Blue swimmer Crabs

Last week I did a night dive at Shiprock and I was surprised to see several Blue Swimmer crabs and a large number of Blue Swimmer crab shells.  I dive Shiprock fairly often and had not seen blue swimmer crabs for a long time. I saw three running along the bottom, claws out looking very ferocious and two that appeared to be mating. Apart from these live crabs I also noticed a lot of Blue Swimmer crab shells littering the bottom. I was slightly concerned with all the shells, was someone killing them and throwing the shells away? Was another fish eating them? Were they eating each other? Where were the poor crabs that had lost their shells? After the dive I was on a mission to discover the truth behind the Blue Swimmer Crabs.

This is what I found.

DISTRIBUTION

 This species is found throughout Australia (0-60m depth) and also overseas through the Indo-Pacific region from east Africa to Japan, Tahiti and northern New Zealand. They prefer shallower waters in estuaries but move to deeper water as they age and in response to water temperatures and inshore salinity.

APPEARANCE

These crabs are good swimmers (thus their name!) and the last pair of legs has been modified into swimming paddles, which assist them when swimming. When not active they tend to bury themselves in the bottom sediment, leaving only their eyes, antennae and gill chamber opening exposed. The carapace (the main shell part of the body) can grow up to 21cm and they can obtain a weight of up to 1kg. It has a long spine projecting out of each side of the carapace, and of course the Blue Swimmer Crab is usually a blue colour. Although the colour can range from brown through to blue or even purple. The males have longer nippers than the female and are usually a deeper blue colour. These crabs are not afraid to use their nippers to defend themselves. These crabs shed their shells regularly, this is called moulting.

SEX LIFE

Information I found stated that the crabs form breeding pairs and mate during the late summer (January- March) moult of the females. The mature males moult some weeks before the mature females and each male carries a female clasped beneath him for 4-10 days before the female moults. The actually mating occurs immediately after the female has moulted and while her shell is still soft. So were the crabs I saw mating or was the male just holding onto the female waiting for her to shed her shell? The time frame is also wrong according to the information I found, maybe the crabs are moulting and mating late this year?

BABIES

The female blue swimmer spawns up to 2 million eggs. The eggs are planktonic and hatch within about 15days. During the egg and larval stage they can wash out to sea until they start to grow and make their way back into shore.

CATCHING AND EATING

The Blue Swimmer Crab is caught commercially, inshore areas in southern Queensland account for approximately half the commercial catch. This crab also forms a significant part of the by catch of many prawn trawlers. These crabs are supposed to be very good eating with a delicate, fishy taste and a fine texture. In NSW the bag limit for each individual recreational fisherman is 20 crabs, the legal length is crabs with a carapace of 6cm. All females carrying eggs must be returned to the water immediately. The catching of Blue Swimmer Crabs in Intertidal Protected areas (IPAs) and in some zones within Aquatic Reserves is banned. Therefore it is illegal to take any Blue Swimmer Crabs from Shiprock.

Now that I know a bit more about the creatures I saw on my last dive I am keen to get back in the water and have a better look at them. Why not join me for a night dive at Shiprock soon. Check out the web page for details of when I will be night diving at Shiprock next.