Mud crabs genus Scylla (S. serrata, S. tranquebarica, S. olivacea, and S. paramamosain) is an important member of mangrove/estuarine saline water ecosystems than other crustaceans due to its major activities (biological burrowing and bioturbation creation) in protecting and spreading mangrove forests. Mud crabs are generally found in estuaries, especially in mangrove forests of India, Taiwan, Japan, China, South Africa, Indonesia, and the Philippines of Indo-Pacific places. Similarly, Malaysia, Singapore, Western Samoa, Salmon Island, Fiji, and New Caledonia are big mud crabs habitats.
1. Direct Contribution of Mud Crabs into Habitat
With the above economic values, mud crabs also contribute to the mangrove or estuarine water management indirectly. They form a bioturbation structure in sediment soil that helps in trapping the seeds of mangrove plants. It increases the chance of a mangrove forest area, and this has a positive impact on the management of water quality in the area as it leads to a green ecosystem in the area. Mud crabs play a significant role in changing nutrients, increasing mineralization, the oxygen-carrying capacity of the soil, and providing support for other aquatic organisms
[1]. Extended fishing and dependency on natural sites gradually damage the number of crabs and natural habitats for other organisms. The purposive sampling method is generally used to analyze abundance, the frequency distribution of carapace, and the growth parameter of crabs by using FISAT 111 and Bengen statistics. Additionally, the carapace takes 4 and 6 months to mature in males and females, respectively
[2]. Thus, extended fishing of mud crabs on a commercial basis should be avoided in their natural habitat. The exact role in protecting the mangrove ecosystem is quite interesting.
Mud crab plays a key role in balancing ecosystems by using their biological burrowing activity on the soil, making soil porous, laid to aeration, and nutrient flow in soil. They make burrows where the water level is below 100 cm, and the percentage of burrows increases by more than 40% with a lack of shade
[3]. In the natural habitat, the porous soil makes mangrove forest conservation as the soil holds the seeds of the plant (bioturbation), which greatly impacts forest making and coastline protection
[4]. Another dimension of facilitating aquatic life by mud crab is that they produce a large number of pelagic larvae that provides a great source of food for planktophagus aquatic organisms. Thus, from the above data and observation, it has been clear that mud crab plays a vital role in the food web by directly controlling the complex mangrove ecosystems.
The mangrove mud crabs that contribute to world fisheries are under-threat in many places due to varied water physicochemical factors, overfishing, pathogens, heavy metals, and chemical toxicants in water. Along with environmental factors, such as temperature and salinity, the effects of xenobiotics, heavy metals, and other toxicants must be checked in their habitat water and soil for their better growth, production, and reproduction
[5]. Their omnivorous food habits have been experimentally proved, so the larval and adult care of these species under a suitable environment is suggested for their health management. Different behaviors of mud crabs, such as migration, reproduction, and breeding, are exclusively hormonally and environmentally regulated as a function of age
[5]. Finally, mud crabs and their bio-waste are also used for various purposes, such as environmental monitoring, analyzing toxic loads, and in clinical and pharmaceutical sciences, indicating their demands. Therefore, the ecological interaction of these species during their life stages is environmentally important
[5].
2. Role of Habitat Water on Ecology and Life Cycle of Mud Crabs
The lifecycle of mud crabs such as
S. serrata comprises three primary stages: the dispersing larvae phase, the benthic juvenile stage, and the adult stage. In order to mature into adults, mud crabs generally migrate from the seawater to estuaries during their benthic juvenile stage
[6]. Usually, in these stages, they inhabit a muddy mangrove forest with changing temperatures and salinities
[7][8].
S. serrata in Okinawa inhabits marshy mangroves, and in Taiwan and the Philippines, it prefers sandy, muddy bottoms of seaward water
[9]. According to some studies, they prefer varied habitats at various stages of their life cycle, from larvae to adults. Its larvae prefer stenohaline water and structurally complex habitats, which contain both refuge and food, but the seagrass habitat is preferred by crablets of
S. serrata [7]. Extensive studies in this field proved that water physicochemical factors play a huge role in maintaining the variation among these habitats (
Table 1).
Specifically, in India, it is noticed that mud crabs inhabit a variable benthic coastal region of different estuaries with fluctuating several abiotic and biological factors in the water of coastal sites. They can sustain in a varied range of soil sedimental and physio-chemical water parameters, such as pH, organic carbon, turbidity, temperature, and salinity affecting their growth and survivability (
Table 1).
Scylla sp. can thrive well in water temperatures ranging from 18–31 °C, 1–33 ppt of salinity range, alkalinity range from 70 to 119 mg L
−1, and the dissolved oxygen concentration in water fluctuating between 4–10 mg L
−1 [10]. Tidal heights ranging from 8.60 to 72.52 cm are optimum for crab survivability and growth. Additionally, organic matter content in water between 1.91% to 3.25% and a slightly basic pH with an average pH of 7.04 is optimum for
Scylla sp.
[11]. Food availability also plays a major role in their survivability in varied environmental factors and habitats depending on their life cycle.
Table 1. Effect of pH, temperature, and salinity on the physiology of mud crabs.
Water Physicochemical Factors |
Location |
Ranges |
Duration (days) |
Effects on Crab |
Reference |
pH |
Coimbatore, Tamil Nadu, India |
8.2 |
60 days |
Normal growth, feed intake, and survival rate |
[12] |
7.8 |
7.6 |
Decrease in growth rate, survival rate, and feed intake |
7.2 |
7.0 |
Chantaburi, Thailand |
|
|
Hemolymph osmolality (%) |
[13] |
4–6 |
10 days |
11% decrease |
6–12 |
10 days |
15% increase |
Temperature |
|
|
|
Growth rate (%) |
[14] |
Terengganu, Malaysia |
24 °C |
45 days |
7.28 ± 1.31 |
28 °C |
45 days |
9.69 ± 0.75 |
32 °C |
45 days |
7.83 ± 0.56 |
27–30 °C |
45 days |
9.48 ± 1.02 |
Northern Territory of Australia |
20 °C/20 ppt |
1 day |
7.75 ± 1.28 |
[15] |
25 °C/20 ppt |
1 day |
12.68 ± 0.77 |
30 °C/20 ppt |
1 day |
15.98 ± 0.36 |
35 °C/20 ppt |
1 day |
12.59 ± 0.60 |
Salinity |
Queensland, Australia |
|
|
Hemolymph osmolality (mOsm kg−1) |
[16] |
4 ppt |
NA |
415 ± 12 (hyperregulated) |
12 ppt |
312 ± 8 (hyperregulated) |
20 ppt |
194 ± 15 (hyperregulated) |
28 ppt |
122 ± 12 (hyperregulated) |
Iilan, Taiwan |
14 ppt |
1 day |
772.38 (stabilized) |
[17] |
24 ppt |
3 days |
803.50 (stabilized) |
34 ppt |
0 day |
1034.50 (stabilized) |
44 ppt |
1 day |
1274 (stabilized) |
Queensland, Australia |
30 ppt |
4 days |
968.73 ± 8.85 (stabilized) |
[18] |
Odisha, India |
|
|
Mitochondrial respiration rate complex I and II (nmol) |
[19] |
10 ppt |
21 days |
4.42 ± 0.88 and 6.41 ± 1.69 |
17 ppt |
21 day |
1.69 ± 0.41 and 4.04 ± 0.58 |
35 ppt |
21 day |
2.19 ± 0.55 and 4.42 ± 0.88 |
This entry is adapted from the peer-reviewed paper 10.3390/w15112029