Assistant Professor, Christ College, Cuttack, Odisha, India
Scylla serrata and Scylla tranquebarica are economically valuable mud crab species that inhabit estuarine and mangrove ecosystems. Chilika Lake, Asia's largest brackish water lagoon, provides a habitat for these species, supporting biodiversity and local fishing industries. This analysis explored the morphometric features, behavior, and ecological importance of these species in Chilika Lake. Morphometric studies have revealed differences in carapace width, body mass, and claw structure, affecting their growth, reproduction, and predator-prey dynamics. S. serrata has a wider carapace and stronger claws for competitive advantage, while S. tranquebarica features an elongated carapace and lighter claws for mobility. Both species showed territorial behavior and nighttime activity. S. serrata displays more aggressive behaviour, while S. tranquebarica adapts through escape responses and flexible habitat choices. Their feeding strategies varied with seasonal changes in salinity and prey availability. Climate change, excessive fishing, and habitat loss threaten the crab populations. Conservation efforts, including controlled harvesting and habitat restoration, are crucial for the preservation of biodiversity and fisheries. This review emphasizes the importance of these species in the Chilika Lake. Understanding these adaptations can assist in sustainable management and conservation policies. Further research is needed to study the genetic diversity and responses to environmental changes.
1.1 Overview of Chilika Lake
Chilika Lake, situated on India's eastern coastline in Odisha, is Asia's largest brackish water lagoon, covering an area that ranges from 900 to 1,165 square kilometers, depending on the season. This lagoon, shaped like a pear, extends about 64.5 kilometers in length, and its width fluctuates between 5 and 18 km. It traverses the districts of Puri, Khordha, and Ganjam, receiving water from 52 rivers and streams including the Daya River, which empties into the Bay of Bengal. In 1981, Chilika Lake was designated India's inaugural Ramsar site, acknowledging it as a wetland of global significance [1,2]. It is the largest winter refuge for migratory waterfowl on the Indian subcontinent, attracting over a million birds from more than 200 species during peak migration periods. The distinctive brackish environment of the lake fosters a diverse array of life forms, including several rare, vulnerable, and endangered species listed on the IUCN Red List. Economically, Chilika Lake is crucial to the area, providing a livelihood for over 150,000 fishermen living in 132 villages along its banks and islands [3,4]. The lake's rich fishery resources play a significant role in the local economy, making it an essential resource for both biodiversity preservation and sustainable economic development.
1.2 Importance of Mud Crabs in the Ecosystem
Mud crabs (Scylla spp.) play a crucial role in estuarine and mangrove ecosystems and significantly affect both ecological stability and economic livelihoods.
1.3 Ecological Importance
Mud crabs are both important benthic predators and scavengers. Their burrowing actions improve sediment aeration and nutrient cycling, aiding the breakdown of organic material and fostering plant growth. This bioturbation process is vital for sustaining the health of mangroves and estuarine habitats. Furthermore, by feeding on a range of organisms, mud crabs help control species populations, thereby preserving the ecological balance [5,6,7].
1.4 Economic Value
From an economic perspective, mud crabs are highly valued in the fisheries and aquaculture industries. In India, mud crab farming has become a lucrative business because of the strong market demand and export potential of the species. These farming practices are especially advantageous for coastal communities, offering job opportunities and helping to alleviate poverty. The ability of the species to adapt to various farming systems and withstand environmental changes makes it ideal for sustainable aquaculture development [8,9,10,11].
2.1 Taxonomic Classification
The genus Scylla, a member of the Portunidae family, includes four acknowledged species: Scylla serrata, Scylla tranquebarica, Scylla olivacea, and Scylla paramamosain. These species are mainly found throughout the Indo-West Pacific area and have considerable ecological and economic significance, especially in estuarine and mangrove habitats [12, 13]. Scylla serrata, often referred to as the giant mud crab, is noted for its sturdy body and is commonly found in South Africa, Southeast Asia, northern Australia, and Pacific Islands. Scylla tranquebarica, recognized by its slender body shape, is predominantly located from Taiwan to the Malay Archipelago and other Indo-Pacific areas. The taxonomic distinction among these species relies on morphological traits such as the shape of the carapace, spination of the frontal lobe, and coloration. Precise identification is vital for effective management and conservation efforts as well as for aquaculture practices. Understanding the taxonomic differences and distribution patterns of Scylla species is crucial for the sustainable use and preservation of these valuable crustaceans [14,15].
Table1 Comparative summary of the taxonomic classification and distribution of Scylla serrata and Scylla tranquebarica
|
Parameter |
Scylla serrata |
Scylla tranquebarica |
|
Common Name |
Giant Mud Crab |
Slender Mud Crab |
|
Family |
Portunidae |
Portunidae |
|
Genus |
Scylla |
Scylla |
|
Species |
S. serrata |
S. tranquebarica |
|
Carapace Morphology |
Broad and robust |
Elongated and slender |
|
Claw Strength |
Strong and large |
Lighter and more slender |
|
Primary Distribution |
Southeast Asia, Australia, Pacific Islands |
Indo-Pacific: Pakistan, Taiwan, Malaysia |
|
Habitat |
Estuarine, mangrove, coastal lagoons |
Estuarine, mangrove |
|
Behavioral Trait |
Aggressive, territorial |
Adaptive, quick escape response |
|
Economic Importance |
Highly valued in aquaculture |
Moderately valued in local fisheries |
2.2 Global and Local Distribution
Scylla serrata and Scylla tranquebarica are distributed across the Indo-Pacific region in estuarine, mangrove, and coastal habitats. These species occur in India, Sri Lanka, Malaysia, Thailand, Indonesia, Philippines, and northern Australia. Their distribution depends on salinity gradients, sediment type, and the availability of shelter and food in brackish environments [16,17]. In India, Chilika Lake, a brackish water lagoon on the eastern coast, supports wild populations of both the species. Studies have shown that S. serrata occupies the southern and central sectors of Chilika Lake, where salinity is higher due to marine ingress. S. tranquebarica dominates the northern sector, which has lower salinity due to freshwater inflow [18,19]. During the post-monsoon period (October–January), juvenile crab numbers increased due to breeding and larval recruitment. In the pre-monsoon and summer months, adult crabs migrate to deeper saltier waters to avoid freshwater dilution from monsoonal runoff [20]. These movements are related to reproduction, prey availability, and environmental stressors.
2.3 Identification and Differentiation of Scylla serrata and Scylla tranquebarica?
Accurate identification and differentiation of Scylla species are essential for fisheries management, aquaculture, and ecological monitoring. Scylla serrata and Scylla tranquebarica, although closely related, exhibit distinct morphological characteristics that enable field identification, particularly in mixed populations, such as those found in Chilika Lake. Coloration is the primary distinguishing feature. S. serrata typically displays a dark greenish-brown coloration with more uniform carapace shading, while S. tranquebarica exhibits a slightly lighter olive or mottled brown appearance, often with irregular spotting on the carapace and chelae [21]. Frontal lobe spine configuration offers critical taxonomic clues. S. serrata possesses broadly separated frontal lobes, shallow notches, and four distinct spines. In contrast, S. tranquebarica has more pronounced lobes with deeper notches and relatively longer frontal spines, giving the frontal margin a more jagged appearance [16]. The carapace texture and granulation provided additional criteria. S. serrata tends to have a smoother and more evenly curved carapace, while S. tranquebarica features a slightly more granulated texture, particularly along the anterolateral spines. Morphometric ratios, such as carapace width to length and chela size, also differ significantly between the two species, with S. serrata often having a broader and more robust body frame, aiding in its territorial dominance and feeding strength [16, 22].
Table 2: Morphological Differentiation Between S. serrata and S. tranquebarica
|
Feature |
Scylla serrata |
Scylla tranquebarica |
|
Coloration |
Dark green to brown |
Olive to light brown with mottled spots |
|
Frontal Lobe Spines |
Four spines; shallow notches; broader separation |
Four spines; deeper notches; sharper and longer |
|
Carapace Texture |
Smooth, slightly convex |
Granulated with more defined lateral spines |
|
Chelae (Claw) Structure |
Heavier and bulkier |
More slender, elongated |
|
Carapace Width: Length Ratio |
Lower; more compact |
Higher; more elongated |
|
Behavioural Aggression |
More territorial and aggressive |
Less aggressive; relies more on escape mechanisms |
Morphometric Differentiation of Scylla serrata and Scylla tranquebarica in Chilika Lake
Morphometric analyses are essential for distinguishing between closely related mud crab species, particularly Scylla serrata and Scylla tranquebarica, which coexist in Chilika Lake. Key parameters, such as carapace dimensions, abdomen width, chela size, and total body mass, provide insights into their growth patterns and ecological adaptations.
Carapace Dimensions: S. serrata typically attains a larger size, with males exhibiting a greater carapace width (CW) and length (CL) than S. tranquebarica. This size advantage may confer dominance on territorial disputes and mating opportunities.
Abdomen Width (AW): Females of both species have broader abdomens to facilitate egg carrying. However, S. tranquebarica females tend to have a slightly wider AW than their body size, which may influence their reproductive capacity.
Claw (Chela) Size: Males of S. serrata possess more robust chelae, which are advantageous for territorial and mating disputes. In contrast, S. tranquebarica males have comparatively slender claws, which may favor agility over brute strength.
Total Body Mass: The larger carapace dimensions of S. serrata contributed to a higher overall body mass, influencing its dominance in certain ecological niches.
These morphometric distinctions are instrumental for species identification under field conditions, aiding in the monitoring and management of crab populations in Chilika Lake.
Comparative Morphometric Table
|
Parameter |
Scylla serrata |
Scylla tranquebarica |
|
Carapace Color |
Dark green to blue-green |
Olive to purple-green |
|
Frontal Lobe Spines |
Prominent, sharp |
Blunt, moderate |
|
Carapace Texture |
Smooth |
Slightly granular |
|
Male Carapace Width (CW) |
Larger |
Smaller |
|
Female Abdomen Width (AW) |
Narrower |
Wider |
|
Male Claw Size |
Larger, stronger |
Smaller, slender |
|
Growth Pattern |
Allometric |
Isometric |
|
Maturity Size (Females) |
Larger |
Small |
Behavioural Characteristics of Scylla serrata and Scylla tranquebarica in Chilika Lake
The behavioral ecology of Scylla serrata and Scylla tranquebarica in Chilika Lake reveals distinct patterns in activity, territoriality, feeding, and burrowing, influenced by environmental factors and interspecific interactions [23,24,25]. ?
Activity Patterns: Both species exhibited predominantly nocturnal behavior, with increased activity during night-time hours. This nocturnality is believed to be an adaptive strategy to avoid diurnal predators and exploit nocturnal prey availability. Seasonal variations, particularly in salinity and temperature, influence their movement and sheltering behaviours, with crabs seeking deeper or more stable microhabitats during adverse conditions [24].?
Territoriality and Aggression: S. serrata is noted for its aggressive and dominant behavior, often engaging in confrontations to establish territory, especially during mating seasons. In contrast, S. tranquebarica tends to be more evasive, employing rapid escape responses and utilizing a broader range of habitats to avoid direct competition. These behavioral differences are crucial for resource partitioning and coexistence within the same ecosystem [26].
Feeding Behavior: Both species are omnivorous, consuming a diet comprising crustaceans, mollusks, fishes, detritus, and plant material. Seasonal shifts in prey availability driven by monsoonal changes lead to variations in dietary composition. For instance, during the monsoon season, there is an increased intake of crustaceans and mollusks due to their abundance, while detritus and plant matter become more prominent in the diet during other seasons [27]. ?
Burrowing and Shelter Preferences: Burrowing behaviour is integral to both species for protection and thermoregulation. S. serrata prefers fine sand substrates for burrow construction, thus creating deeper and more stable burrows. S. tranquebarica, however, exhibits flexibility in substrate preference, often utilizing a variety of sediment types. The presence of shelters, such as mangrove roots and artificial structures, significantly reduces aggressive encounters and cannibalism, thereby enhancing survival rates [28]. ?
DISCUSSION
Scylla serrata and Scylla tranquebarica are two significant mud crab species inhabiting estuarine and mangrove-rich ecosystems such as Chilika Lake. This study examined morphometric variations and behavioral strategies that enable their coexistence in a shared habitat. Morphometric differences between species, particularly carapace width, claw robustness, and abdominal structure, contribute to niche differentiation. S. serrata, with wider carapace and stronger chelae, shows enhanced competitive ability in agonistic interactions [16,29]. These features support defence, mate competition, and territory acquisition. S. tranquebarica's streamlined body and lighter claws suggest adaptation for mobility and opportunistic feeding, enabling different niche exploitation. Both species exhibit nocturnal activity and burrowing behavior for thermoregulation, predator avoidance, and molting. S. serrata's territorial nature contrasts with S. tranquebarica's evasive behavior, highlighting species-specific behavioral plasticity in adapting to environmental stressors [30]. Both species contribute to nutrient cycling and food web stability in Chilika Lake through their foraging behavior. However, overfishing, climate change, and habitat degradation threaten the population. S. serrata shows declining trends in capture-based fisheries [31], while S. tranquebarica may be vulnerable due to habitat specificity and competition. Sustainable management through harvesting regulations, closed seasons, and aquaculture-based enhancement can mitigate these threats. Research on genetic diversity, reproduction, and environmental tolerance is essential for its conservation. Understanding the traits of these species provides insights for habitat management and sustainable fisheries development.
CONCLUSION
The environmental and economic importance of these two species of Scylla serrata and Scylla tranquebarica in Chilika Lake warrants the development of management plans. Morphometric variation in these species is indicative of evolutionary responses to success in the transient estuarine system. Nevertheless, these regions are increasingly threatened by climate variability, overharvesting, and habitat degradation. Knowing their behavior, habitat preferences, and morphological characteristics represent starting points to apply sustainable fisheries management and conservation measures. Further studies on genetic diversity, reproductive biology, and long-term environmental stability should be conducted to ensure the sustainability of these keystone species and the communities dependent on them in Eastern India.
REFERENCE
Pradipta Pati*, Environmental Challenges and Morphometric Diversity of Mud Crabs in Chilika Lake, Eastern India, Int. J. Sci. R. Tech., 2025, 2 (6), 17-23. https://doi.org/10.5281/zenodo.15566220
10.5281/zenodo.15566220
