REPRODUCTION

Male hermit crabs are generally larger than the females and are therefore able to grasp the outside edge of the female's shell and drag it around  for several days until the female molts. When this occurs the male deposits  sperm on the female's abdomen. The female later uses the sperm to fertilize her eggs.   The fertilized eggs are retained in the shell by the female until they hatch as free swimming larvae.

GROWTH

Hermit crabs are encased in a skeleton which does not grow.  It must be shed, or molted.  The new skeleton can initially stretch, but because it is soft provides little protection from predators.   Hermit crabs have an advantage over other crabs in having a second line of defense, normally a snail shell, in which they can hide.  This shell still protects hermit crabs once their own shell is hard.

However, hermit crabs eventually outgrow their snail shell.   If there are lots of empty shells in the area,  hermit crabs will inspect new potential homes then quickly switch shells.   However, problems can arise if shells are scarce, resulting in one of several  courses of action 

 #1: KEEP OLD SHELL  

Occasionally a big  hermit crab can be seen with a tiny shell covering the tip of its abdomen.

#2: ABANDON SHELL  

This is an even more dangerous choice.  Sometimes "naked" hermits are found hiding in mussel beds.

 #3: MAKE DO  

Hermits have been found using all kinds of things for protection including:  dead barnacle shells, worm tubes, bottles, cans and in one case an ancient oil lamp!

Larva of a true crab (not a hermit crab)