The property market in Dublin has nothing on the cut-throat battle for suitable homes faced by the hermit crab. Yet it shows a particular preference for new mollusc shell homes even when these are in short supply.
Dr David K. A Barnes, of the Department of Zoology and Animal Ecology at University College Cork, has studied hermit crabs for years and has learned much about house-hunting crustacean-style. His latest findings are published today in the journal Nature.
The crabs prefer new mollusc shells when searching for the perfect home. However, he discovered Coenobita rugosus, a large tropical, semi-terrestrial hermit crab living along the coasts of Madagascar, will resort to using fossil shells washed out of seaside limestone if the number of suitable shells falls too low.
The crabs put a great deal of effort into finding a suitable shell, inspecting many before choosing. The hermits use the shells to protect against predation, against the risks of mechanical damage due to life on the surf's edge and also from desiccation, so a good fit is important.
The search is a lifelong endeavour because larger shells are needed as the hermit crab grows. This can put pressure on the availability of suitable shells with much competition for a good home.
Coenobita rugosus has found a way around this problem by occasionally making use of ancient fossil shells that break free along the shoreline.
"These unlikely mobile homes fall out of coastal limestone as it is eroded by the sea in southwestern Madagascar, placing the occupants alongside Homo sapiens as resourceful exploiters of prehistoric animal remains," Dr Barnes writes in Nature.
"Hermit crabs often suffer from a shortage of shells, however, and so are forced to make do with less suitable shells or even with artificial materials. Coenobita rugosus is one species that uses fossil shells as a substitute."
The hermits will shun these fossils even when they are plentiful, provided suitable empty shells in any locality do not fall below 13 per cent of all shells.
When the proportion of usable empty shells goes below 9 per cent, Dr Barnes found that the fossil shells begin to look attractive to some hermit crabs.
This expedient seems to be a particular trait of Coenobita rugosus. "One other local hermit crab species (of seven species investigated) was seen to use fossil shells but only on one occasion," he said. "Explanations for this behaviour are complex, as the crab is unlikely to appreciate the nature of a shell until it has inspected it."
It is definitely not a matter of any port in the storm for the house-hunting hermit. Their preference is clearly for newer shell material, according to Dr Barnes' results.
He found that the great majority of the fossil shells that break free and become available to the crabs are wholly unsuitable, mainly because of blockages that prevent the crustaceans from entering them.
"But the rarity of exploitation by hermit crabs of fossil shells cannot simply be due to the scarcity of usable fossil shells, because otherwise the extent of usage would be three times higher," Dr Barnes said.
Typically about 1 per cent of all shells that are usable are fossils, but the hermit crabs chose these shells only 0.3 per cent of the time.
Obviously new is better for the hermit crab who will wait until it can get the modern detached bijou res it desires rather than make do with an older, classic fixer-upper.
Dr Barnes notes that in southwestern Madagascar as in many parts of the world, shell collecting on the shoreline is an important resource for local craft workers.
Larger mollusc shells are preferred and smaller shells are often discarded. He speculates that this might increase the availability of select housing for the hermit crabs, making the use of fossils a thing of the past.