Markham, J. C. 1968. Notes on growth patterns and shell-utilization of the hermit crab Pagurus bernhardus (L.). Ophelia 5:189-205.

Hermit crabs live inside empty gastropod shells and use them mainly for protection. These shells are often switched out for larger ones to accommodate the growth of the crab, but what effects do shells of different sizes have on growth rates of the Pagurus bernhardus that inhabit them?

When some species of Pagurus known as dwarf crabs in certain regions were introduced to larger shells, they grew to maximum sizes never before seen in that area. Due to their hard exoskeleton, Pagurus must molt just as other crustaceans do. The growth of each molt and their frequency vary, but overall body growth is still continuous.

Markham collected 34 crabs of similar size and separated them into 3 groups. One group received shells that were too small, another kept the shells they were found in, and the last group was given shells large enough that they could completely retreat into them and they were provided with larger shells as they grew. By measuring their size after each moult and to determine growth rates, it was determined that the Pagurus in the larger shells grew both larger and faster than the other two groups.

The study confirmed that the size of the shell does have an effect on the growth rates of the crabs, if a shell is too small it can constrict the crab and stunt growth. The maximum size of Pagurus bernhardus is partially determined by the sizes of available shells.

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(posted Dec. 24 2015)