The Lifestyle & Lifecycle of a Sand Crab
Did you know juvenile sand crabs have tails? Or that female Sand Crabs mate only once in their lifetime, but release a hatch of eggs on several separate occasions in their lifespan?
The resources on this website relate to the crab species Portunus Armatus, Portunus Pelagicus, and Callinectes Sapidus.
They are known throughout the seafood-loving world by a number of common names, including the Blue Crab or Blue Swimmer Crab.
Here are a few other names they are known by:
* Australia: Sandy, Bluey, Sand Crab, Blue Manna Crab, Blue Swimmer Sand Crab
* Philippines: Alimasag
* Malaysia & Singapore: Sand Crab
* United States: Pacific Sand Crab, Maryland Blue Crab, Chesapeake Blue Crab
* Indonesia: Rajungan
In the wild, they live in saltwater rivers, bays, and oceans, and are blue-green-white in colour with a white or yellow underside, depending on where they have been living and what they have been eating.
Their back two legs have paddles on them, allowing them to swim forwards, backward, and in either direction sideways… even up to and on the surface of the water.
Usually, though they swim along or relatively close to the sandy bottom, often burying themselves in the sand almost completely, with just their eyes and antenna poking out… laying in wait as they ambush seafood passing by.
They eat small fish, snails, worms, and mussels and scavenge on other dead bits of sea creatures.
Females take around a year to a year and a half from hatching to become sexually mature, while males can take up to two years.
The females mate only once with one male in their entire life, with male crabs mating potentially multiple times.
When a male crab finds a suitable female crab to breed with he climbs on top of her and cradles her below him for a number of days until they find themselves in a suitable location, and she moults.
At this stage, he deposits his sperm into a sack, called a spermatophore then puts the sack inside her abdomen. She carries this reserve of sperm her entire life, using portions of it when she is ready to fertilise a batch of eggs.
The female sand crab can carry up to two million fertilised eggs per batch beneath its abdomen.
Once she releases them into the ocean or estuary near the coast, the baby blue crab eggs hatch into free-swimming larvae called zoea. They are very small and transparent in appearance, with a flattened body and a long spine. Drifting around feeding on plankton, they grow over several weeks.
As they grow, they moult repeatedly and become a crab with a tail. At this stage in their development, they are called megalops. They continue growing and moulting, still free-swimming, they begin to take on characteristics of adult crabs, developing walking legs and claws.
They continue feeding and moulting and their tail curls under their body completely, becoming a juvenile crab. Once the juvenile crab is fully developed it will sink to the floor of the ocean and live out the majority of its life on the seafloor.
As they grow bigger you can tell the gender of the crab by the shape of what used to be their tail. The boys have a pointy one and the girls have a more rounded one.
When blue swimmer crabs are fully grown they can weigh up to 1 kilogram (2.2 pounds), with a shell (carapace) up to 20 centimeters (8 inches), though it’s very unlikely for you to find one that large.
The precise timing of different developmental stages varies based on the availability of food, the salinity of the water, and water temperature. Most blue crabs that reach adulthood have a lifespan of between two and three years.
All throughout their lifecycle, from when they are tiny all the way up to full size – fish love to eat blue crabs. The bigger the crab, the bigger the fish needs to be to eat them though as they have very tough shells most of the time.
Live blue crabs of all sizes are used as bait for fishing and a variety of imitation sand crab jigs or lures are available.
As all crustaceans grow, including blue swimmer crabs, they get too big for their shell or exoskeleton. So they grow a smaller soft shell inside, drawing some salt and other minerals from their hard shell into their body.
This softens their outer shell and it cracks when the time is right (nighttime), shedding their hard shell or ‘malting’ it.
Until they grow into and expand their new soft shell and it hardens up – they are vulnerable to predation from just about every fish in the sea, including other blue crabs.
This cycle of getting too big for their shell repeats around ten or eleven times a year, mainly in the summer as they bulk out into a larger and larger shell.
Every time they malt, the crab emerges 30% bigger than its old shell. They use the minerals and salt to harden their soft shell again over the course of a few days.
Prawns malt for the same reason, so do lobsters and all other crabs you can eat.
Sand crabs are fished and farmed successfully in many countries. They are sold as either soft-shelled or hard-shelled crabs.
Soft shell crabs are adult sand crabs that are harvested just after they have malted. Typically they are frozen or cooked and then sold.
Hardshell sand crabs are sold in three different states: live, green (raw), or cooked.