The Smooth-handed Crab, Pilumnopeus serratifrons, lies in moist depressions under boulders lying on the sandy shore. When the protective rock is turned over, this crab does not attempt to run away, but folds its legs and claws tightly under its body and pretends that it is a pebble.

This is an excellent example of camouflage.

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The planktonic stage of a parasitic shell-less barnacle can swim through the water and lodge itself under the tail-flap of adult male Smooth-handed Crabs. The barnacle then burrows through the body of the crab, eating its host. This also makes the male crab unable to produce offspring.

Here we can see the egg-sack of the next generation of shell-less barnacles ready to be released into the water to find other male Smooth-handed Crabs.

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References:

Davey, K. 1998. A Photographic Guide to Seashore Life of Australia.
New Holland press, Sydney.