All crabs belong to the group called the Decapoda – Latin for ‘ten legs’.

There are lots of crab species in the Gulf, and they range in size from tiny to quite large. Most have hard shell (carapace) unless they are moulting. This happens as the crab is growing. Calcium is reabsorbed from the old shell, which gradually becomes softer, and is used to grow its next shell. When the new shell is ready the old shell is cast off, and the new shell hardens quickly. Sometimes the soft, old shell is washed up. Hermit crabs don’t have a hard carapace, and they use empty marine snail shells to protect their soft bodies. As they grow they need to find larger snail shells. If you are collecting shells on the beach, be sure to check that there is not a hermit crab in residence!

The Blue Swimmer is probably the best known crab, and is sought after because it is abundant and tasty! If you see one in the water while you are swimming, or in a tidal pool, don’t get too close – a nip from their claws is painful.

Sand crab carapace – note the two dark red sots

Sand crabs are pale reddish with two darker spots on the carapace.

 

 

 

 

 

 

 

 

 

Spider crabs range greatly in size, but characteristically have really long legs.

Spider crab near Henley Jetty – approx 20 cm across the carapace

 

 

 

 

 

 

 

 

 

 

Tiny spider crab – approx 1 cm across the carapace

 

 

 

 

 

 

 

 

 

 

 

Rock crabs are smaller and chunkier than the swimmers and spider crabs, but are really good at wedging themselves under rocks and in cracks.

4 Responses to Crabs

  • arek says:

    Dear Friends of St Vincents Gulf

    My wife and i made the usual annual pilgrimage to Stansbury in February with the usual array of activities.
    One of my favourite rituals there is to do some raking for blue swimmers behind the caravan park.
    I always seem to catch a few, and sometimes even plenty. Nearly always i will get at least a 6 or 7 big keepers, some i will fail to hang on to, others i will release due to being to small while i always return the girls back in the water.
    This year i was shocked not to catch one crab. Whats worse is that i did not even see any sign of a crab. Further more there were no signs of the usual plentitude of other little fish, flathead or small rays which often tend to lurk in between the weed patches. What has happened at Stansbury ? Why has the place become a stark desert with no sign of life ? has it become a victim of commercial over fishing ? is there something more sinister which is destroying our marine environment ? I have so many questions because i am deeply disturbed by this. I hope you can shed some light on this event. I await your reply and i thank you for giving an opportunity for people to see the gulf not as a resource but as a fragile life source that can be disrupted forever by our stupid actions.

    Regards

    Arek

    • admin says:

      Hi Arek,
      I will forward your query to some of our marine experts to see if they can offer any ideas. The publication “Natural History of Gulf St Vincent” has a great chapter on Blue Swimmer Crabs, which summarises most of what is known about them currently. They are very mobile, and can travel large distances! It is surprising that you describe the Stansbury sea grass beds as a desert – sea grasses are usually full of life – as you have previously observed. One possible explanation that springs to mind is that it has been quite hot, so the fish and crabs may have moved to deeper, cooler water. Perhaps one of our Yorke Peninsula friends would like to comment on local conditions and Blue Swimmer catches this summer. I have spoken to a couple of people crabbing successfully off Henley/Grange using kayaks in recent weeks. With regard to your question about over-fishing, this is certainly a concern with popular marine species. At our Forum last October we had a presentation from Sean Connell, who described a fisheries threshold phenomenon, which suggests that if a species is fished beyond a certain level the populations can collapse very suddenly. The problem is that threshold is pretty much unknown. I guess this should be a caution to all of us not to take the abundance we grew up with for granted! Stay tuned for more information. And thanks for caring!

    • admin says:

      Arek,
      Further to this discussion, a marine researcher has advised that:
      “SA biologists have reported that abundances of blue swimmer crabs (prawns also) have declined very markedly this year. Commercial crab fishers (and prawners) stopped fishing late last year.

      Two causes are thought to be operating:
      1. There could be a lower recruitment (breeding) of crabs than is usual;
      2. Another contributing factor is that snapper numbers have greatly increased. They are significant predators of crabs and prawns and undoubtedly have contributed strongly to the steep decline in crab (and prawn) numbers.

      Marine food webs are most interesting, and the above illustrates the ‘cascade’ effect when a higher level of the food web causes dramatic changes at lower levels. Hence, the underlying aim of fishery management must always be ‘ecosystem management’ in which a balanced ecosystem is maintained.”

      The researcher also said that up to 50% of the Blue Swimmers caught in the gulf each year can be taken by amateurs (you and me crabbing) so we DO contribute to the decline in numbers. It is interesting that even though the commercial operators have stopped, there has been no call to the wider community to play its part.
      Thanks again for your question!

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