Open Architecture Network

The Hand Up Project: Attempting to Meet the New Needs of Natural Life-Forms
Elizabeth Demaray

Right now, 30 percent of all hermit crabs on our shorelines are living in shells that are too small for them. In the springtime, when the animal has its growth spurt, this shortage skyrockets to 60 percent. Hermit crabs, whose own bodies provide only thin exoskeletons, must scavenge and appropriate hard–walled shells abandoned by marine gastropods for shelter. The problem is that there currently are not enough shells left on our beaches for hermit crabs to use. This situation is not only uncomfortable but dire. Marine hermit crabs depend upon properly fitting shells for protection from predators (Hazlett, 1981), mating success (Hazlett, 1989) and reproduction (Childress, 1972). The present lack of housing is so severe that biologists now routinely find land hermit crabs attempting to shelter themselves in glass jars and whatever other ill–fitting forms of refuse they may find at their immediate disposal.

The reason for this housing shortage is generally assumed to be pollution and the collection of seashells by humans. However, because scientists have a difficult time asserting causal relationships in uncontrolled (that is, natural) models, we are unable to state specifically all the causes of this lack. On the other hand, due to the fact that hermit crabs exhibit choosing behavior in relation to selecting shelters, this population has been studied extensively. In controlled situations that offer ample housing, biologists have been able to identify the exact features that crabs find most desirable when they are assessing and selecting dwellings.

Shelter: A Crab's Perspective
The shells that hermit crabs seek are made by marine gastropods that secrete calcium carbonate from their mantel–the organ that covers their soft bodies. The shell is built up in deposits until the calcium carbonate becomes a crystalline structure held together via thin membranes of organic material. Depending upon the crystalline structure and the type of animal making it, the shell differentiates into numerous forms. The univalve-type shells that hermit crabs prefer to adopt are spiral in shape. The marine gastropods that make these shells form them in layered bands. These bands build a cavity that spirals from the shell's small center to successively larger areas of internal volume at the periphery. This formation affords the growing gastropod within the hard shell an ever–increasing area in which to expand.

Hermit crabs are scavengers and often locate these borrowed dwellings by smell, when the original gastropod inhabitant dies and begins to decay. Once a hermit crab adopts a shell, it will keep it until the shell is outgrown, carrying it continuously as a shield, wherever it goes. This is no easy feat, considering that a properly fitting shell must be larger than the hermit crab that wears it, and will often significantly outweigh the crab itself. In order to carry its home, one of the crab's front claws is completely dedicated to clutching the shell. This claw bends backward and holds on to the spool of calcium carbonate at the shell's center. In order to move, the animal must first use this claw to lift the shell and heave it onto its back. In spite of such difficulties, the drive to remain housed is so strong in this species that a typical hermit crab would rather be torn limb from limb than be pulled out of its shell. The only time that the animal will willingly leave its shell is a) if it locates another, more suitable one, or b) if it is shedding its exoskeleton– a process which can only be accomplished by fully exiting its dwelling just long enough to wriggle out of its own exfoliated shell casing.

When a hermit crab that has grown too large for its current home locates a new one, it determines the structure's suitability via a process called fondling. During this act