There are spiders called crab spiders which do look like crabs.

Usually white, cream, light yellow, or pale green in color. They try to blend in with the plant tips or flowers they sit on, and grab bugs as they try to land or pollinate. Sort of neutral in the garden, since they tend to take bees as well as pests.

Lovely things, crab spiders. They can change colour to almost exactly mimic the colour of the flower on which they find themselves. We get pretty primrose yellow ones on the early Rhododendrons each spring.