Select the search type
  • Site
  • Web

Student Project

Scylla serratta

Luke Dalton 2016


Scylla serrata is a large portunid crab found around the Indo-Pacific. Its common names include the giant mud crab, mangrove crab and in some locations the green mud crab. These crabs are found mainly in the intertidal zones but have been found deeper into the mangrove swamps of some regions (Department of Primary Industry New South Wales, 2010). Scylla serrata’s shell varies from a deep mottled green to very dark brown. It is prized as a highly commercial food source in many countries and for this reason has been a focus in recent years for aquaculture purposing (Shelley & Lovatelli, 2011). A short study was done on the species to observe the effect of weight of the species and it relationship between the size and weight of the claw. The results showed that the roughly crab claws have a continuous growth with body size usually making up 40 to 50% of the total body weight of the individual.

Physical Description

Scylla serrata are known as swimmer crabs and are recognized by the back appendages that have been flattened in segmented paddles. The carapace of this crab is smooth and varies from a deep, mottled green to very dark brown (Smithsonian Marine Station, 2007). It diverges from most portunid crabs in the robust nature of its claws as well as having numerous spines on the oral end of it carapace. It is the largest species of the Portunidae, reaching up to sizes of 3 kg with a carapace width of 28 cm. It is more commonly found at 15-20 cm and weighing in around 1 kg.

Figure 1



Scylla serrata are most commonly found with sheltered water in particular, estuaries, tidal flats and mangrove areas. They can tolerate a wide variation of conditions including temperatures between 12-15 degrees Celsius and salinity varying between 2-50 % (Keenan & Blackshaw, 1997). Whilst they can stand these variations, in recent studies it has been shown that their activity and feeding is greatly reduced in temperatures below 20 degrees.  They prefer muddy substrates as this is the easiest location for them to burrow during the day before coming out at night to feed (Department of Primary Industry New South Wales, 2010). Their diet mainly consists of barnacles, mollusc and bivalves but being a known an omnivorous scavenger, it is not uncommon to find them consuming plant material, other crabs and dead fish.

Association Ecology

Whilst having no obligate associations, Scylla serrata is known to play host to cyprid larvae of stalked barnacles usually belonging to the genus Octolasmis (Ihwan, et al., 2014)(see figure 2). These larvae attach to the gill chamber of the crabs though it is not known to the extent of which the can compromise respiratory system. The larvae of these stalked barnacles are also found on other Portunidae such as blue swimmer crabs. Though their relative affect is not known the relationship between the two may be extremely important in future aquaculture programs.
Figure 2

Life History and Behaviour

General Information

The complex life cycle of Scylla serrata can be split in two distinct stages, a dispersing larvae phase, and benthic juveniles and adults. In their larvae form, Scylla serrata are known to be stenohaline, depending on high-salinity conditions to survive (Hill, et al., 1982). The latter stages, their juvenile and adult stages are physiologically well adapted to conditions that typically occur in mangrove habitats including the changing temperatures and salinities. 


Studies suggest that Scylla serrata become reproductively mature starting at around 9 cm carapace width, often within the first year of life (Robertson and Kruger 1994). Males crabs usually approach females before the reach sexual maturity and will carry the female around till the females moults into a reproductive stage (Hyland, et al., 1984). Once this moulting occurs the male will then initiate copulation delivering non-motile spermatozoa that the females may retain for a number of months to fertilize multiple clutches of eggs of up to 2 million eggs. Females of the species then undertake long movements from brackish inshore waters to waters with oceanic conditions for spawning. 

Larval Development

The larvae of Scylla serrata hatch in offshore locations where the undergo their developmental stages. The developmental time and survival of larvae is strongly dependant on water temperature and salinity (Smithsonian Marine Station, 2007). The relatively stable conditions of the open ocean compared to the mangroves is the most likely explanation for female mud crabs spawning in this habitat. The development of the larval stages is usually split between 5 distinctive stages but in all cases a reliance on high salinity is need for survival (Alberts-Hubatsch, et al., 2016). It is not until the juvenile stage after larval stage that crabs show preference for habitat nor gain any benefit of different locations. After molting in the first crab stage, the species has the ability to survive in large salinity ranges. 

Behaviour: Canibalism

Cannibalism is a behaviour very poorly understood about mud crabs. Unlike other crabs, Scylla serrata show no change in their behaviour even around juveniles nor injured members of it population (Wall, et al., 2009). There is a current belief that cannibalism may have a genetic basis and if so may be breed out for aquaculture purposes. However further research is needed to determine if this is possible.

Behaviour: Movement

Majority of the time Scylla serrata have been found to be nocturnal. Their movements mainly consisting of moving from the mangroves during the day, to the reef flats and seagrass beds at night to feed. Combine their nightly activity, with their choice to burrow in sediment and as well at time expose themselves to the open air, these crabs minimise build-up of epibionts on their carapace. The crabs undergo only small migrations within their environment (Hyland, et al., 1984). Only when females are about to release their eggs is there any form of large migration with the species leaving to the open ocean (Alberts-Hubatsch, et al., 2016). As stated earlier during this time the larvae will remain out in the open ocean in a planktotrophic state before returning to the coastline in search of mud substrates once at their first crab stage.

Anatomy and Physiology

External Anatomy

Scylla serrata have two eyes on stalks, much like most crustaceans, at the oral end of it carapace as well as two pairs of antennae. The mouth of Scylla serrata is covered by six layers of paired appendages with the out fish used to locate and catch food items. It is from the exterior that the sexual dimorphism is easily seen in this species with the males be much larger then female counterpart (Department of Primary Industry New South Wales, 2010). The male of the species on their ventral side, have a triangular abdominal flap under which they have a pair of large tubular pleopods used for copulation. This species also has in its possession some of the strongest claws of the Portunidae (Department of Primary Industry New South Wales, 2010). The claws, which are highly prized for their meat content, are what allow this organism the ability to capture and eat larger food items.
Figure 3
Figure 4

Claw Size

As commercial fisheries now look at aqua culturing as a possible future. Research into the possible meat yield of crabs become more and more important. Crabs are usually harvested once large enough to have undergone at least one mating season. At harvest, the crabs can be in a large range of weights, approximately 800 grams to 2-3kg (Keenan & Blackshaw, 1997). Therefore, for a species, like Scylla serrata, which are highly prized for the meat in their claw, it becomes paramount to know the relationship between weight of the crab and the yield or weight of the claw. In the same thought process, the relationship between the size of the crab’s claw in width and the weight of the crabs claw also becomes an important detail in deciding whether a crab claw would contain sufficient meat.

Crabs were collected from Moreton Bay, Queensland using standard crab pots with fish as bait. The crabs were then assessed to determine sex and if female were released back into the water. 20 males were then weighed using a standard scale, claws were weighed in a similar manner with the claw placed as dead weight on scale and then were measured using a ruler to determine width at longest part of the claw (See figure 6). The data was then analysed using linear regression techniques 

The results showed that as weight increased so did weight of the claw (P>0.05) as can be seen in figure 7. The claw weighed in a range of 40 to 50% the body weight of the crab. The crabs weight had a significant effect on the claw size with heavier crabs having larger claws (p>0.05) as can be seen in figure 5.

The claw size and weight were both affected by the weight of the crab. The relationship seems to be linear at the larger sizes indicating that the continuous growth of the crab may leads to both larger claws and weight. This may continue till the crab reaches a large size and may be more commercial valuable to those wishing to use these crabs for aquaculture purposing (Shelley & Lovatelli, 2011). In a similar study, the weight of the claw seemed to be related to weight of the animal in an exponential growth. Further study is need to analyse the trend to test for individuals of smaller and larger size.

Figure 5
Figure 6
Figure 7

Digestive system

The digestive tract of Scylla serrata is the same as any other member of the crustaceans. It is divided into three parts; foregut, midgut and hindgut. The foregut comprises of the mouth oesophagus and stomach. The midgut is known to be a tube with anterior and posterior caecum and midgut gland known as a hepatopancreas (Mantel, 1983). The hindgut from the is a simple straight tube that finishes at the anus. 

Figure 8

Circulatory System

Scylla serrata’s circulatory system has always been classed as an open system like other crustaceans, but some suspect that the circulatory system of many crustaceans may be more complex than previously thought (McGaw, 2005). Like all decapods the heart of crab is single chambered with several arteries leaving it, these arteries feed throughout body of the crab the demand for oxygen (McGaw & Reiber, 2002). The heart is located on the dorsal side toward the posterior of the crab and is feed by a series of ostia. Out of the several arteries that leave the heart, only one artery, known as the descending artery, feeds the appendages. The rest spread out to other important feature of the crab such as the nerve cord and digestive tract.

Nervous system

The central nervous system is composed of a dorsal brain connect to a ventral nerve cord. This brain can be divided in three region known as the tritocerebrum, deuterocerebrum and protocerebrum. The ventral nerve cord is unlike other decapods as the subesophageal, thoraic and abdominal ganglia seem to be fused (Mantel, 1983). This may be due to the reduction of the abdominal segment as well as tail that all but non-existent in the crabs.

Respiratory System

Gas exchange in Scylla serrata, like in other crustaceans, occurs across the surfaces of gills which are found in branchial chamber on the dorsal side of the body. Scaphognathites pump seawater into and through the chamber (Keenan & Blackshaw, 1997). Seawater enters the chamber at the posterior end and over each of the walking legs.  A series of bristles can be seen hanging down from the lower edge of the carapace to stop entry of unwanted organisms and for screening out particulate matter. On the interior these crabs have gills on both sides with epipods of the first maxilliped being specialised as gill cleaner for the crabs. These feature can be seen below in figure 9.

Figure 9

Biogeographic Distribution

Scylla serrata's range exists over the majority of the Indo-Pacific but is restricted mainly to the coastal regions as can be seen below in figure 1. The crabs exist down into southern New South Wales in Australia around to Broome in Western Australia. The are found throughout the Indonesian islands across to China, Japan, India and around to north of South Africa. The species may also be found in sustainable population around Hawaii but is not native to the location. 
Figure 10

Evolution and Systematics


Phylum         Arthropoda
Subphylum Crustacea
Class         Malacostraca
Subclass         Eucarida
Order         Decapoda
Suborder         Reptantia
Infraorder Brachyura
Family         Portunidae
Genus         Scylla
Species         serrata


Scylla serrata is situated within the genus Scylla which has four representatives. The three other being located in similar locations are as follow Scylla olivacea, Scylla paramamosain and Scylla tranquebarica see figure 1. These species belong to the family Portunidae, also known as the swimming crabs all of which belong to the order decapoda (Karasawa & Schweitzer, 2006)

Conservation and Threats

Scylla serrata are under no threat of extinction like many other animals and the only potential towards their survival is overfishing. Scylla serrata are highly prized for their meat yields, even with this being the case the species enjoys a reprieve in overfishing of it wild stock. Thanks mainly to their large distribution and ability to live in a number of conditions around the indo-pacific region the crabs are highly prized aquaculture species. Those caught in the wild are protected by numerous laws which are common-place among countries that intentionally harvest these organisms. For example, in Queensland Australia, a maximum yield of ten individuals, over 15 cm width of the carapace and only males are allowed to be harvested in the wild (Keenan & Blackshaw, 1997). This is common practice among all countries that eat this species and allows for sustainable harvesting.

In turn though, this species could be a detriment to species around it. A number of the species have been released in countries where they had never existed before in an attempt to create commercial and recreational crab fishing hotspots. Scylla serrata were intentionally introduced to Hawaii between 1926 and 1935, with established populations then resulting observed in 1940 (Edmondson and Wilson 1940). Established populations now reportedly occur off of Maui, Hawaii, and Kauai (Eldredge and Smith 2001). In 1962, approximately 30 pairs of Scylla serrata were intentionally released into the Gulf coast of Florida in an effort to establish a commercial crab fishery (Smithsonian Marine Station, 2007). This introduction however failed to lead to an established population and the present status of the species in Florida is currently unknown. No negative large-scale economic impacts have been reported however nor has any significant sign of decline between native species be observed.


Alberts-Hubatsch, H. et al., 2016. Life-history, movement, and habitat use of Scylla serrata (Decapoda, Portunidae): current knowledge and future challenges. Hydrobiologia, 763(1), pp. 5-21.

Department of Primary Industry New South Wales, 2010. Giant Mud Crab (Scylla serrata). [Online]
Available at:
[Accessed 22 May 2016].

Edmondson, . C. & Wilson, I., 1940. The shellfish resources of Hawaii. Sixth Pacific Science Congress, Berkeley: University of California Press.

Eldredge, L. & Smith, C., 2001. Guidebook to the Introduced Marine Species in Hawaiian Waters. Honolulu: Bishop Museum Technical Report 21.

Food and Agriculture Organization of the United Nations, 2016. Aquatic Species Distribution Map Viewer. [Online]
Available at:
[Accessed 23 May 2016].

Hill, B. J., Williams, J. & Dutton, P., 1982. Distribution of Juvenile, subadult and adult Scylla serrata (Crustacea: Portunidae) on tidal flats in Australia. Marine Biology, 69(1), pp. 117-120.

Hyland, S. J., Hill, B. J. & Lee, C. P., 1984. Movement within and between different habitats by the portunid crab Scylla serrata. Marine Biology, 80(1), pp. 57-61.

Ihwan, . M. et al., 2014. Present Distribution of Pedunculate Barnacle of Wild Mud Crab Scylla olivacea from Setiu Wetland, Terengganu Coastal Water, Malaysia. Poultry, Fisheries & Wildlife Sciences, 2(2).

Karasawa, H. & Schweitzer, C. E., 2006. A new classifi cation of the Xanthoidea sensu lato (Crustacea: Decapoda: Brachyura) based on phylogenetic analysis and traditional systematics and evaluation of all fossil Xanthoidea sensu lato. Contributions to Zoology, 75(1-2), pp. 23-73.

Keenan, C. P. & Blackshaw, A., 1997. Mud Crab Aquaculture and Biology: Proceedings of an international scientific forum held in Darwin, Australia,. Darwin, Australian Centre for International Agricultural Research.

Mantel, L. H., 1983. The Biology of Crustacea, Vol. 5. New York: Academic Press.

McGaw, I., 2005. The decapod crustacean circulatory system: a case that is neither open nor closed. Microscopy and Microanalysis, 11(1), pp. 18-36.

McGaw, I. J. & Reiber, C. L., 2002. Cardiovascular system of the blue crab Callinectes sapidus.. Journal of Morphology , 251(1), pp. 1-21.

Shelley, C. & Lovatelli, A., 2011. Mud crab aquaculture, Rome: FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS.

Smithsonian Marine Station, 2007. Scylla serrata. [Online]
Available at:
[Accessed 21 May 2016].

Wall, D., Paterson, B. & Mohan, R., 2009. Behaviour of juvenile mud crabs Scylla serrata in aquaculture: Response to odours of moulting or injured crabs. Applied Animal Behaviour Science, 121(1), pp. 63-73.