Tip:
Highlight text to annotate it
X
The red-clawed crab is highly adapted to life on land, able to inhabit seaside forests and other places if there is even the slightest big damp.
While red-clawed crabs are known to dig their own nest, they often intrude on holes dug by sesarmid crabs that share a similar ecology.
Sesarmid crabs aren’t as well adapted to land as red-clawed crabs, so they never fare too far from the seaside.
Red-clawed crabs, on the other hand, would be capable of living deep in the forest if they wanted to.
So, why is it that the red-clawed crab never ventures far from the ocean?
The river mouth comes to life at dusk when the spring tide arrives between July and August.
Inherently cautious red-clawed crabs begin a mass exodus to the river over dangerous obstacles including roads, scrambling for the river in a short-lived spectacle that lasts less than an hour.
Once at the riverside, red-clawed crabs size up the waves generated by high tide and wade their way into the water where they carefully drop off the larvae attached to their abdomen.
This act is sometimes referred to as egg laying, but in the case of the red-clawed crab the already hatched eggs are “released” into the water.
Crab larvae are divided into two stages of growth, with the newly-hatched larvae called zoea.
For reasons not yet fully understood these zoea need a salt water environment to grow, a fact that also applies to the shore-going red-clawed crab.
Close inspection of larvae releasing behavior reveals several fascinating phenomena.
If there are man-made objects on the riverside, red-clawed crabs only use those going right up to the water’s edge as if to evaluate the tide.
It’s thought that this is to prevent them from falling into the water and drowning.
Whatever the case, they must have superb eyesight to gauge the water’s surface under a new moon.
Even when entering from the shore, red-clawed crabs only ever jump in towards the flow of the high tide.
They may be sensing the temperature difference between seawater and the river water coming from upstream, an amazing feat, if true, as they manage to do it from a distance without touching the water at all.
After releasing the larvae, female crabs rush back to the forest only to be ambushed by males seeking to reproduce.
The reproduction cycle from mating to hatching of larvae is around 1 month, meaning that a single female releases zoea into the river 2 or 3 times each season.
The biggest mystery of all is their ability to detect the spring tide while living in the forest.
There may be grassland on the way from the forest to the river mouth that leaves them highly vulnerable to predators such as birds.
It is thought that the releasing of larvae takes place between July and August as the spring tide and sunset coincide.
This goes well beyond the realm of visual and temperature sensors and would not be possible without a sensor for precisely measuring the tide.
There are few places now where red-clawed crabs can be seen releasing larvae as it is rare to find forest connected to the sea without obstructions like roads.
Once upon a time it was normal to see countless red crabs crossing seaside roads on their way to the ocean.
The behavior of some of the most familiar animals still remains a mystery.
There is still much speculation about what sensors red-clawed crabs use to measure the world around them.
Humans do not have such refined sensors with which to measure the natural world.
However, humankind posses the power of analysis, one that goes beyond our natural limitations.
The protection of this diverse and beautiful earth through the analysis of nature is a mission that has been entrusted to humankind.