This is a segment from BBC's Life Story, and it captures some pretty extraordinary animal behavior. When a new shell washes up on shore, these hermit crabs measure each other and then line up from biggest to smallest until a crab that’s the right size for the new shell comes along. Then the crabs pass their old shells down the line and grab a new, slightly larger one. That way, each hermit crab in the line finds its new home with little hassle.

“Staghorn Hermit Crab” (Manucomplans varians)

…a unique species of Pagurid hermit crab that occurs in the northeastern and southeastern Pacific Ocean, ranging from the Gulf of California to Panama.. Like other hermit crabs M. varians will find and use the shells of snails to house its soft body parts. True to what their common name suggests staghorn hermit crabs are almost always seen with a specific species of coral (Nanaria mirabilis) growing on their back. The corals typically grow quite large and create several striking spirals all while being carried around by M. varians. It is unknown if the coral being present has any negative effects on M. varians and it might even serve as a sort of defense/camouflage as M. varians can retreat into the coral and ‘seal’ itself inside using its claw. 


Animalia-Arthropoda-Crustacea-Malacostraca-Eucarida-Decapoda-Anomura-Paguroidea-Paguridae-Manucomplans-M. varians

Image: Ryanphotographic

In another times, hermits crab used shell from another mollusc groups, not just gastropods, but also ammonoid shells. As shows one of the few (if it isn´t the unique)  fossil pagurid inside a cephalopod shell

Ammonoids are an extinct group of cephalopods, unique in having full exoskeletal spiraled shells, as opposed to squids and octopuses.  However, despite having an external shell like modern Nautiluses, they are evolutionarily more related to the squids and octopuses.

Modern hermit crabs always use empty shells and never kill the original occupant,  probably these paguridos did the same.

Common Hermit Crab (Pagurus bernhardus)

Also known as the soldier crab, the common hermit crab is a species of pagurid hermit crab which is distributed along most of Europe’s Atlantic coasts. Common hermit crabs are typically seen in pools on the upper to lower shore regions in rock/tide pools. Common hermit crabs chiefly detritivores but they will also scavenge and filter feed if necessary. In typical hermit crab fashion P. bernhardus will use the shells of gastropod species, like Littorina sp. and Buccinum sp. for protection. 


Animalia-Arthropoda-Crustacea-Malacostraca-Decapoda-Paguridae-Pagurus-P. bernhardus

Image: Hans Hillewaert