The study of onotgeny seeks to understand the developement of an organism throughout its lifecycle. An ontongisnistic perspective is crucial for hermit crabs and shell selection because new shell habitation and transfers are key for hermit crab development and reproductive success.

Starter Shell

After spending several weeks in the ocean as a larvae, the larval hermit crab, also called a glaucothoe, eventually reaches a point in development where it can colonize its first shell.  This first colonization is very similar to succeeding colonizations of new shells: the hermit crab finds the appropriate empty shell, flips the small shell upside down, enters the shell, and then extends its body long enough to be able to right the shell and continue movement(Reese, 1969).

Figure 1. The shell provides important room for growth and development.

Shells Limit Hermit Crab Growth

The hermit crab then enters the adult stage of its life and will continue to grow under the found shell’s protection.  It will develop a pair of lungs and begin foraging for detritus in the inter-tidal zone. This rich environment contains large quantities of nutrients, providing easy food sources for the hermit crab. However, although there is an abundant and diverse array of food, there is also a diverse array of predators.  Larger crabs, fish, and birds all present threats to developing hermit crabs. Not only does its original small shell present a relatively easy and soft target, as the hermit crab grows its shell provides it less overall protection. Thus there is pressure to find new, larger shells either through finding an empty suitably sized shell, or by challenging other crabs for there existing ones.

Figure 2. Crab size versus protection when crabs have free access
to shell they grow faster than in wild conditions. Finding the right size
shell is crucial for successful hermit crab growth.

The figure shows that shells are a limiting resource to crab size. If there are a wide variety of shells available a hermit crab will inhabit a larger shell than if there is a shortage of shells, as found in the natural condition. Thus a large part of a hermit crab’s success is in out competing other hermit crabs of both the same and different species for larger shells.

Ill-fitting Shells Increase Aggressiveness

Obviously larger hermit crabs will have an advantage in shell competition over smaller hermit crabs. However, larger hermit crab’s are not more aggressive (Hazlett, 1981), rather there is evidence to point to the ratio between size of hermit crab and the size of its shell in determining its aggressiveness (Grant, 1974). A large hermit crab with an insufficient shell has more to gain from an aggressive encounter than either a small hermit crab with a small shell or a large hermit crab with a large shell. Thus a crab’s shell competing strategy will change both with its absolute size, but also with it size in relationship to it’s current shell.