SHORE TO LANDWARD TRANSECT BURROW DIVERSITY OF FIDDLER CRAB IN A TROPICAL INTERTIDAL COAST OF CHITTAGONG IN BANGLADESH Mst

Burrow characters Saltmarsh Mangrove Mangrove pool Burrows indicate the abundance and distribution of fiddler crab in an intertidal coast that varies with structure and morphology within intertidal habitats. We observed fiddler crab burrow density and characters (burrow length, depth, diameter and volume) within randomly selected six 1m quadrate from three intertidal habitats: higher saltmarsh, mangrove pool (a small ditch distributed within mangrove) and mangrove land through field surveys in a coast of Chittagong, Bangladesh. Burrows were observed and counted for density estimation, and burrow characteristics were studied through excavating 10 representative burrows from each quadrate of each habitat. Spearman correlation was used to relate between the distances (from shore towards land) and burrow characters. Transect starting from saltmarsh as base towards mangrove land showed burrow density decreased from shore to higher intertidal habitat. Simultaneously, higher burrow length and diameter were observed landward and contrariwise shoreward. Burrow prevalence in mangrove pools represents fiddler crabs are abundant within land and shore interface presumably due to the dual privilege of easy burrowing and moist condition required for gill ventilation.

Burrows indicate the abundance and distribution of fiddler crab in an intertidal coast that varies with structure and morphology within intertidal habitats.We observed fiddler crab burrow density and characters (burrow length, depth, diameter and volume) within randomly selected six 1m 2 quadrate from three intertidal habitats: higher saltmarsh, mangrove pool (a small ditch distributed within mangrove) and mangrove land through field surveys in a coast of Chittagong, Bangladesh.Burrows were observed and counted for density estimation, and burrow characteristics were studied through excavating 10 representative burrows from each quadrate of each habitat.Spearman correlation was used to relate between the distances (from shore towards land) and burrow characters.Transect starting from saltmarsh as base towards mangrove land showed burrow density decreased from shore to higher intertidal habitat.Simultaneously, higher burrow length and diameter were observed landward and contrariwise shoreward.Burrow prevalence in mangrove pools represents fiddler crabs are abundant within land and shore interface presumably due to the dual privilege of easy burrowing and moist condition required for gill ventilation.

INTRODUCTION
Fiddler crabs of the genus Uca Leach, 1814 are variably distributed over tropical to temperate mudflats, salt marshes, mangrove swamps, and sandy beaches (Crane, 1975;Hodgson, 1987;Peer, 2015).This genus is noteworthy with sexual dimorphism with male having one astoundingly enlarged and one minor cheliped, whereas females possess two equally sized chelipeds (Peer, 2015).About 97 species of Uca were recorded in the tropical and temperate climates throughout the world (Rosenberg, 2001) and by and large they live in the intertidal zone, actively create burrows during low tide as they stay inside their burrows during high tide (Wolfrath, 1992).Burrowing begins when crabs are very small (Herrnkind, 1968) and is influenced by a variety of conditions, such as water availability, food availability, substratum, ground temperature, tidal periodicity, reproductive activity, threat by potential predators, seasons and mate activities (Bertness, 1985;Genoni, 1991).
Burrows are exigent to fiddler crabs that support to adapt in a semi-terrestrial life to get rid of excessive wave action and environmental stresses to relief from harsh ambient temperatures and desiccation (Bertness and Miller, 1984;Lim and Diong, 2003).Burrows provide a shelter and escape route from terrestrial predators during exposed at low tide and from aquatic predators during at high tide, water for physiological processes, sites for molting and reproductive needs (Hyatt and Salmon, 1979;Christy, 1982;Thurman, 1984;Christy, Backwell et al., 2001;and Milner et al., 2010).
Fiddler crab creates burrows in intertidal zone with patchiness in distribution and diversity in burrow morphology.Within intertidal zone, salt marsh and mangrove habitats are the most productive ecosystems in tropical and subtropical regions lying between land and sea (Chowdhury et al., 2011) and these habitats provide shelters for fiddler crabs.By means of burrow creation, fiddler crab work for nutrient recycling and energy flow in coastal ecosystem (Qureshi and Saher, 2012), translocate oxygen into the anoxic layers and promote aerobic respiration, iron reduction and nitrification thus play an important role in biogeochemical cycle of mangrove sediments (Mokhtari et al., 2016).Likewise, they convert organic matter into small sized packs for others organisms and being preyed upon by predators and transfer energy both in terrestrial and marine environment (Skov and Hartnoll, 2001;Tina et al., 2015).
Spatial distribution of fiddler crabs was studied in different coast of the world depends on the availability of vegetation, substratum, food, salinity, tide, and presence of other animals (Icely and Jones, 1978;Nobbs, 2003;César et al., 2005).Despite of huge ecological significance in intertidal habitat and large coastal region, research regarding spatial distribution, burrow characteristics of fiddler crab are scanty and infancy in Bangladesh.Therefore, with a view to understanding burrow distribution pattern in intertidal habitats of Bangladesh, we surveyed a coast of Chittagong for the first time, which will be an effigy of other coasts and will open a horizon for future research.

Sampling area
Sampling took place in a tropical intertidal coast during low tide at South Kattoly sea beach (Figure 01), locally known as Dakshin Kattoly sea beach, Salimpur, Chittagong (Latitude 22 0 35'50" N, Longitude 91 0 75'40" E) along the Bay of Bengal.This coast was planted by mangrove forest under the Asian Development Bank green belt project on 1997-1998 and characterized by a long intertidal salt marsh merged with mudflat and mangroves stretches about 200 m from the shore line.Tides are semi-diurnal with high tide and low tides in every 6 hours interval.
The characteristic of the coast is muddy or sandy coast dominated by saltmarsh Porteresia coarctata (Aysha et al., 2015), and mangrove was dominated by Avicennia spp.Fiddler crab was found in higher marsh but not in lower saltmarsh.A special feature of around 1ft deep irregular shaped ditch (round or oval or Lshaped,), mangrove pool was observed within mangrove land adjacent to salt marsh (Fig. 3A).Pools remain water logged as it receives water during spring tide, but usually get dry when normal tidal water cannot reach.

Sampling method and data collection:
Sampling was followed by transect from saltmarsh towards mangrove by selecting three habitats (saltmarsh, mangrove pool and mangrove) and data were collected from November 2015 to February 2016.As burrow count provide quick and reliable estimate (Warren, 1990), burrow abundance and burrow characteristics (diameter, length, depth, volume and shapes) were determined in three intertidal habitats from shore towards land viz.higher saltmarsh, mangrove pools and mangrove higher intertidal land.In each habitat, we selected six 1 m 2 (1 m ×1 m) quadrates randomly to count burrows visually.But three quadrates in saltmarsh were chosen intentionally along the same line to consider them as base point to measure landward distance of other quadrates.Burrow length, depth and shape were measured with measuring tape after excavating 10 representative burrows from each quadrate using soil corer up to 30 cm depth.Burrow diameter was measured by digital venire calipers.Burrow volume was calculated using the formula (πr 2 h, where π = 3.1416, r = radius, h = length) assuming a cylindrical hole.The distance of each quadrate was measured from the first three quadrates which were situated in saltmarsh.In order to find correlations between distances (shores to landward) and burrow characters, each quadrate distance was measured from the quadrate positioned in salt marsh considering as the base point (0 m) towards mangrove, so that we can understand sea to landward burrow characters.

Data Analysis
Data were checked for homogeneity with Levine's test and for normality with Shapiro-Wilk test.We compared means of burrow characters among three habitats using one-way ANOVA through SPSS version-16.0.Significance was assigned at the 0.05% level.Bivariate Pearson correlation analysis (2-tailed) was used to find correlation between shore to landward distance and burrow characters.

RESULTS
Fiddler crab burrows were diversely distributed among three habitats in the intertidal zone, and the burrow characters differed considerably (Table 1).Considering burrow density, significantly higher (P<0.05)number of burrows (83.67±16.33)was found in mangrove pool and the highest abundance was 115 (Table 1).Within other habitats, relatively higher amount of burrows were observed in saltmarsh areas (76.67±6.67)than in mangrove intertidal high land (42.00±3.79).And, the highest number of burrows was 90 and 49 in those two habitats, respectively.
There was no significant difference among the shelters regarding burrow length and depth.Minimum and maximum length was 26.0 and 58.2 mm in saltmarsh areas, 44.80 and 89.60 mm in mangrove pool, and 55.0 and 112.0 mm in mangrove high land.In these sites, average burrow length was 43.4±9.4,69.5±13.1, and 90.0±17.7 mm, respectively (Table 1).Burrow length increased with increasing closeness to high intertidal land, and reciprocal observation in seawards boundary towards saltmarsh (Figure 3).Likewise, depths of the burrows were recorded lowest 38.0±8.1 mm in saltmarsh, highest 76.7±13.6 mm in mangrove land and 60±10.1 mm in mangrove pool.Similarly, burrow diameter was significantly higher (P<0.01) in mangrove land (24.0±2.1 mm) and mangrove pool (18.9±2.0 mm) than in higher saltmarsh (12.3±0.3 mm).Therefore, burrow lengths, depths and diameters were higher in mangrove higher intertidal land than the other habitats.
Burrows were also observed adjacent to a 2-5 feet water channel with patchy distribution that remains dry in low tide and drain tidal salt water upward during spring tide or downward during low tide.Four distinct shapes of burrows (I, J, C and L shaped) were observed with predominant 'I' shaped burrows in saltmarsh and, I, J and C shaped burrows in mangrove pool and mangrove higher intertidal habitat (Figure 4).Burrows with higher length had the enlarged ground in the bottom (Figure 4B and 4C).

Correlation
A Pearson product-moment correlation coefficient was computed to assess the relationship between shore to landward distances and burrow characteristics.Burrow lengths and burrow diameters were significantly positively correlated with landward distance (r=0.774,p=0.014, and r= 0.811, p= 0.008, respectively).Contrarily, negative relation was found between distance and burrow length (Table 2).Both the burrow length and burrow diameter were negatively related with burrow numbers.

Burrow abundance
Fiddler crab burrows were found in all the three sampling areas viz.saltmarsh, mangrove pool and mangrove highland with variable densities.Burrow distribution showed higher density (nos.) in mangrove pool and in saltmarsh than in mangrove high land (Table 1).Highest number of burrows (115 burrows m -2 ) was found in mangrove pool (Table 1), where soil was relatively wet due to regular tidal splash and water containment, therefore, was soft for burrowing.Similarly, higher burrow density was also found in salt marsh which is relatively unstable area in terms of sedimentation due to regular tidal action.This area was usually occupied with early aged smaller fiddler crabs with higher density.In contrast, low burrow density was found in mangrove high land where soil was harder to make burrows.Mangrove high land is usually dried place inundated only in spring tide or highest high tide.So, fiddler crab distribution showed higher burrow prevalence in moist areas possibly due to the privilege of easy burrowing and wet condition required for gill moisture (Wolfrath, 1992;Lim and Diong, 2003).Soil characteristics (sediment) have important influence regarding burrow creation.Burrow density matches with the observation of Zeil and Hemmi (2006) who found fiddler crab live in large colonies that of all ages and sexes can reach densities of 50-100 individuals per square meter.

Burrow characters and morphology
Burrows in the saltmarsh are characterized by lower depth and smaller diameter.On the contrary, burrow depth and burrow length increased towards mangrove pool and mangrove high land (Figure 3).In mangrove high land, burrow length was higher (90 mm) with significantly higher burrow diameter, but burrow density was lowest among the studied habitats.Qureshi and Saher (2012) reported burrow length 105.4±45.9mm and mean burrow diameter 13.7±3.0mm for U. annulipes, which is similar with the present study.Therefore, with closeness to land, water availability decreased as this area is inundated only in spring tide and the soil become harder to burrow.In contrast, burrow depth increased with decreasing burrow density (Edmond et al., 1996;Qureshi and Saher, 2012;Tina et al., 2015).It reflects larger holes are relatively in highlands towards land (Lim and Diong, 2003) and perhaps burrows were created by larger individuals for breeding purpose (Hyatt and Salmon, 1979).Breeding burrows are relatively large and with hoods (Christy, 1982).Deeper burrows in high shores might be a way of maintaining lower burrow temperatures in areas that are exposed to sunlight for greater periods of time (Wolfrath, 1992;Lim and Diong, 2003).Reciprocally, in salt marsh (lower shore), burrow depth was lower along with significantly smaller burrow diameter which indicate smaller individuals were distributed close to the shore with higher densities.Therefore, these smaller burrows in anoxic sediments are significantly shorter in depths, which would help to improve aeration after every tidal flushing (Lim and Diong, 2003).Similar to mangrove land, mangrove pool was dominated with burrows with higher burrow length and diameter due to availability of water and easy burrowing.Therefore, water and sediment characteristics influence burrow density (Genoni, 1991).

A E D C B
Shape observation revealed Uca produce nearly vertical and straight and unbranched, mostly typical I-and J-shaped burrows (Tina et al., 2015) and few L and C-shaped burrows with lower region enlarged (Montague, 1980).Similar burrow shapes were also observed in vertical and complex branching morphologies with a single and multiple entrances by Qureshi and Saher (2012).Different species of fiddler crab produce simpler L or Jshaped (Katz, 1980;Genoni, 1991, andMontague, 1980) burrow or even U-shaped (Montague 1980) and Yshaped (Qureshi and Saher, 2012) complex burrows.Saltmarsh had the most I and J shaped burrows with simple structure possibly for easy burrowing by smaller fiddler crabs and short time availability within tidal action.Mangrove pool and high land had mostly I, and J-shaped burrows with few L and C-shaped burrows.Complexity of burrow structure were found in mangrove high land with higher burrow length and diameter represents larger fiddlers make deeper burrow in firm soil.

Burrow diversity along transect
Considering transect from salt marsh to mangrove land, burrow density decreased with increasing distance from shore toward mangrove (Edmond et al., 1996).Reversely, burrow length and diameter increased landward, presumably larger individuals create stable burrows for breeding purpose in the higher intertidal zone and smaller individuals dominate in saltmarsh near water area.Moreover, burrows were deeper in drier upper shore land than in moist lower shore sediment (Takeda and Kurihara, 1987;Wolfrath, 1992;Lim and Diong, 2003).Burrow diameter increased with increasing distance and correlation analysis between burrow diameter and landward distance showed strong relationship (R=0.771,y = 0.1157x + 12.06).So, closer to saltmarsh, lower the burrow diameter and reciprocally near the land, higher the burrow diameter.Likewise, borrow length increased with increasing distance from shore showed positive correlation (Table-2).Thurman (1984) also showed that burrows of U. subcylindrica increase in depth with increasing distance from low water mark in Laguna Salado, Mexico.Therefore, shore to landward distance comparison explicit positive relation among distance, burrow length and burrow diameter.Contrarily, correlation between burrow number (density) and distance was negative.Therefore, higher number of burrows in saltmarsh was closer to sea water that decreased toward mangrove land.But, the higher numbers of burrows were abundant in mangrove pools.Pools are characterized by a small ditch contains salt water after being inundated with flood tide.Therefore, fiddler crab burrows were dominated in the margin of salt marsh and mangrove pool.So, water content (Frith and Brunenmeister, 1980; Ewa-Oboho, 1993) salinity and substratum (MacIntosh, 1989;Frusher et al., 1994) of habitat have role in the distribution of fiddler crab burrows.Costa (2000) found positive correlations with water content and distribution of burrows for U. thayeri and Tina et al. (2015) observed burrows were shorter in length, shallower in depth, and smaller in volume at 2 m distance, but these measures increased with increasing distance from the river edge.In fine, it reflects complex interaction of multiple factors play role in burrow structuring, ie.soil texture sediment characteristics, water availability and burrowing individuals size (Thurman, 1987;Costa, 2000).

CONCLUSION
Fiddler crab burrow distribution and characters differed in different habitats of the intertidal coast of Bangladesh with more burrow density in shoreward and contrariwise towards landward.The number of burrows reduced towards higher intertidal zone but burrows of this area had higher burrow depth and diameter which represents water availability, substrate characteristics and size of burrowing individuals determine the burrow characters.Contrarily, smaller and temporary burrows were denser in saltmarsh towards shore.Highest burrows in mangrove pool indicated crabs prefer to stay in moist condition, where water availability support burrowing and gill activity.Fiddler crabs are ecologically important bio-turbator contribute in nutrient replenishment and support for nutrient enrichment in the intertidal habitat.The current study focused on fiddler crab burrows at the scale of 1 m 2 sampling plots.For better understanding of burrow characters, a more detailed study with a finer sampling design and massive burrow observation along with more intensive ecological study is required.

Figure 1 .
Figure 1.Geographical location of the sampling site with map from Google earth and Asiatic Society of Bangladesh

Figure 2 .
Figure 2. Schematic diagram of sampling sites in the saltmarsh, mangrove and mangrove pool.First three quadrates in saltmarsh were intentionally selected along the same line.Circle indicates pools intermingled with mangrove forest and squares (1m 2 ) are randomly chosen quadrates

Figure 3 .
Figure 3.Comparison of Burrow characteristics along transect from saltmarsh (0 m) towards mangrove (every point indicates mean with error bar).Burrow density (blue) and burrow length (red) decreased with increasing distance, but burrow diameter (green) increased with increasing distance

Table 1 .
Burrow characteristics (Mean ± SE) in salt marsh, mangrove pool and mangrove land.The range of observed values is given in the parentheses.Means with the superscripts are significantly different (P<0.05) based on Duncan's test.N=6 quadrates (10 burrows from each)

Table 2 .
Bivariate analysis of Pearson correlation between sea to land ward distance and burrow characters (N=18 quadrates, 10 burrows from each quadrate)