Sea urchins are present in every major marine habitat worldwide, making them an ideal group to use in comparative studies. Here, we look at the relative impacts of habitat variation and phylogenetic relatedness on the density of spines on sea urchins. Spines may be used for defense, locomotion, and a variety of other behaviors, but they may also play a role in the unusual vison of urchins. Behavioral data show that urchins have spatial vision, with spatial acuities (measured in angular resolution) ranging from 10-33º, depending on the species. Acuities measured via behavioral assay match closely with the angular distance of spines across the body, and it has been suggested that the spines screen off-axis light and restrict the “view” of the patch of photosensitive skin between the spines. Essentially, the animal may function like a large, compound eye. In order to investigate the role of habitat and phylogenetic constraint on spine density and vision, we measured multiple individuals from 33 urchin species, representing every family of regular urchins within the order Echinoidea. We determined the density of spines around the equator of the animal, since this is the region of the body that views the horizon. We measured spine density by analyzing images of museum and other collected specimens and converted spine density measurements into angular resolutions. While we found little evidence of a phylogenetic signal in the trait, it does appear that spine density is more variable in deep, dark habitats than well-lit ones. This suggests that spine density may be subject to specific ecological pressures in well-lit environments (wave action, active predation, etc.) which can have a downstream effect on the visual acuity of the species.