An illustration of hermit craps shedding their shells and finding new ones. As hermit crabs grow they require larger shells. Since suitable intact gastropod shells are a limited resource, there is frequently strong competition among hermit crabs for the best available shells. The availability of empty snail shells at any given place depends primarily on the relative abundance of gastropods in the right range of sizes, compared to the demographics of the population of hermit crabs.
Harper's New Monthly Magazine (New York, NY: Harper & Brothers, 1871)