A VERY SPECIAL CRAB

There are at least 500 different species of crab in Queensland, and more than 100 of these live in mud. We could not single out one of these and call it 'The' Mud Crab unless it was a very special crab. The Mud Crab is Queensland's largest crab. It is also one of our five big edible species, and yields large quantities of what must be the most delicious crab meat in the world.

The most common edible crabs belong to one family - the swimming crabs (Portunidae) - which can be recognised by the flattened swimming paddles on the last pair of legs. This feature, plus the nine even-sized teeth on each side of the body behind the eyes, makes it easy to identify the Mud Crab, and to distinguish the juveniles from the common species of fiddler and marsh crabs which are so abundant on mangrove mud banks. These saw-like teeth are responsible for the last half of the Mud Crab's scientific name, Scylla serrata, which was given to it in 1755.

A WELL ORGANIZED ANIMAL

As you prepare your next Mud Crab, take a minute to think of it as a highly organized animal rather than just another gastronomic delight. Remember that before it was cooked it was a well camouflaged dark olive-green with mottled legs and orange tips to the claws. See how each joint in the claws and legs hinges in only one direction, but in combination they allow a wide range of movements. The eyes are on stalks, but they can be folded back neatly into the pro­tecting eye sockets. The two pairs of antennae between the eyes detect minute changes in water currents and water chemistry, and just below the antennae there are two small openings through which urine is excreted . It is quite common to see crabs squirting jets of urine while they are being removed from the traps.

The mouth of the Mud Crab is covered by six layers of paired appendages. The outer five pairs may be used directly to locate, catch, and manipulate small food organisms such as those encrusting man­grove roots. Larger food organisms, many of which live below the surface of mangrove mud, are detected and retrieved by probing move­ments of the walking legs. The tips (dactyls) of the walking legs, like the outer mouthparts, are highly sensitive to touch and taste. With its large and powerful claws, the Mud Crab is particularly well adapted to consume large food organisms encased in hard protective shells such as molluscs (oysters, mussels, pippies, winkles etc.) and hermit crabs which abound in .mangrove estuaries. It is therefore not surprising that these and other slow moving or immobile animals form the bulk of the crab's diet rather than fast moving fish or crustaceans.

Once the shells of larger food organisms have been crushed by the claws, they are passed to the outer mouthparts where hard indigestible fragments are sorted and discarded. The remaining soft choice tissues

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are then passed to the inner (sixth) pair of stout jaws (mandibles) where pieces are bitten off and swallowed. The stomach, a complex grinding mill, is connected to the mouth by a short gullet. With the aid of digestive juices (acid and enzymes) produced by the liver, the stomach grinds the food into a thin watery paste. From the stomach, useful materials (fats, proteins, carbohydrates) pass into the short mid-gut and thence to the tubules of the liver which open into the mid­gut. While useful materials are absorbed in the liver, waste materials travel directly through the hind-gut, a simple tube which runs right around to the tip of the flap, where the faeces are expelled. The whole digestive process is quite rapid. The crab, which is most active at night, has two major feeding periods - one in the early evening, the other before dawn.

Underneath the triangular abdominal flap in the male there are a pair of large tubular pleopods, each with a smaller one inserted into its base like a plunger. These are used to transfer sperm to the females during mating. The mature female has a much broader abdomen, which covers the paired female openings and carries four pairs of forked pleopods with thick hairy edges to which the eggs are attached when laid.

When the carapace is lifted off the crab the most obvious structures are the eight 'feathers' or gills which lie in large chambers at each side. These gills extract oxygen from water which is pumped into the gill chambers through openings above the bases of the claws and out through openings at each side of the mouth. Carbon-dioxide is removed from the blood; oxygen is passed into it, and bonded to a blue respiratory pigment. The blood is circulated through a series of large connecting spaces (sinuses) by a simple heart pumping through opened arteries. The blood clots very quickly on exposure to air. If this were not so, even small fractures in the shell would soon drain the large body sinuses.

Provided the gills do not dry out, mud crabs can live for several days out of water.

HOW CRABS GROW

A Mud Crab cannot grow while it is enclosed in its hard shell. To increase in size it must shed this outer shell, and expand its body and limbs before a new shell hardens. This process is called moulting; the periods of static size between moults are known as intermoults.

Just prior to moulting, the membranes joining the legs to the body swell and begin to form obvious bulges. A hair-line crack appears along the sides of the gill chambers. This rapidly opens around the back of the carapace and allows it to be lifted from the abdominal attachment. The crab can then back out of the old shell. The bulk of the body swells whilst fluids are reduced in the extremities to allow the legs etc. to be withdrawn. The outside walls of the gills, some of the gut lining, and the eye lenses are all part of the shell and are lost at this time. Once the crab is free of the old shell, fluid is t.aken up to expand the new larger shell while it is still soft.

Moulting is a critical time in a Mud Crab's life because it is almost completely helpless during the actual shedding of the shell, and being soft it is easily eaten by other animals, including other Mud Crabs. If it is not quick enough in withdrawing its legs , they may swell inside the old shell and would have to be broken off so the crab could escape from its old shell.

The very complicated processes involved in moulting are con­trolled by hormones. The hormone which triggers these processes is held in check by a moult-inhibiting hormone during intermoult periods.

GROWING NEW LEGS

A special type of growth process, regeneration, allows lost or damaged parts to be replaced. These replacements are needed when legs or claws are lost in fights with other animals. Most fishermen know, to their loss, that Mud Crabs have the ability to cast off one or most legs (autotomy) if they are handled roughly or damaged . If similar losses occur in nature, a membrane grows over the stump; this is then distended as a small finger-like projection which enlarges as a limb bud. A folded, newly-formed leg develops within this bud. At the next moult this regenerated leg will be released, although it may take more than one moul t to reach a size and shape equivalent to the other legs.

Regeneration: I. the cast she ll of a Mud Crah showing the limb bud (I) wh ere the

right claw had bee n lost. 2, the same cra harte r moulting showing a new righ t daw.

Note also the incre ases in size during the moult.

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One of the Mud Crab's claws is a heavy 'crusher', the other is a much lighter 'cutter'. If a crusher is lost, the remaining cutter develops into a crusher at the next moult whilst the lost crusher is replaced by a cutter. Deformities frequently occur, particularly in the claws (e.g. double pincers) and legs (e.g. double swimming legs). These probably do not have much influence on the crab's life .

REPRODUCTION

Females produce eggs in paired ovaries. These open externally through tubes (oviducts) which incorporate a storage sac where sperm are held after insemination. Males produce sperm in paired testes which open through coiled tubes (vasa defferentia) which package mature sperm into gelatinous bundles (spermatophores) for transfer to females.

A male can only inseminate a female while her shell is soft, during the period of about 48 hours following her moult, but pairing occurs some time previously. Unlike the female Sand Crab the female Mud Crab is very active in her approaches, and actively works her way under the attendant male. Normally such behaviour would stimulate an aggressive response, but the male straddles the now passive female and will actively defend her against intruders. This situation is main­tained until the female moults and sperm transfer takes place. Spermatophores are transferred, via the male's pleopods, to the female's oviducts, where they are stored and can remain viable for many months. Copulation may take more than eight hours, after which the female rights herself but remains beneath the male for a further few days until her shell is substantially hardened. In nature it is possIble that some of this mating behaviour takes place in the burrow, the pre-moult females seeking out the burrows which normally contain males.

Male crab cradling and protEcting a female prior to her moulting and copulation (le ft).

Copulatory crahs (douhlers). The newly moulted female is inve rted beneath the mal e and her ahdome n, which is lifted during copulation, can he seen protruding above the male. The cast shell is in the foreground (right).

When the eggs are ripe, they are passed down the oviducts, fertilized by the stored sperm, and extruded onto the pleopods. The eggs adhere to the pleopod hairs and the female is said to be 'in berry'. From one to eight million eggs are extruded, depending on the size of the female. Berried females are rarely seen in Australian populations. Because they reduce their feeding activity, the chance of catching them in the normal baited traps of the inshore fishery is remote, and it is also possible that they may move offshore, since they are netted in quantity offshore in other countries.

The fertilized eggs hatch into planktonic larvae (zoeae) which swim by beating brush-like extensions of some of their mouth-parts . Zoeae grow by moulting four times over 12 to 15 days, and in the fifth moult they change into a bottom-dwelling stage which is transitional between zoea and crab, called a megalopa. On the subsequent moult, larval characters are lost completely, and the post-larvae (3-4 cm carapace width) progressively assume the adult crab characters over the first few of the following 15-17 post-larval moults.

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La rval 'Lage,. I , Zoea 1; 2, Zoe a 4; :3, Mega lopa. Scale line = 1 mm.

In juvenile crabs, which are up to 1 year old (3-10 cm carapace width), the female abdominal flap progressively broadens into a triangular shape. In both the male and the female the claws are about 22% of the body weight. Juveniles frequent intertidal regions associated with mangroves although larger crabs are found more often in deeper sub-tidal regions.

In sub-adults (l0-15 cm carapace width) the male claws become much heavier in relation to the body (30-35% ), while the female claws remain about the same.

In adult females (14-20 cm carapace width) the abdomen is broad, semicircular, and of the same colour as the legs (green with black mottling). The claws of adult males (14-19 cm carapace width) are quite massive (40-45% of total weight). Contrary to popular belief, adult males rarely inflict injuries on rivals. Disputes are usually settled by claw displays and claw pushing - ritualized tests of strength which end with the hasty retreat of the smaller or weaker crab.

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The time taken to reach maturity varies quite markedly according to prevailing water temperatures, higher temperatures tending to increase the growth rate and therefore decrease the time to maturity. In Moreton Bay, for example, the winter temperature falls below 20°C, which seems to be the minimum necessary to promote most of the crab's activities. They are therefore comparatively inactive during this period, and the majority do not moult until temperatures increase in spring. During September to October most crabs caught in crab pots are smaller than 15 cm (the 'legal' size). They are full of meat and ready to moult. During October and November, when water tem­peratures have risen, most crabs caught have moulted and attained the legal size, but do not yet have a full complement of meat. These 'floaters' should be released and given a chance to replace the post­moult water content of their bodies with firm meat.

In the hot tropics, maturity should be reached after about 18 months. In Moreton Bay, which is near the colder end of the crab's range, it probably takes about 36 months.

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THE BURROW

Burrows are usually associated with marsh-grass or mangroves and tidal creeks. They become less common inland , and are seldom found beyond the landward limit of the mangroves. Most burrows extend into the ground at approximately 30 degrees to the horizontal to a depth of at least 0.8 m below the surface. They extend below the low-tide water table, and always contain some water. This water issually more saline and cooler than adjacent creek water, and is nearly devoid of dissolved oxygen.

Burrows appear to be permanent structures used by successive generations of Mud Crabs. They are usually occupied only during the warmer months when they may act as havens of cooler, more saline water. They probably also protect the crabs during moulting and mating periods, when they are more vulnerable. Further studies will be needed before the role of the burrow can be fully understood.

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MUD CRAB ENEMIES

Zoeae, because they are planktonic, will form part of the food of plankton-eating fishes, jellyfish, etc. Post-larvae and juveniles would be eaten by many species of fishes that frequent mangrove areas. Adult crabs have fewer predators because, as anyone who has caught Mud Crabs will verify, a large 'mudd ie' is a most formidable creature. Turtles and large fish, such as barramundi and groper, are known to eat adult Mud Crabs, but man is probably one of the crab's major predators, at least in areas near centres of population. Probably the Mud Crab's most effective way of avoiding predators is its ability to bury itself quickly and deeply in soft mangrove muds .

FISHING FOR MUD CRABS

Mud Crabs are highly prized as food wherever they occur, and are therefore often subjected to heavy fishing pressures by amateur as well as professional fishermen. Most of the Australian catch is taken using the following methods.

Crab Pots. These are wire-netting cages with one or more funnel­shaped entrances. Crabs are enticed into the trap by a centrally located bait and cannot find their way out again. These traps are usually set overnight in mangrove creeks. The cane bee-hive trap is one of the many variants of crah pots that are used successfully.

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Some type, of <Tah pots. 1, , teel a nd wire me,h pot; 2, traditional heehive pot ; ;1, trap dill, or dill y pot.

Dilly Nets. The dilly net is simply a circular bag of netting attached to a ring of galvanized rod which is attached, by bridles, to a pulling cord. Baited dillies can be set from creek banks or from boats. Each net should be pulled at 20 to 30 minute intervals using a rapid, steady pull so that the feeding crabs drop into the bag of the net.

A variant of this, the suicide dilly, cannot be recommended on any grounds. Large crabs break the lighter netting and escape, whilst the many females and undersized crabs caught will be damaged or killed while being removed from the tangled net.

Hooking. A steel rod about 4 m long and hooked at one end is inserted into a crab burrow. Inhabitant crabs can then be dragged from the burrow in the bend of the hook. As this fishing method requires expert knowledge, and as unskilled 'hookers' succeed only in destroying burrows and killing the occupants, this method is also not recommended.

Light and landing Net. On dark, still nights, crabs can often be spotted with a pressure lamp or sealed beam spotlight, and scooped by a long-handled landing net. However this method is not very produc­tive because it yields mostly females and males below 15 cm carapace width which are protected under Queensland's fisheries regulations.

Once crabs are caught it is essential that their claws are restrained by tying with a stout cord. Unrestrained crabs will fight, resulting in lost claws and crushed shells. Also remember that those powerful claws are a very real menace to your own well-being and cou ld cause severe inju ries if you are careless. If you are bi tten, quickly break the claw from the crab. It will remain firmly locked by reflex action and one of the pincers must be broken with pliers, or the whole claw smashed with a hammer. Don't panic and act quickly. The best way to handle Mud Crabs is to grasp both of the swimming legs firmly in one hand, place a well shod foot on the crab's back, and tie its claws.

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FARMING MUD CRABS

Because Mud Crabs are so delicious, so accessible, and so easily caught using simple gear, they are subject to heavy fishing wherever they occur - fr6m the east coast of Africa through India, Malaysia and the Philippines to Japan and Tahiti, and around the northern coast of Australia from Broome to Sydney. Although they are found in the open sea and in estuaries almost up to fresh water, they are most common wherever there are mangroves. The crabs may be seldom seen during the day, but their large oval burrows in estuarine mud banks or swamps are unmistakable.

The aquaculture of Mud Crabs throughout southeast Asia has long been carried out in conjunction with the farming of milk-fish (Chanos chanos) and prawns (principally banana prawn, Penaeus merguiensis, and the Giant Tiger prawn Penaeus monodon. By Australian standards the methods developed so far for fish, prawn and crab farming in these countries appear primitive. Fish ponds are enclosed by simple man-made earthen dykes. Local cheap materials, notably bamboo, are used in the construction of rigid structures such as weirs and fish traps.

The stocking of these ponds is almost fully dependant on the natural addition of small fish, prawns and crabs which enter on flood tides. Successful stocking is thus unreliable as there is no control on the numbers of either desirable or undesirable species entering the ponds. The yields of these ponds are generally low, and highly variable. Fertilizers or supplementary foods are rarely used to increase production as both are thought to be too expensive.

The natural stocking of these ponds with crabs is sometimes supplemented by collecting small crabs (juveniles of 2-7 cm carapace width) on local mud-flats. Severe losses usually result from cannibalism because the crabs are poorly fed and have few safe pla ces in which to moult.

In contrast to the underdeveloped countries, aquaculture in Australia has a low priority. Australians do not need to rely on fish protein; productive inshore and offshore fisheries exist; and aqua­culture, being a labour intensive industry, tend s to have very high operating costs. Aquaculture is limited to the farming of luxury species such as oysters and trout. The techniques employed are largely adap­tations of those proven in other developed societies such as North America and western Europe.

While techniques for the intensive culture of Mud Crabs have not been developed overseas, successful hatchery and growth trials have recently been conducted in Queensland. Extension of this work to pond-production trials now depends on the investment of large amounts of capital to develop efficient pond designs and management techniques. Cannibalism is probably the largest single restraint on high stocking densities and hence on high production per unit of invested capital and labour. The selection of a suitable site for crab farming involves many considerations including accessibility to labour, transport, power, topography, soil structure and chemistry, quality of tidal water, and annual temperature and rainfall patterns.

RESEARCH ON MUD CRABS

With the exception of northern Australia, the major Mud Crab populations of the Indo-Pacific occur in Indonesia, Malaysia, The Philippines, Thailand, Vietnam, Sri Lanka and India. In these countries there have been heavy demands on the coastal fisheries as a source of dietary protein and the Mud Crab has suffered centuries of heavy fishing pressure. It is not surprising that most research on this crab has been sponsored by national and international food and fisheries agencies, and has been directed towards the aquaculture of the crab as a means of supplementing repeatedly low fisheries yields.

At the University of Queensland research studies into several aspects of the mud crab's biology have been in progress for a number of years. Studies already completed deal with growth and development of wild and laboratory reared crabs, the size at which crabs become sexually mature and the nature and extent to which wild crabs lose and regenerate claws throughout their lives. Investigations into the effects of seasonal temperatures and salinities on growth and survival rates of larval crabs are planned for the near future.

Investigations on the fisheries and fisheries biology of the Mud Crab are being conducted by the Queensland Fisheries Service.

The following references give some examples of the research which has been carried out on the Mud Crab.

ARRIOLA. F .. J. 1940. A preliminary study of th e life hi,tory of Scylla serra/a (Forskal). Phiti,),)ine J Sci. 73: 4:)7 -456.

DELL. R.K. 1964. The large Indo-Pacific ;;wimming crah Scyl/a serra/a Fon;kal in northern New Zealand. Records of the nominion Museum 5 (Il): 59-62.

ESCRITOn. G. L. 1970. A report on experiments in the cui ture of the mud crab Scylla serra/a. F.A.O. Indo-Pacific Fi,herie, Council, 14th Ses,;ion, Bangkok, Thailand. Symposium Pape r No. 46.

ESTAMPADOR. R. P. 1949. Stud ie, on Scylla (Crustacea; Portu nida e) I. R ev ision of the genus. Philippine J. Sci. 78 (I) : 95-109.

HILL, BA. 1974. Sa I inity and tempera ture tolera nee of weae of the portu nid era b Scylla serra/a . Marine Bi()logy 25: 21-24.

1975. Abundance, breeding and growtb of the crab Scylla serra/a in two South African estuaries. Marin e Bi()'()~v 32: 119-126.

1976. Natural food, foregut clearance-rate and activit\' of the crab Scylla serra/a. Marin!! Biology 34: 109-116.

ONG KAH SIN. 1964. The earJ.v deve lopmental s tages of Sev//o serra/a Forskal (Crustacea; Portllnidae) reared in the lahoratory. F.A.O. Indo-Pacific Fi,herie, Council, 11th Ses,;ion, Kuala Lumpur, Mala.v;;ia. : 135-146 ,

1966. Ohservations of the post-larval life hi s tory of Scylla serra/a (Forskal) reared in the laboratory. Malaysian Aj:ric J 45 (4): 429-44:3.

RA,/A BAI NAfDU, KG. 1955. The early development of Scylla serra/a (Forskal) de Haan and Nep/unus sanguinolen/us (HerhsL). Indian J Fisherie, 2 : 67-76.

RAPHAEL, Y.!. 1970. A preliminarv report on the hrackish water pond culture of 5<:y/la serra/a (Forska I) in Ceylon. F.A.O. I nd o-Pacific Fisheries Cou ncil, 14 th Session, Bangkok, Thailand. Sympo,ium Paper No. 21.

SEPHENSON, W. 1972. An annotated chel'k li;;t and ke.v to the Indo-West-Pacific swimming crabs (Crustacea Decapoda Portunidae). Proc. Roy. Soc. New Zealand Bull. 10: 64 pp.

VARIKUL. V., PHUMIPHOL, S., and HONGPROMYART, M. 1970. Preliminary experiments in pond rearing and some biological studie, of Scylla serra/a (For,kal). F.A.O. Indo-Pacific Fisheries Council, 14th Se"sion, Bangkok, Thail a nd . Symposium Paper No. 49.

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COOKING MUD CRABS

Crabs should be cooked as soon as possible after they are caught. Stored crabs, even when their claws are tied, may damage themselves. They will also defaecate, and the faeces, apart from being unpleasant in their own right, will attract flies and are liable to be blown. The maggots usually get into the crabs' gills and kill them, and the death of one crab in a container usually means that others will die quickly as a consequence.

Crabs can be cooked in boiling water or by steaming. A crab dropped directly into boiling water will probably throw its claws. This can be avoided if the crab is first placed in the freezer compartment of a refrigerator for about 30 minutes.

Salted water should always be used for boiling crabs - ideally some of the water from which the crab was taken. Freshwater tends to make the crab meat watery and reduces its flavour, but if the water is too salty the meat will be tough and salty. If seawater is not available, add cooking salt to tapwater (25 gm/l) in a large pot and bring to the boil. Drop the crab into the pot on its back, bring to the boil again, and cook for 20 to 25 minutes. The water may be seasoned with sliced lemon, black pepper, celery, garlic, and onion, or in the 'Creole' style by adding a bag of 'crab boil'. 'Crab boil' is a mixture of mustard, coriander and dil seed, cayenne pepper, bay leaves, allspice, and cloves in a small muslin bag. Experiment with this recip8 to suit your taste­it is probably wise to be frugal with the cayenne pepper initially. A couple of tablespoons of vinegar added to the water makes the cooked crab easier to pick.

To steam a crab, first kill the crab in the freezer, remove the limbs and carapace, clean out all soft parts, halve, and steam the limbs and body halves in a collander for 8 to 10 minutes.

Once cooked, the crab should be drained, cooled, and then chilled in the refrigerator. The edible meat, found in the legs and as a block along each side of the body, can now be removed and eaten as is, or incorporated into a more ambitious dish such as one of the following.

Jellied crab (six to eight servings)

2 envelopes unf1avoured gelatine 1 cup cold water '/, tspsalt 1 tsp grated lemon pee l II i cup freshly squeezed lemon juice

:2 ('up~ mayonnal~e :2 tsps instant minced onion 400 g drained crab meat 1 cup finely chopped ce le ry lemon wedges

In a small saucepan, soften the gelatine in cold water for 5 minutes; heat slowly, stirring constantly, until thoroughly dissolved. Add salt, lemon peel and juice, mayonnaise and onion. Chill until slightly thickened, but not set. Coarsely flake crab meat; fold into chilled mixture with celery. Pour into a quart jelly mould; chill until firm. Unmould onto chilled plate; garnish with lemon wedges.

Crab and avocado(eight servings)

100 g crab meat 4 small avocados lime or lemon juice

hollandaise sauce CUlT,. powder butten:d bread numl"

Cut avocados lengthwise, pit, and brush surfaces with lime or lemon juice. Mix crab meat with 200 ml hollandaise sauce. Stir in dash of curry powder. Heat mixture until hot and spoon into avocado halves. Top crab meat with buttered crumbs, place avocados under grill for 2-3 minutes until browned.

Crab and mushrooms in wine sauce (four servings)

450 g crab meat 100 g sl iced mushrooms :2 tblsp butter :2 tblsp butter to sauce fresh mushrooms :2 tblsp f10ur ,/" cup milk ,/" cup white wine

\/'2 t~p dr," nlu~tard 1/1 t~p dr,\' tarragon sa II to last" pepper to task hot :--;au('t:' to ta~t('

'I , CLIP bread numb,

Saute mushrooms in butter. Make a cr.eam sauce, blending melted butter, flour and milk, wine, mustard, tarragon, salt, pepper and hot sauce. Cook 2-3 minutes, then add crab meat and mushrooms. Place in casserole; sprinkle top with bread crumbs and dot with butter. Bake 200 0 C for :30 minutes uncovered. Cover before serving.

Sand-crab or shelled prawns could also be used as the seafood base in all of these recipes.

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