In honour of Mandurah’s iconic Crab Fest, Fauna Friday features the star of the attraction: the Blue Swimmer Crab (Portunus armatus). You may have enjoyed crabbing and cooking up crabs for a seafood dinner, but how much do you really know about these amazing creatures…?
For starters, they have quite a bizarre reproductive strategy: the crabs mate in early winter and the young female stores the male’s sperm through winter while her ovaries mature. And mature they do – releasing approximately two million eggs in early spring! She fertilisers her eggs using the stored sperm and incubates them for about three weeks in the protection of special abdominal structures called pleopods.
A Blue Swimmer Crab’s lifecycle has several stages and involves many changes in form. The female releases her eggs after incubation in spring and they hatch shortly after into tiny (~1mm), vulnerable larvae known as zoea that float on the ocean’s surface. Those that survive make their way to shallower coastal waters where they enter their megalopae growth phase, growing rapidly and shedding their shell (carapace) several times but still only about 2cm.
The juvenile phase follows, and for two months, the crab continues to grow and moult while sheltering on the coastal seabed, reaching a width of ~60mm. winter sees the crabs reach maturity and they moult for the last time, reaching an average adult width of ~90cm.
This complex life cycle is why it is so important to be a responsible crabber; observing the size limits, returning “berried” females (those incubating eggs) and seasonal closure so the crabs have an uninterrupted reproductive cycle to recruit new individuals to the population with each generation.
Blue Swimmer Crabs, like most crustaceans, are opportunistic scavengers – they form an important ecosystem function by consuming dead organic material from shallow coastal systems and estuaries. Adults will also actively prey on small fish and marine worms.
So Crab Fest is so much more than consuming and celebrating a menu item; it is also a great time to acknowledge and appreciate the fascinating creature that is the Blue Swimmer Crab – an integral and beautiful part of our estuarine environment.
I’d like to acknowledge the Western Australian Museum for some of the technical content this week 🙂