Blue Crabs

Portunus pelagicus

 
General
Physical Characteristics
Reproduction
Catch Limits & Legal Lengths
 



General

The blue swimmer crab is essentially a tropical species of crab that has adapted to the cooler conditions of southern Australia.  South Australia is probably near the lower limit of the crab's temperature range. Blue Crab


In South Australia, blue swimmer crabs are widely distributed throughout the inshore waters,  particularly in areas with extensive sandy bottom and seagrass meadows in Gulf St Vincent and Spencer Gulf.

The blue swimmer crab, as it's name suggests, is a swimming crab.   Swimmer crabs are generally distinguished by the fact that their last pair of legs are modified as swimming paddles.  Blue swimmer crabs are active swimmers but when inactive, they bury themselves in the sediment leaving only their eyes, antennae and gill chamber opening exposed.

To grow, it is necessary for crabs to shed their hard external shell.   Just before moulting, the skin under the hard shell (exoskeleton) secretes substances which sever the connections between it and the shell.  A thin layer of new shell is excreted just underneath the old shell.  The crab then takes in water to cause a split in the old shell at the point where the carapace joins the flaps.  The animal then climbs backwards out of the old shell.  The crab must swell rapidly to stretch the wrinkled new skeleton out to its full size before it hardens.  This is done by taking in water.  After the shell is adjusted to its new size, the skin secretes substances which oxidise and harden the shell.

During the moulting process, prior to hardening of the new shell, the crab is extremely vulnerable to predation  and will often go into hiding.  

 

 



Physical Characteristics

Male

The ‘flap’ on the underside of the male is narrow. From the top, the male is a bright blue colour. The pincer claws of the male are a blue colour and are much longer than those of the female.

Female

Crabs, unlike other crustaceans such as lobster and prawns, have small abdomens (tails or flaps) that are folded up under the body.  The ‘flap’ on the underside of the female is wide. From the top, the female is a mottled grey/brown colour. The pincer claws of the female are a grey/brown colour and are much shorter than those of the male. During spawning, there may be an egg mass under the flap of the female.

To conserve this species, blue crabs with eggs attached are totally protected in South Australian waters.

 


Reproduction

Courtship and mating of the blue crabs generally occurs in February and March.  The male will court the female crab for four to ten days before mating.   Courtship involves the male crab carrying the female crab beneath him during this period.  While courting, the male is extremely aggressive threatening or striking with its claws at anything that moves. Copulation takes place when the female is in the soft-shell condition.  The female retains viable sperm within an internal structure called a spermatheca until the following summer.

The broad female ‘flap’ which bears appendages is where her eggs are attached after spawning. When the eggs hatch, they become minute, transparent larvae which drift in the sea. The larvae pass through many different forms (metamorphosis) before assuming the typical adult shape.

 

crab_egg

Female with eggs 



Catch Limits & Legal Lengths


Minimum legal length: 11 cm measured across the carapace from the base of the largest spines A Measured Blue Crab

Catch limits apply to a combined catch of blue swimmer crabs and sand crabs
Personal daily bag limit: A total of 40 (including blue swimmer and sand crabs)
Daily boat limit: A total of 120 (including blue swimmer and sand crabs)

Restrictions:

Females with external eggs are totally protected and must be returned to the water immediately.

 




Recreational fishers are not permitted to sell or trade their catch.