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FISH FACTS: Mud crab migrations

HAS anyone stopped to ponder the life of a mud crab? These delectable creatures can be encountered in mangrove estuaries throughout Australia’s tropical north from Exmouth Gulf in Western Australia to as far south as the Bega River in NSW. At least four closely related species of swimming mangrove crabs occur throughout the Indo-Pacific region, but in Australia we only have two species, namely the giant mud crab (Scylla serrata) or muddie, which occurs along the east coast and as far west as the NT, and the orange (or brown) mud crab (S. olivacea) which occurs in northern WA and across the top end as far east as the Gulf of Carpentaria. The orange mud crab can be identified by its orange/reddish colour (compared to the greenish brown of the giant mud crab).

The giant mud crab is the largest of the swimming crabs, growing to around 28 cm shell width and nearly 3 kg. Male crabs (bucks) tend to grow larger than females (jennies), the latter which can be easily identified by their broad abdominal flap and smaller claws. Adult muddies can live in either the intertidal or subtidal zones, usually inside mud burrows or buried in the mud, whilst juveniles and crablets tend to live mainly in the shallower intertidal zone under rocks and other benthic shelters which provide them with protection from predatory fish, other crabs and birds. In contrast, adult mud crabs have relatively few predators, although they may be eaten by crocodiles, sharks, and large estuary cod, and they are a known favourite diet of the Queensland (Giant) grouper and even cobia.

Their large size, aggressive nature and very large powerful claws makes adult muddies formidable omnivorous scavengers. They are highly cannibalistic, eating other crabs as well as their usual diet of barnacles, gastropods, bivalves and dead fish. Studies of giant mud crabs have found around half of the material in their guts were molluscs, 20-22% crustaceans, and the remaining 28-30% consisting of plant material (mangrove leaves being a favourite) and debris.

Orange crab, left, with a giant crab on the right.

Despite their large size, muddies are fast growing and short lived. Aquaculture study of mud crabs has found that moulting occurs every week when they are crablet size (1-2 cm carapace width), decreasing in frequency to every 2 or 3 weeks when they are early juveniles (4-6 cm carapace width), and decreasing further to a moult every 4-6 weeks once they reach 6-8 cm in carapace width. Growth actually occurs immediately after crabs shed their old hard exoskeleton, while the new shell is still soft.

Moulting is a high risk period for muddies. Not only are they particularly vulnerable to predation or cannibalism when their new exoskeleton is soft (which is probably why moulting usually occurs at night, generally during high tide periods during neap tides), they also tend to stop feeding a few days before moulting, and cannot recommence feeding until the mouthparts have hardened 2-4 days afterwards.

Moulting is also important for reproduction, as the female mud crab needs to be soft-shelled before it can be inseminated by the male. Both male and female mud crabs mature at between 9 and 12 cm carapace width (often at the smaller size in regions where they are more heavily fished).  Maturity is usually reached within 12 to 15 moults, a process which can take as little as 6-8 months in tropical areas with high water temperatures which do not fall below 20°C at any time of year.  In contrast, in more temperate areas, feeding and growth activity drops off markedly when water temperatures decrease below 20°C, which means muddies may require up to 2 years to mature in cooler locations such as the north coast of NSW.

The moult that marks the transition from a juvenile to a mature state is called the final, pubertal or terminal moult. Most muddies do not grow any further after this terminal moult, and death is thought to occur in 3 years or at most, a maximum age of 4 years. However, some studies have found that a small proportion (about 3%) of female crabs do moult once more, and hence could potentially be fertilized and spawn more than once before they die.

The distribution of muddies in estuaries is strongly affected by salinity, with the crabs generally avoiding waters less than 10 ppt (1/3 strength of seawater). This places a limit to their upstream movements in large rivers and means they are often flushed to the mouths of estuaries after rainfall events. It is known that the early larval and juvenile stages of mud crabs also cannot survive salinities of 15 ppt (around ½ strength seawater), which is probably why their lifecycle has evolved to require female mud crabs to migrate out of estuaries to spawn offshore. Once spawning occurs offshore, the various larval stages are carried inshore with currents until the juvenile mud crablets eventually settle out of the plankton onto the bottom after around 20 days, after which they begin to move into nearby estuaries to repeat the life cycle.

A recent electronic tagging study conducted by researchers in northern NSW has found that sudden drops in salinity or water temperature are the main triggers for the offshore movement of female mud crabs during their spawning event. In a study published in the journal Estuaries and Coasts, female mud crabs were captured in the Clarence and Kalang Rivers and tagged with electronic acoustic “pinger” tags which were super-glued to their carapaces. They were then released and their movements were monitored by an array of listening receivers located within both rivers and along the NSW coast. The study found that river “flush events” after periods of high rainfall during the summer and early autumn months stimulated downstream migration of fertilised female crabs, with their movements usually coinciding with runout of larger spring tides (a timing thought to aid rapid downstream migration).  

“Berried” female mud crabs (so called as they hold between 2 and 5 million fertilized eggs under their abdomen) only hold their eggs for around 2 weeks and do not feed during this time. The tagging study found that during this time of year in northern NSW, the berried females were “flushed out” and migrated downstream out of the river mouth, then all headed northwards along the coast. This northward movement is similar to the “compensatory migrations” made by other species (such as tailor), which are thought to swim north to spawn because they are compensating for the southward flowing East Australian Current which otherwise would carry their planktonic larvae into the oblivion of cold southern NSW waters. Interestingly, the tagging study did not detect any female crabs returning to the estuaries once they had migrated to sea, which could suggest that the seawards movements of female mud crabs represents a “terminal compensatory spawning migration”. However, for that very small percentage of female crabs known to moult more than once following maturation, the researchers pointed out that this may not necessarily be the case.

The study which electronically tagged muddies in the central/northern NSW coast can be found in the journal Estuaries and Coasts, while a good recent reference summarising the biology of mud crabs can be found in the Journal Hydrobiologica.

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