WANTED: 

 Used Snail Shells 



ALAN SOLEM, Curator, Lower Invertebrates 



What happens to dead sea snails? 

 The soft parts are either eaten by 

 the predator that killed the snail or 

 quickly cleaned out by the busy scaven- 

 gers of the ocean. Remaining is the 

 rock-like shell. Eventually it is battered 

 about and ground into fragments by 

 waves, dissolved by boring organisms, 

 or, together with coral and clam frag- 

 ments, cemented into coralline rock. All 

 this takes time, however, and meanwhile 

 the empty shell has probably been put 

 to good use by several generations of 

 hermit crabs. 



These decapod crustaceans, despite 

 their name, are more closely related to 

 shrimps and lobsters than to true crabs. 

 More than 1,200 species are known, and 

 hermit crabs are common shore inhabit- 

 ants of most oceans of the world. The 

 front portion of the crab and its append- 

 ages are heavily armoured with calcified 

 plates. However, the rear portion of the 

 body is soft and would be highly vulner- 

 able were it not for the fact that the crab 

 wedges his tail and abdomen into an 

 empty snail shell. Once inside, hook- 

 like structures on the crab's tail append- 

 ages cling to the inside of the shell, mak- 

 ing it difficult to pull the animal out. 



Nearly all snail shells coil to the right 

 and the abdomen of most hermit crabs 

 is flexed to the right to permit easier en- 

 trance into the snail shell. Young her- 

 mit crabs use small shells, changing to 

 larger ones as they grow. Often a pro- 

 posed new home will already have an- 

 other hermit crab inside, and occa- 

 sionally two crabs will fight over an 

 unoccupied shell. If no suitable shell 

 can be found, the hollow tubes con- 

 structed by marine worms, bamboo, or 

 even a discarded metal cold cream tube 



Above: The large claws 

 of a hermit crab peer out 

 of the mouth of a Florida 

 fighting conch shell. 



or glass vial will be used to protect the 

 crab's soft abdomen. 



The most commonly seen type of her- 

 mit crab appears to be no more than 

 "big claws and skinny legs" peeking out 

 of a snail shell. If the shell is large 

 enough, the crab can withdraw com- 

 pletely out of sight inside the shell. 

 Many shell collectors have picked up an 

 "empty" shell and placed it carefully on 

 the beach in a pile to be taken home 

 later, only to have it walk away because 

 of a hidden hermit crab. 



Other hermit crabs are more special- 

 ized. The Pacific Ocean snail, Merita 

 polita, lives on rocks in shallow ocean 

 waters and has a thick calcareous plate, 

 the operculum, on its foot. When the 

 snail retracts into its shell, the operculum 

 tightly closes the opening, protecting the 

 snail both against drying out and being 

 eaten by enemies. Empty Nerila shells 

 are inhabited by hermit crabs of the 

 genus Coenobita, whose claws are modi- 



Left: Claws of the her- 

 mit crab, COENOBITA, 

 {left) seal shell opening 

 as effectively as the orig- 

 inal inhabitant's opercu- 

 lum (right). 



fied to form almost as effective an aper- 

 tural seal against enemies. This is a 

 quite marvelous set of parallel adapta- 

 tions against predators. Both snail and 

 crab seal the shell opening, but the snail 

 uses a disc on its foot and the crab its 

 claws. 



Undoubtedly this use of empty snail 

 shells by crabs has been going on for 

 millions of years. An example of the use 

 and reuse of the same shells by successive 

 generations of hermit crabs is seen in 

 Bermuda. There, a large snail, Livona 

 pica, became extinct apparently during 

 glacial times. But a large hermit crab, 

 which can use only the shell of the ex- 

 tinct snail for protection, is still common 

 in Bermuda. Apparently this crab will 

 exist until the last empty shell of Livona 

 pica has been cemented into rock or 

 broken into fragments. This is a striking 

 example of how the utmost use is made 

 of every substance and structure in the 

 economy of nature. 



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