Whilst rockpooling on Rottingdean beach, east sussex, we found what we think was a decorator crab. It was covered in red seaweed and at first glance looked like a sea anenome. We have indetified it but discovered that they live on the coasts of Australia. So had it got lost or was it something else? Can anyone help us?
Worldwide there are numerous species of crabs that may be described, and even called decorator crabs. Most of them are certainly Majoidea, or spider crabs - they deliberately attach pieces of seaweed to their bodies as camouflage and thus blend in with their surroundings. In so doing they are "decorating" themselves.
Some crabs have taken to wearing an anemone on each of its claws which appear as boxing gloves. Another, the sponge crab will skillfully attach a living sponge to its back. It is not impossible to have had an anemone on its back. There is a UK species of hermit crab which does exactly that, both for camouflage and for protection.
With the amount of international trade and more specifically the amount of sea shipped trade which goes on around the world there are more and more unusual marine species hitching lifts and finding themselves in new places. I guess it is not out of the question for a small crab to have hitched a ride from as far away as Australia. Water temperature may prove a barrier to successful breeding but it is not neccessarily a barrier to sustaining life, unless we are concidering extremes in temperature. Even Australian water temperatures can be decidedly "chilly". However, maybe an easier explanation may come from one of our own temperate spider crabs. Take the local spider crab Inachus dorsettensis - it is quite easy to see how this may decorate itself in the way you describe.
I hope this helps. Well done for finding it. These disguised crabs are not easy to find.