Empty hermit crab shells encrusted with coralline. As hermit crabs grow they need to switch to bigger shells and for this reason it is important to have empty shells in your aquarium. This particular group of shells are encrusting with coralline algae making them more attractive plus the benefit of spreading the coralline to your aquarium. Image is a representation. Size & shape will vary as well as the coverage of coralline on each shell. Some will be fully encrusted while others will have smaller amounts.