Ryan Photographic - Coenobitidae - Land hermit crabs
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Family Coenobitidae
The land hermit crabs are a wildly successful family despite only being represented by 17 species in two genera. They are typically tropical species and may be encountered in great numbers in most tropical supratidal regions. Apart from coconut (robber) crabs, they all protect their soft abdomen with a molluscan shell or some other hollow receptacle (in some of the Solomon Islands they squeeze themselves into left over empty WW2 cartridge cases). Below I show one in an empty light "socket" and another in a plastic cap - they are nothing if not adaptable.
As they grow, the hermits require larger shells which obviously puts a limit on the size which they can achieve. They often fight over empty shells - and the bigger they get, the fewer the potential "homes" are available. The coconut crabs are basically the meanest mothers on most of the islands where they live - so they only use shells when they are small. As they get bigger their tough exoskeleton and powerful pincers protect them against most threats.
In a fascinating study on the tiny island of Ugaga in Fiji's Beqa lagoon, Katherine Szabo of the University of Wollongong investigated shells from an ancient midden (essentially a rubbish dump). She found that 63% of all shells in the midden had been inhabited by hermit crabs and that figure reached 85% of the four most preferred species. Here is a quote from the abstract of her paper.
At Ugaga, hermit crabs were found to have removed the majority of shells from the midden and had deposited their old, worn shells in return. The behavioural ecology of genus Coenobita suggests a mutualistic interaction whereby humans make available shell and food resources to hermit crabs, which in turn provide a site cleaning service by consuming human and domestic waste.
This is an extraordinary finding. Of course, the mutualism is unintentional, but it is a fascinating concept. Not only did the midden provide the hermits with food but also a continual, reliable supply of shells of just the right type. (Szabo was able to distinguish shells that had been occupied by hermit crabs from those that hadn't by changes to the shell structure).
All the land hermit crabs possess reduced strengthened gills in a highly vascularized branchial chamber that basically acts as a lung which must be kept moist for gas exchange to work. Robber crabs, and some others, may pump air through the chamber by muscular contractions. They also pass air into the chamber in the same way that marine crabs move water. They have flaps, called scaphognathites, on their second maxillae which fan air into the chamber. Other than the need to return to the sea to release larva and for their larval existence they function very well terrestrially.
Hermit crabs mate on land. They both partially leave their shell and the male transfers a spermatophore to the female. When she is ready, she ruptures the sac and fertilises her eggs which she carries for around a month. When they are ready to hatch she returns to the sea where the released eggs emerge as zoea. These undergo several molts until they become megalopae, after which they move back onto land and become tiny crabs (looking for tiny shells!).
Although hermit crabs are popular pets most of them don't survive long in captivity. The pressure group PETA (People for the Ethical Treatment of Animals) counsels against keeping these fascinating beasties. Recently, some advances have been made in captive breeding of hermit crabs. But if you are interested in one as a pet you need to be there for the long-haul. Some captive hermit crabs have lived as long as 40 years.
Birgus latro Coconut or robber crab
Birgus latro, Kri Island, Raja Ampat, West Papua IMG_0824
Birgus latro, Robber or coconut crab, Kri Island, Raja Ampat, West Papua IMG_0827
Birgus latro, Kri Island, Raja Ampat, West Papua IMG_0829
Birgus latro Coconut crab head shot
Birgus latro, Coconut crab showing pincers (chelipeds), Western Samoa
Birgus latro Coconut crab Rennell Island
Coenobita perlatus "Strawberry" hermit crab
Coenobita perlatus "Strawberry" hermit crab
Hermit crab with plastic scoop, Kadavu, Fiji
Hermit crab in light socket, Magic Bay Rao, Morotai IMG_7447
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