Uca lactea, MUD-MOUNDS

De, C, 2015, Burrowing and mud-mound building life habits of fiddler crab Uca lactea in the Bay of Bengal coast, India and their geological and geotechnical importance, Palaeontologia Electronica (Oxford, England) 24 (1), pp. 1-22 : 5-11

FORMATION OF UCA LACTEA MUD-MOUNDS

Mode of formation of Uca lactea mud-mounds , their morphological variations with respect to high and low grounds, common association of mangroves and other associated biogenic activities have been closely studied with especial reference to sample area ( Figure 2.1-2 ).

Special Substrate Condition

Uca lactea mud-mounds are produced in a very special substrate condition that exists over a large area in the estuary bank where semi-rigid and fairly dry sand layers (a few cm to 20 cm thick as understood from shallow trenching) cover thick layers (few meters) of soft and water-rich mud ( Figure 1.3 ). This mud represents mudflat facies conspicuously developed along the Bay of Bengal coast, especially around estuary mouth areas ( De, 2009). Mud-mounds are produced when the depth of the mud layers subsurface is well within the penetration limit (about 20 cm) of the crabs as understood from random burrow casting.

Burrowing Process and Mud-Mound Formation

While burrowing vertically down they penetrate through the top sand layers and eventually reach subsurface moist mud layers. At the burrow base, due to lateral seepage of water, mud slur is produced that tends to accumulate within the burrow cavity. The burrowing crab with the help of its larger claw pushes up the mud slur that oozes out of burrow mouth in repeated pulses so as to form a conical mud-mound on the ground around the burrow opening. The burrow base water serves their purpose of frequent gill moistening for survival. Removal of mud slur keeps the tube unblocked and clear for easy dwelling and passage for the burrowing crab. The height as well as radius of the mud cone increase with repeated oozing out of mud slurs ( Figure 5.1-6 ). The position of the burrow opening is gradually raised so as to accommodate growth of mud-mound and keep burrow mouth clearly open on the surface.

A single conical mound with only one central burrow opening ( Figures 3.3 , 4.3 , 5.1 , 6.1 ) is referred to here as a simple mud-mound. Such mounds predominate in the elevated portions of the estuary bank ( Figures 2.1 , 6.1-4 ) and the central burrow is dwelled by a single crab. The outer surface of the cone becomes stepped and knobby ( Figure 5.4-6 ) suggesting repeated pulses of oozing out of mud slur. Mound surface with projected down mud tongues ( Figures 3.6 , 5.4 ) suggests higher water content of the internal mud slur. On the contrary, discrete mud lumps and even viscous mud road ( Figure 3.7-8 ) are formed on mound heads where internal mud is more viscous and lower in water content. A compound mound is composed of a main mound, commonly the highest one, and several (three to five) lower sub-mounds around, each of which has a central open burrow opening ( Figures 3.4-5, 5.5-6, 6.5 -11). The burrow openings are close enough to amalgamate individual mounds into a compound mound ( Figure 6.6 - 10). Compound mounds are formed predominantly in low ground where population density is high ( Figures 2.1 , 6.5 -11) and are dwelled by several individuals of crab. Higher moistness of the subsurface mud layers in low grounds also contribute to the formation of compound mud-mounds.

The transformation of simple to compound mud-mounds as observed in the field is photographically illustrated in Figure 5.1-6 and the overall mechanism of mound formation is schematically explained in Figure 7 . This transformation produces several transitional morphotypes, all having a conspicuous conical shape, made up of mud and a stepped or knobby outer surface with axial burrow tube(s) inside ( Figure 6.1 -12). Burrow casting ( Figure 3.1-2 ) and serial horizontal sectioning ( Figure 3.9 -11) of a mound suggest that the burrow shafts are discrete and not connected to each other subsurface, unlike the Uca marionis compound mud volcanoes reported from nearby area ( De, 2009). Active formation of mounds, simple or compound, is indicative of the presence of a burrowing crab inside. The mound building process changes a flat substrate to micro-mountainous or rugged topography ( Figure 6.1 -12) within a few days in the same way as the ocypodid Heloecius cordiformis ( Milne-Edwards, 1837) burrow-head mounds do in mangrove swamps near Sydney, Australia ( Warren and Underwood, 1986).

Influence of Tidal Action

During high tide the area remains inundated for a few hours and biogenic activity ceases completely. Just before inundation the burrow opening gets plugged by sediment collapse and a tubular air bubble is formed subsurface within which the crab takes shelter. This biophysical mechanism for intertidal quasiterrestrial crab burrowing is well explained by De (2005) from this region. On recession of water during low tide the substrate gets subaerially exposed for several hours, the burrowers come out of burrows and start biogenic activity afresh either by renovating the earlier domicile or by constructing a new one. Field observations (total six times) immediately after high tide suggest that about 5% to 10% of mounds collapse after each high tide forming shallow bowl shaped depressions ( Figure 8.1-2 ), mud lumps ( Figure 3.7 ), rolled-out mud balls ( Figure 8.3 ) and broken mud clasts ( Figure 4.2 ). During low tide new mounds were observed to be formed to compensate the loss of burrowing habitat. The crabs try to maintain a balance between loss of dwelling place during high tide and formation of new burrows during low tide.

The above observations were made during the winter month of December when sea (tidal range between 4 m to 5 m and centimetric wave amplitude in mud-mound region) and climatic (no rainfall, temperature range 19–28 ° C and wind velocity ~ 30 km /hour during day time as per Port Trust of India database) conditions remained fairly normal. Detailed characterization and replication of impacts of tide level due to climatic changes (e.g., during summer, monsoon and pre-monsoon storm events) on the mud-mound-building processes of Uca are important, but constitute altogether a different and much complex and wider aspect of neoichnological study, which is beyond the objectives of the present paper .

Influence of Ground Level

Micro-topographically the study area ( Figure 2.1-2 ) contains distinctive high and low grounds. The measured level difference between high and

PALAEO- ELECTRONICA.ORG low grounds varies between 10 and 20 cm. The low ground represents the base of small runnels or creeks flowing down into estuary water. A quantitative survey on mound heights and types (total 149 numbers; Figure 9 ) across high and low grounds within the sample study area of about 300 sq m ( Figure 2.1 ) reveals that simple mounds dominate over compound mounds (105 vs. 8) in high grounds and vice versa in low grounds (30 compound vs. 6 simple mounds). The average population density of simple mounds in high ground is 2- 3/sq m area, while it is 7-9/sq m area for the compound mounds in low ground ( Figures 2.1 , 6.12 ). Moreover, average vertical height of simple mounds is always less than that of compound mounds irrespective of ground height (11 cm against 14 cm in low ground vs. 4 cm against 7.5 cm in high ground; Figure 9 ). These readings also suggest that height of mounds, simple or compound, is greater in low ground ( Figures 2.1 , 6.12 ). In other words the volume of oozed out mud is more in low ground than that in high ground. This is explained by the fact that the local ground water level drops with receding tidal water level and the subsurface sediment (mud layer in present case) gets dewatered or dried up earlier below high ground than below low ground. This is evidenced by rapid and continuous oozing out of mud slur and consequent growth of compound mounds in low ground during neap tide interval despite cessation of mound building process in high ground ( Figure 7 ). So, the volume of mound is a measure of the subsurface moistness of the mud layer (or formation of mud slur) rather than the amount of vertical excavation. This is contrary to the idea that the volume of the burrow mouth sediment heap is a measure of subsurface burrow volume as in the case of Ocypode burrows in nearby dry sands beach. The size of the mud-mounds, thus, formed was observed to be largest in low ground during neap tides ( Figure 6.5 -10). This is true for the sediment mounds grown around burrow mouths of intertidal benthic crabs in Korean mud flats ( Lee and Koh, 1994).

Influence of Mangrove Vegetation

Uca lactea ecologically belongs to mangrove communities. But, they construct mud-mounds in open substrate having thinly populated mangrove vegetation ( Figure 2.1 ). There may be many reasons for this. Firstly, they avoid subsurface obstruction of root systems for quick burrowing and removal of mud slur. This suggests that vegetation does not provide structural support for their burrows or a refuge from predators. Secondly, these crabs need visual landmarks as well as employ visual signals and hence avoid thickly vegetated habitats that hinder visibility. Thirdly, they avoid living within forest because their waving behavior during mating makes them conspicuous to predators, as in the case with U. marionis in this area ( De, 2009). Effects of vegetation on burrowing differ among different species of Uca ( Nobbs, 2003) . Being Australian intertidal mangrove communities, Uca flammula Crane, 1975 ), Uca signata ( Hess, 1865) and Uca elegans ( George and Jones, 1982) live in forest, edge and clearing habitats, respectively ( Nobbs, 2003). Mud-mounds are absent in open sandy beach surface wherein several species of the crab genus Ocypode , polychaete Diopatra cuprea ( Bosc, 1802) , bivalves and gastropods thrive in large numbers because of the absence of exploitable muddy substrate on or subsurface within the burrowing limit of Uca .

Figures

FIGURE 7. Schematic illustration of Uca lactea mud-mound building process as controlled by ground elevation and tidal recession. HTL stands for high tide level.

FIGURE 9. Vertical height variation curves of simple and compound mud-mounds (total 149 numbers) of Uca lactea in low and high grounds. Curve 1 - compound mounds (30 numbers with average height 14 cm) in low ground; Curve 2 - simple mounds (6 numbers with average height 11 cm) in low ground; Curve 3 - compound mounds (8 numbers with average height 7.5 cm) in high ground and Curve 4 - simple mounds (105 numbers with average height 4 cm) in high ground. Note shorter height of simple mounds relative to compound mounds irrespective of ground height and higher population of simple mounds in high ground and compound mounds in low ground.

FIGURE 2. 1. A field photograph of referred sample area for detailed study showing prolific growth of Uca lactea mudmounds on substrate having varied height, presence of mangrove bushes and spread of living Turritella spp. shells and distal creek. 2. Map prepared from sample study area shows distributions of simple mounds in high ground, compound mounds in low ground, Turritella spp. trails in low ground, mangrove bushes/plants and creek.

FIGURE 1. 1. Regional geological setting of the Bengal Delta Complex with reference to down drift coastal plains incorporating large area covered by modern mangrove forest of Sundarbans, the Bay of Bengal Sea and Quaternary stratigraphy with time-successive palaeocoastlines (modified after Mallick et al., 1972). Note position of studied Bakkhali beach sector. 2. Magnified view of coastal beach sectors with location of study area (marked by red) by the side of a mighty estuary and position (*) of reported Uca marionis mud volcanoes (De, 2009). 3. A field excavation site from study area showing occurrence of soft mud layers below relatively dry and rigid sand layers with sharp contact as specific substrate condition.

FIGURE 5. 1-6. Photographic illustrations arranged in order of gradual morphological transformation of simple (1) to compound mud-mounds (5-6) highlighting some possible morphological variants. Note greenish brown biomat coating on mud-mounds (1 and 5) that are microbially stabilized. The grey coloured mounds are yet to be stabilized.

FIGURE 6. 1. A cluster of predominantly simple mud-mounds produced in high ground. 2-4. Photographs showing field occurrences of predominantly simple mud-mounds in higher ground. 5-11. Photographs showing field occurrences of predominantly compound mud-mounds in low ground. Note greenish brown biomat coating on mud-mounds (6-9 and 11) as a part microbial stabilization process. 12. Photograph illustrating ground elevation-based distribution of simple and compound mounds. (Note size variations, population density variations, surficial features, substrate ruggedness, variation in substrate elevation and different morphotypes of mud-mounds as observed in natural populations.)

FIGURE 3. 1-2. White paraffin wax casts of the Uca lactea burrows in natural position in the field. Note simple unbranched I-shaped burrow morphology with lower bulged portion and narrow apertural neck. 3. Uca lactea simple mud-mound with a central circular burrow aperture. 4-5. Uca lactea compound mud-mounds with multiple circular to elliptical burrow apertures. 6. A compound mud-mound with projected down mud tongues (arrowed) denoting high water content of the mud slur. 7. Mud-mounds with apertural and rolled out mud lumps (arrowed) indicating low water content of the mud slur. 8. Mud-mound with mud rod (arrowed) ejected out of burrow mouth indicating paste-like viscosity of the mud slur. 9-11. Serial horizontal sections of mud-mound showing successive appearance of burrow openings (arrowed): one in top and three in base level showing downward bulging without any sign of convergence.

FIGURE 4. 1. Uca lactea burrow mouth with excavated loose sediment heaves or domes in relatively dry area. 2. Uca lactea mud chimney and surrounding substrate with mud clasts derived from collapsed mud-mounds and sorted out by tidal current, mud cracks and one trace producing crab (arrowed). 3. Ideal simple mud-mound of Uca lactea associated with feeding pellets arranged around burrow openings. 4. Network of Turritella spp. trails as observed after tidal wash out in low grounds. 5. Network of non-branching and radiating worm burrows (finer) in exposed and relatively dry mudflat. 6. Branching worm burrows (thicker) in exposed and relatively dry mudflat.

FIGURE 8. 1-2. Photographic illustrations of collapsed mounds (arrowed; circular to irregular depressions with central burrow opening(s) as observed immediately after high tide. 3. Rolled out mud balls, lumps and clasts derived from collapsed mud-mounds by tidal action.