Elongated Eyes

Fiddler crab compound eyes are not round, but vertically elongated (not quite as extreme as the eye of a ghost crab shown here).

The angle between the directions of view of neighbouring ommatidia depends on the local radius of the eye:

Large eye radius = small interommatidial angles = better resolution.

Figure 1: The elongated eye of the Ghost Crab (left); the relationship between the eye radius and interommatidial angle (right).