For hermit crabs, finding a shell is not an easy process. Unlike snails and mussels, hermit crabs cannot grow their own shell, and must therefore search periodically for empty snail shells to switch into as they grow. Not only must they find a shell that is not broken, but the shell must be the right size, and there is often considerable competition for these new homes. Hermit crabs have developed two separate systems for quickly and efficiently finding the perfect shell. These systems, known as asynchronous and synchronous vacancy chains, allow hermit crabs to trade shells with each other.
In asynchronous vacancy chains, when a hermit crab finds a suitable shell, it switches the old for the new one, leaving the old shell for others. When another individual finds the vacant shell, it switches into it, creating a chain where new crabs switch into each newly available shell. In this system, hermit crabs can take as long as they want to investigate, evaluate, and choose the perfect shell.
Synchronous vacancy chains are quite different. In this system, when a hermit crab finds a shell that is too large for it, it simply waits there. Smaller hermit crabs line up behind the initial shell finder, in descending size order, waiting for the shells to be vacated, while the initial finder stops searching and instead waits for a larger hermit crab to switch into the new shell. In this way, shells are efficiently passed from individual to individual. Smaller crabs have even been observed climbing up on top of the larger individuals in front of them, investigating the shell before it becomes vacant. Unfortunately, multiple chains sometimes occur, where two individuals find the same larger, unsuitable, shell. Only one chain will ultimately benefit. In these situations, the smallest individuals switch lines, vying for a place in the successful line.
Using such systems, crabs are ultimately able to save both time and energy when searching for new shells.
This summary was contributed by Thomas McAuley-Biasi.Edit Summary
“Synchronous vacancy chains occur after several crabs adjacent to an available vacant shell have queued in decreasing size order; as soon as the largest crab switches into the vacant shell, a rapid series of sequential shell switches takes place.” (Rotjan et al., 2010:639)
“In asynchronous vacancy chains, in contrast, individual crabs encountering a suitable vacant shell will switch and later their discarded shells will be discovered and occupied by other crabs.” (Rotjan et al., 2010:639)
“This suggests that waiting allows for a higher number of crabs to benefit from a single, introduced shell resource.” (Rotjan et al., 2010:645)