E and at latitude 24^o46´N (Plate 1). It is most approachable picnic beach of Karachi near Keamari oil terminal. The largest numbers of animals were present in the intertidal zone at Clifton especially during the winter season (Ahmed and Hameed, 1999). It received the sewerage and industrial waste from Layari through harbor (Firdous, F., 2001). Hard texture mica inflamed the sand particles of the shore make it difficult for organism. It was that coastal area surrounding the port of Karachi which had a big environmental disaster last few years (27^th July 2003) (Abbas and Qari, 2010).

Plate 1 Burrowing activity of Ocypodid crabs at Clifton beach

2.1.2 Korangi creek

Korangi Creek (mangrove forest) is situated in the south of Karachi. It is covered by muddy creeks (Plate 2). The north most creeks of Indus delta are Korangi Creek. A Korangi Creek is 12 km from Karachi harbor and 9 km from Quidabad. Korangi Creek is connected at its northeast remain with Phitti Creek and Kadiro Creek while at its sub western end it connects with open sea and with Gizri Creek and is bounded on its sides by extensive mangrove vegetation of Avicenna marina (Saher, 2008). This creek receives effluents from domestic, industrial and oil Refinery. Its domestic, wastes comes from adjoining fishing villages (Waguder, Ibrahim Haydri, and town ship of Korangi). Malir River is another source of domestic and industrial wastes to Creek area. This area receives discharges from Pakistan Refinery, soda ash factory and National Refinery as well as Karachi Electric Supply Corporation Power Plant etc. (Abbas, 2006).

Plate 2 Burrowing activity of Ocypodid crabs at Korangi Creek

2.1.3 Field Work

The samples of Ocypodid crabs D. blanfordi were collected on monthly intervals for the period of March 2007 to May 2008 from supra tidal zone of  Clifton and Korangi Creek (Plate 1 and 2). Sampling of crabs was determined by quadrate method at low tide (Snowden et al., 1991; Suzuki, 1983). Each crab found within quadrate was captured by digging from the sand and mud up to 10cm depth from hand and with the help of forceps.

2.1.4 Laboratory procedures

All crabs were sorted by sieving of sand and mud (0.250 and 1.00 mm mesh size) then washed with tap water and placed first in 5% formalin for 24 hours and then transferred in 70% alcohol. The species and sexes were identified with the help of authentic available literature (Tirmizi and Ghani, 1988; Crane, 1975; Ng et al., 2008).

The sample was also identified for sexes into male and female (ovigerous and non ovigerous). Sex ratio was determined by the criteria of shape and size of the abdominal flaps (Litulo, 2005). The male has cylindrical abdominal flap with first pair of pleopods modified into gonopods and female has semicircular to circular abdominal flap depending on maturation period to which eggs are attached (Qureshi and Saher, 2012). Comparative abundance of ovigerous females was worked out to establish the reproductive period of these crabs.

3 Data Analysis

3.1 Measurement of allometric growth, population structure, abundance and size class

Samples were sorted on the basis of both sexes (male and female). For allometric growth measurements only 100 samples were taken randomLy in each month from the sample collected in ten quadrates from each sites. Carapace length (C.L), Carapace width (C.W), abdominal width (Ab.W), abdominal length (Ab.L) and chelae length (Ch.L) were measured, starting from 0.1mm with the help of centimeter scale under the Nikon binocular microscope. The abdominal width (Ab.W) and abdominal length (Ab.L) were started to measure from 5^th and 6^th abdominal somites. Allometric growth pattern was studied by plotting the C.L, C.W, and Ab.L, Ab.W, and CH.L relationship using the regression method of MINITAB 11.0 software.

After measurements the sample was divided into the separate size class from smallest to largest size and the result was depicted in the form of histogram plotted for showing trend of population structure and size class of each sample.

3.2 Measurement of Sex Ratio

The Chi- squire (X²) test was employed to study of sex ratio of studied crab, where O is the observed value, and E is the expected (1:1) value.

(X²) = ∑ (O-E) ² / E

3.3 Fecundity

For the determination of fecundities of D. blanfordi crab, eggs were counted of different size ovigerous females of the same species. The methodology included the careful removal of eggs from the pleopods of ovigerous females in 150 mL of water contained in a beaker with the help of fine brush. The eggs were carefully separated by tensing out with the help of forceps and needle and then shake well in a beaker in order to obtain their uniform distribution in water column. A counting tray was used for counting eggs. The procedure involved the placing on the counting tray and a 1 mL sample of water placed for the counting eggs. An ordinary 3 mL disposable plastic syringe was used for transferring the aliquots of egg samples to the tray (Siddiqui and Ahmed, 1992). The egg size (µm) of each female sample was measured by the ocular micrometer under the microscope. The relationship between size of ovigerous female (C.W) and their number of eggs was also studied.

4 Results and Discussion

Dotilla blanfordi was recorded throughout the year. Total 3868 individuals of D. blanfordi were recorded from two sites (Clifton and Korangi Creek). There were 2151 individuals of D. blanfordi (1357 male and 794 female individuals) sampled from Clifton whereas 1717 individuals of D. blanfordi were sampled from Korangi Creek (957 male and 766 female individuals). At Clifton the highest number of D. blanfordi were recorded in August (464 m^-2), 332 male individuals m^-2 and 132 female individuals m^-2 whereas at Korangi Creek the highest number of individuals were recorded in February (202 m^-2), 95 m^-2 male individuals and 106 m^-2 female individuals (Figs. 1 and 2 respectively). At Clifton C.W of male individuals ranged from 1.5 to 10 mm (5.67 + 1.71 SD) and female individuals ranged from 2.3 to 8.5 mm (5.46 + 1.06 SD) whereas at Korangi Creek the C.W of male individuals ranged from 2.1 mm to 12.6 mm (6.48 + 2.00 SD) and of female individuals from 2.3 mm to 10.9 mm (5.7160 + 1.0772 SD) (Tables 1 and 2; Figs. 3 - 6).

Figure 1 Seasonal variation in abundance of D. blanfordi at Clifton beach of Karachi coast

Figure 2 Seasonal variation in abundance of D. blanfordi at Korangi Creek of Karachi coast

Table 1 Summary of descriptive statistics (C.L, C.W, Ch.L, Ab.L, and Ab.W) for D. blandfordi at Clifton beach

Table 2 Summary of descriptive statistics (C.L, C.W, Ch.L, Ab.L, and Ab.W) for D. blandfordi at Korangi Creek

The relationship between C.W x C.L of male individuals and C.W x Ab.W of female individuals was positive at both sites Clifton and Korangi Creek (Figs. 7 and 8 respectively). The equation Log C.L = Log - 0.146 + 1.09 Log C.W (P < 0.001) for male individuals and Log Ab.W = Log - 0.351+1.23 Log C.W (P < 0.001) for female individuals in Clifton sample whereas the equation Log C.L = Log – 0.124 + 1.05 Log C.W (P < 0.001) for males and Log Ab.W = Log - 0.107 + 0.902 Log C.W (P < 0.001) for females in Korangi Creek sample (Figs. 7 and 8 respectively).

There was large variation observed in sex ratio of male and female for D. blanfordi at Clifton (1:0.57-1:5.91) and Korangi Creek (1:0.78-1:1.86). The ovigerous females were captured only in a period of March-July 2007 and in February 2008 at both Clifton and Korangi Creek (Figs 9 and 10). Individual fecundity of D. blanfordi ranged from 140 (C.W=4.0 mm) to 583 eggs (C.W=8.5 mm) at Clifton and 145 (C.W=4.2) to 567 eggs (C.W=9.8 mm) at Korangi Creek (Tables 3 and 4; Figures 11 and 12).

Figure 3 Population structure of D. blanfordi (male and female) at Clifton beach (March 2007-August 2007)

Table 3 Variation in carapace width (mm) of non ovigerous and ovigerous females of D. blanfordi at Clifton beach

Table 4 Variation in carapace width (C.W) of non ovigerous and ovigerous females of D. blanfordi at Korangi Creek

Figure 4 Population structure of D. blanfordi (male and female) at Clifton beach (September 2007-February 2008)

Table 5 Seasonal variation in numbers of egg in ovigerous females of D. blanfordi at Clifton beach

Table 6 Seasonal variation in numbers of egg in ovigerous females of D. blanfordi at Korangi Creek

Figure 5 Population structure of D. blanfordi (male and female) at Korangi Creek (March 2007-August 2007)

Figure 6 Population structure of D. blanfordi (male and female) at Korangi Creek (September 2007-February 2008)

Figure 7 Relationship between C.L, C.W. and Ab.W of D. blanfordi from Clifton beach Note: A : Male B : Female

Figure 8 Relationship between C.L, C.W. and Ab.W of D. blanfordi at Korangi Creek. Note: A: Male B: Female

Figure 9 Total number of non ovigerous and ovigerous females of D. blanfordi collected from Clifton beach

Figure 10 Total number of non ovigerous and ovigerous females of D. blanfordi collected from Korangi Creek

Figure 11 Regression of eggs number against carapace width (C.W) in Dotilla blanfordi at Clifton beach

Figure 12 Regression of eggs number against carapace width (C.W) in D. blanfordi at Korangi Creek

The total 18843 number of eggs were counted in ovigerous females of D. blanfordi at Clifton and 32602 at Korangi Creek (Tables 5 and 6). Mean fecundity was 338.1 ± 111.4 eggs / brood at Clifton and 339.12±78.92 eggs/brood at Korangi Creek. The range of egg size was 2.57- 3.5 µm at Clifton beach and the maximum egg size was found in July (3.4-3.5 µm) whereas at Korangi Creek the range of egg size was 2.89- 3.8 µm and the maximum egg size was found in June (3.2-3.8 µm) (Figs. 13 and 14).

Figure 13 Seasonal variation in egg size (µm) of D.blanfordi ovigerous female at Clifton beach

Figure 14 Seasonal variation in egg size (µm) of D.blanfordi ovigerous females at Korangi Creek

The data resulting from present investigation show the high density, distribution and population structure of the D. blanfordi. There was significant variation found in number of individuals, sites throughout the study. There was an apparent temporal variation in the population structure over the sampling period (August) and its appearance was the recruitment of small size crabs.

The morphometric measurements have been used to measure the relative growth in crustaceans (Siddiqui and Ahmed, 1992). During the present study the relationship of carapace length and carapace width for male and female crabs of D. blanfordi is isometric i.e., carapace length is increasing with carapace width but the relationship between carapace length and width is not appropriate to express all biological alternations that occur in crab’s life (Santos et al., 1995).

The present study showed the population of D. blanfordi was present the whole year at Clifton and Korangi Creek. The males were found more abundant as compared to females (Hartnoll, 1973). Small changes in body size can lead to disproportionate increase in the coefficient correlation showing the relationship between carapace length, width and abdominal width is proportional with each other Lavajoo et al., 2012).

In whole study male individuals showed high density as compared to female individuals. High density of male individuals is due to some females dying after breeding. The male individuals were also larger in size as compared to female individuals, which was convenient in reproductive activity (Pravinkumar, 2013; Litulo, 2004; Mouton and Felder 1995). It was noted that the ovigerous females did not occur throughout the year. In present study mostly breeding period occurred from March to July 2007 and February 2008. One previous study described the breeding period from September to March (Prasad and Tampi, 1953). Whereas Rehman (1966) described that the breeding was not limited for few months, and the whole year breeding continued. The presence of ovigerous females in the sample provided an indication of reproductive activity. There were many variations found in numbers, size and colour of egg mass that was due to capturing of ovigerous females at different stages of eggs maturation (Litulo, 2004). The colour of eggs was the sign of maturation. In present study the mature egg mass colour was dark yellow with large numbers of egg in abdominal flip whereas when numbers of egg were small in abdominal flip the colour of egg mass was light yellow.

The results of allometric growth relationship between C.W of ovigerous females and number of eggs were positive in present study are comparable with other studies (Litulo, 2004; Mouton and Felder, 1995). The fecundity of in D. blanfordi crabs found in present study was similar to that studied previously in different areas of the world in their local species (Litulo, 2004; Costa and Soares-Gomes，2009).

It was observed that wet substrate was more favorable for both reproduction and population as compared to dry substrate. Previous studies mention that the environmental factors (temperature, salinity, food availability etc.) might also affect on distribution, density, reproduction and fecundity of macro fauna like fiddler crab Uca annulipes (Pravinkumar et al., 2013; Litulo, 2004) It was tested in the present study at the time of sampling that presence of more sunlight and temperature feeding activity increased with numbers of burrow of crabs, it mean there was positive correlation found in between physical parameters and density and reproduction of crab. It is fact Ocypodid crabs mating in burrow (Henmi et al., 1993).

The present study showed spatial and temporal variation in the occurrence and quantity of crab species. The results showed that both sites Clifton and Korangi Creek were more productive site for the reproduction of D. blanfordi. From the present study it was also concluded that the population of D. blanfordi crabs was affected by both exogenous (temperature, light, food availability) and endogenous (gonad development period, eggs incubation time releasing of larvae and age of attaining sexual maturity). Costa and Soares-Gomes (2009) described in a tropical coastal lagoon, southeast Brazil in their study that the temperature at lagoon seems to favour adequate conditions over all year long to social behavior, feeding, gonadal maturation and egg incubation.

The observations in whole study showed that male individuals showed high density as compared to female individuals. High density of male individuals is due to some females dying after breeding. The male individuals were also larger in size as compared to female individuals, which was convenient in reproductive activity represents the sexual dimorphism. Difference in morphological sexual maturity was observed within sexes also. From the present study it is suggested that Ocypodid crabs can be used as pollution indicators for assessment of environmental health of our coastal areas.

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