Blue legged hermit crab
These guys are very small, so all of the pictures are close-ups. Reef tank owners like them because they never become huge (and aggressive). Here he is looking at me and the camera right through the glass.
My friend, Stacey, tried to get a picture of the crab as it considered whether it wanted to switch to a larger shell. She was never able to get a picture with the crab out of its shell, but this picture is still nice. The brown "wall" in the background is the expanded skirt of a button coral.
A great shot of the blue legged hermit crab perched on a piece of macro algae. The full width of the image is less than one inch.
Here is another one peering out of his shell. The two grasping pincers can be seen in the foreground, more of a mottled brown than a blue.
And here he is, walking away.