Reef Stories
Land & Sea
A show in nature!
Fiddler and Hermit Crabs
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Although Fiddler Crabs are not strictly speaking reef crabs, they often do live in proximity to reefs and are so included here.
The different genera and species of Hermit crabs are found running over beaches, in the intertidal and clambering over the reefs themselves at night.
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Fiddler Crabs
If you have ever walked either on the seaward side or landward side of mangroves or close to tropical muddy beaches as the tide goes out, you can't have failed to notice small, often colourful crabs starting to emerge from their burrows. They are very shy and retreat with lightning speed into their burrows at the first sign of danger. If you very still and patient they will emerge in numbers and miniature spectacle of nature will start.
The claw movements have been likened to a violin player moving their bow, giving the 106 species and genera their common name, fiddler crabs.
The males start to raise and lower their enlarged right or left claw in various patterns to show other males how strong they are and to impress females. The males often have spectacular colour patterns which adds to the show. Several species are often found together
Fiddler crabs feed by scraping detritus up from the mud surface and then eating anything edible found. They turn the sediment around and around in their mouth-parts turning it into a round pellet which they drop when no nutrients are left. The action of feeding and burrowing make fiddler crabs important in bringing oxygen deep into the mud, stopping it becoming anoxic and thus dead of life.
As the tide comes back in the fiddler crabs retreat back into their burrows and carefully seal the entrance against incoming water.
Hermit Crabs
Those of us who wander along shell beachs whether in the temperate zones or the tropics will have seen hermit crabs scuttling about. Unlike other crabs which are completely protected by hard shells, they have a soft abdomen. By slipping their abdomen into a snail shells they gain protection they need.
There are some 1100 species spread across 120 genera and 7 familes. The Coconut Crab is the largest, in fact the largest terrestrial invertebrate on the plane. With a body length of up to 40 cm it can weigh over 4 kg. At the other end of the scale are the tiny filter-feeding pagurid hermits that live not in snail shells but in a dead corallite in a massive coral colony.
Terrestrial species are day-active and can be seen walking around looking for food and new shells. As their bodies grow, they must find a new shell that can accomodate further growth. This leads to heavy competition for shells where they are scarce. The battles for a new shell can be fierce and lead to the death of one protagonist. Aquatic species tend to be nocturnal, but are just as dependant on a supply of shells. Some (Dardanus spp.) go a step further to protect themselves and place stinging anenomes on their shells. These are carefully transferred to a new shell when the time comes.
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Fiddler Crabs
Fiddler Crabs
If you have ever walked either on the seaward side or landward side of mangroves or close to tropical muddy beaches as the tide goes out, you can't have failed to notice small, often colourful crabs starting to emerge from their burrows. They are very shy and retreat with lightning speed into their burrows at the first sign of danger. If you very still and patient they will emerge in numbers and miniature spectacle of nature will start.
The claw movements have been likened to a violin player moving their bow, giving the 106 species and genera their common name, fiddler crabs.
The males start to raise and lower their enlarged right or left claw in various patterns to show other males how strong they are and to impress females. The males often have spectacular colour patterns which adds to the show. Several species are often found together
Fiddler crabs feed by scraping detritus up from the mud surface and then eating anything edible found. They turn the sediment around and around in their mouth-parts turning it into a round pellet which they drop when no nutrients are left. The action of feeding and burrowing make fiddler crabs important in bringing oxygen deep into the mud, stopping it becoming anoxic and thus dead of life.
As the tide comes back in the fiddler crabs retreat back into their burrows and carefully seal the entrance against incoming water.
Hermit Crabs
Hermit Crabs
Those of us who wander along shell beachs whether in the temperate zones or the tropics will have seen hermit crabs scuttling about. Unlike other crabs which are completely protected by hard shells, they have a soft abdomen. By slipping their abdomen into a snail shells they gain protection they need.
There are some 1100 species spread across 120 genera and 7 familes. The Coconut Crab is the largest, in fact the largest terrestrial invertebrate on the plane. With a body length of up to 40 cm it can weigh over 4 kg. At the other end of the scale are the tiny filter-feeding pagurid hermits that live not in snail shells but in a dead corallite in a massive coral colony.
Terrestrial species are day-active and can be seen walking around looking for food and new shells. As their bodies grow, they must find a new shell that can accomodate further growth. This leads to heavy competition for shells where they are scarce. The battles for a new shell can be fierce and lead to the death of one protagonist. Aquatic species tend to be nocturnal, but are just as dependant on a supply of shells. Some (Dardanus spp.) go a step further to protect themselves and place stinging anenomes on their shells. These are carefully transferred to a new shell when the time comes.
