Species Description: The giant mud crab, also known as the serrated swimming crab, Scylla serrata, is a non-native species in Florida whose current status in the state is uncertain. It is a robust crab belonging to the family of swimming crabs (Portunidae) to which the familiar blue crab, Callinectes sapidus, also belongs.
The carapace has four blunt frontal teeth and each anterolateral margin has nine similarly sized broad teeth. The chelipeds (claws) are robust with several well developed spines, and the rear legs are flattened into swimming appendages, as is typical of members of the portunid family. Individuals are grayish green to purple-brown and variable in color with small irregular white spots on the carapace and swimming legs (Motoh 1979, Perry 2007).
Potentially Misidentified Species: Although Scylla serrata is unlikely to be confused with other portunid crabs found in Florida, taxonomy of the species in its native Indo-Pacific range is confusing, with several genetically distinct Scylla species (i.e., S. serrata, S. tranquebarica, S. oceanica, and S. paramamosain) commonly all being lumped together as S. serrata (Fuseya and Watanabe 1996, Tamaki et al. 2001).
Regional Occurrence: Scylla serrata inhabits muddy bottoms, mangrove marshes, and river mouths in estuarine environments (Motoh 1979). It is native to the Indo-Pacific and has been introduced to Florida, Hawaii, and elsewhere, most often intentionally in attempts to establish populations of this commercially important species.
S. serrata have been reported from south Florida several times, although a 2000 study failed to locate any specimens (Baker et al. 2004). The population status of the species in Florida is currently unknown.
IRL Distribution: Although Scylla serrata occurrence in the IRL has been noted in the literature (e.g., Poss et al. 2000), there is no evidence of established populations currently occurring in the region.
Age, Size, Lifespan: This large crab can exceed 18 cm in carapace width (Stephenson and Campbell 1960, Eldredge and Smith 2001). The FAO Species Identification and Data Programme suggests a maximum male carapace width of 25-28 cm and a maximum weight of 2-3 kg (FAO/SIDP undated).
Abundance: Scylla serrata does not currently appear to occur in appreciable numbers anywhere in Florida.
Reproduction: Studies indicate Scylla serrata become reproductively mature starting at around 90 mm carapace width, often within the first year of life (Robertson and Kruger 1994, Knuckey 1996).
Male crabs approach female crabs before the females have undergone a precopulatory molt, grasping them with their chelipeds and first pair of walking legs and carrying them around for up to several days until the females molt. On molting, males turn the females over and initiate copulation, delivering non-motile spermatozoa that may be retained by the females for up to several weeks to months before being used to fertilize multiple clutches of up to 2 million eggs each (Chen 1976). Hill (1996) notes that females bearing egg masses on their pleopods migrate offshore where the eggs hatch in a few weeks.
Embryology: Fratini and Vannini (2002) report an extended larval duration for Scylla serrata. Experimental work by Nurdaini and Zeng (2007) reveal a mean larval development time to the megalopa stage ranging from 20.6-22.6 days at 25°C, shortened by several days at higher developmental temperatures. These authors also noted 100% larval mortality at 15 ppt salinity and high survival at salinities above 20 ppt. This finding is consistent with observed migration of egg-bearing females to high-salinity offshore waters prior to spawning.
Temperature: Adult and subadult Scylla serrata are broadly eurythermal, while larvae exhibit a somewhat narrower tolerance. Islam and Bhuiyan (1981-82) report an impressive tolerance range of 3-45°C for Scylla serrata in the Karnafully River estuary, Bangladesh. Mwaluma (2002) indicates that successful pen culture of the species could be carried out in waters that ranged seasonally between 25°C and 36°C. Hill (1974), however, notes considerable larval mortality at temperatures above 25°C. The author also reports larval tolerance of temperatures as low as 5°C, although individuals become inactive below 10°C.
Hydrology: Adult Scylla serrata are broadly eurohaline, although individuals other than spawning females preferentially inhabit brackish inshore habitats. Chen and Chia (1996) report specific metabolic responses allowing animals to persist at low salinities (i.e., amino acid catabolism and formation of ammonia to reduce osmolality at 10 ppt) as well as high salinities (i.e., initiation of urea synthesis and moderation of nitrogen excretion at 40 ppt).
Trophic Mode: Scylla serrata is principally a carnivore, preying on small invertebrates such as molluscs, crustaceans, polychaetes, and on small quantities of detritus and plant material (Eldredge and Smith 2001).
Associated Species: No obligate associations with Scylla serrata are known, although infestation of the gill chambers of the crab by cyprid larvae of stalked barnacles of the genus Octolasmis has been documented (Jeffries et al. 1992).
Invasion History: Eldredge and Smith (2001) report a native Indo-Pacific distribution of Scylla serrata as likely encompassing East and South Africa to Tahiti, north to Okinawa, and south to Australia and New Zealand.
In 1962, Approximately 30 pairs of S. serrata were intentionally released to coastal waters in Collier County on the Gulf coast of Florida in an effort to establish a commercial crab fishery (Perry 2006). This introduction failed to lead to an established population and the present status of the species in Florida is currently unknown.
S. serrata was also intentionally introduced to Hawaii between 1926 and 1935, with established populations resulting from these efforts noted by 1940 (Edmondson and Wilson 1940). Established populations now reportedly occur off of Maui, Hawaii, and Kauai (Eldredge and Smith 2001).
Although most initial introductions of S. serrata were intentional releases for the purposes of establishing commercial fisheries, the protracted larval period likely confers high dispersal potential to populations of new recruits (Fratini and Vannini 2002). The species has successfully spread through most of the Indo-Pacific, now occurring in Japan, the Philippines, Indonesia, East and South Africa and the Red Sea (Eldredge and Smith 2001).
Potential to Compete with Natives: Ecological impacts resulting from introduction of Scylla serrata into areas in which the species has become established have yet to be assessed. The animal has been described as an active, aggressive species (Motoh 1979) and some degree of competition with co-occurring native species is likely.
Possible Economic Consequences of Invasion: Scylla serrata is economically important as both a wild-harvested stock and a commercial aquaculture product and it commercially harvested in those areas to which it has been intentionally introduced and where established populations have resulted (Samonte and Agbayani 1992, Perry 2006). Large-scale negative economic impacts resulting from introduction of this species have not been reported.
Edmondson C.H., and I.H. Wilson. 1940. The shellfish resources of Hawaii. Sixth Pacific Science Congress, University of California Press, Berkeley, p 241-243.
Eldredge, L.G. and C. Smith (eds.). 2001. Guidebook to the Introduced Marine Species in Hawaiian Waters. Bishop Museum Technical Report 21. Bishop Museum, Honolulu. FAO Species Identification and Data Programme (FAO/SIDP). Undated. Scylla serrata Species Identification Sheet. Available online.
Chen T.P. 1976. Aquaculture Practices In Taiwan. Fishing News Books Limited, 1 Long Garden Walk, Farnham, Surrey, England. 162 p.
Chen J., and P. Chia. 1996. Hemolymph ammonia and urea and nitrogenous excretions of Scylla serrata at different temperature and salinity levels. Marine Ecology Progress Series 139:119-125.
Fratini S., and M. Vannini. 2002. Genetic differentiation in the mud crab Scylla serrata (Decapoda: Portunidae) within the Indian Ocean. Journal of Experimental Marine Biology and Ecology 272:103-116.
Fuseya R., and S. Watanabe. 1996. Genetic variability in the mud crab genus Scylla (Brachyura: Portunidae). Fisheries Science 62:705-709.
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Hill B.J. 1996. Offshore spawning by the portunid crab Scylla serrata (Crustacea: Decapoda). Marine Biology 120: 379-384. Islam M.J., and A.L. Bhuiyan, A.L. 1981-82.
Temperature tolerance and its impact on the distribution of mud crab in the Karnafully River estuary. Bangladesh Journal of Agriculture 6,7:38-46.
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Knuckey I. A. 1996. Maturity In male mud crabs, Scylla serrata, and the use of mating scars as a functional indicator. Journal Of Crustacean Biology 16: 487-495.
Motoh H. 1979. Edible crustaceans in the Philippines, 11th in a series. Asian Aquaculture 2:5. Mwaluma J. 2002. Pen culture of the mud crab Scylla serrata in Mtwapa Mangrove System, Kenya. Western Indian Ocean Journal of Marine Science 1:127-133.
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Perry H. 2006. Scylla serrata. USGS Nonindigenous Aquatic Species Database, Gainesville, FL. Available online.
Poss S.G.. Aguirre, W., Crochet, N., Nates, S., Hard, S.D., and O'Connell, A.M. 2000. Nonindigeous species in the Gulf of Mexico ecosystem. Gulf Coast Research Laboratory Museum, Univ. Southern Mississippi.
Robertson W. D., and A. Kruger. 1994. Size at maturity, mating and spawning In The portunid crab Scylla Serrata (Forsskål) In Natal, South Africa. Estuarine, Coastal And Shelf Science 39:185-200.
Samonte G.PB., and R.F. Agbayani. 1992. Pond culture of mud crab (Scylla serrata) and economic analysis. SEAFDEC Asian Aquaculture 14:3-5.
Tamaki M., M. Minagawa, T. Hayashibara, M. Sano, K. Fukuoka, E. Quinitio, J. H. Primavera, H. Imai, and K. Numachi. 2001. Identification of four species of Scylla spp. by pcr-rflp analysis and species composition of Scylla juveniles in Ishigaki Island. Workshop on Mud Crab Culture, Ecology and Fisheries, Can Tho University, Vietnam, 8-10th January, 2001.