What Is This, an Overpass for Crabs?

Millions of red crustaceans cross bridges like this in Australia to reach their fertility festivities.

byKristin V. Shaw|
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You've heard about the success of the wildlife overpass in Utah and the 150-foot-wide Robert L.B. Tobin Land Bridge over Wurzbach Parkway in San Antonio, Texas. These clever creations were designed with both animals and motorists in mind; depending on where you live, you might encounter any numbers of critters of all sizes. You might hit one with your motor vehicle and it could walk away from a collision while you'll be left with a totaled car. 

Today I learned such a thing exists as a crab overpass. A crustacean highway, if you will, that gives the red crabs a fighting chance to meet someone special, hook up, and keep the species going. In Australia, millions of crabs emerge from the forest on Christmas Island to breed in the ocean, and the government will stop traffic and close roads (sometimes on short notice, the parks department says) to provide safe passage. The overpasses help route the crabs up and over moving vehicles to avoid the crunch of knobby tires. 

Parks Australia

"How do the crabs know to use the bridge?" is a popular response to a tweet from Parks Australia, which is celebrating the return of the red crabs. I guess my answer to that would be that a tiny crossing guard directs traffic. No, actually animals are pretty smart and they're constantly assessing risk and route, so this probably seemed like the better alternative to getting crushed by what might be seen as giant, scary cars from a crab's perspective. 

Each female crab can produce up to 100,000 eggs. She holds them in her brood pouch, and when the moon reaches its last quarter, the females leave their burrows and gather en masse on the shoreline. Parks Australia says you might see up to 100 crabs per square meter of beach or rock. No, they are not creating a remix of Noisestorm's "Crab Rave" from 2018. 

"The exact timing and speed of the migration is determined by the phase of the moon," says Parks Australia. "Red crabs always spawn before dawn on a receding high-tide during the last quarter of the moon. Incredibly, they know exactly when to leave their burrows to make this lunar date. However, because crabs wait until the first rainfall to start their trek, they sometimes have to hurry. If the rains arrive close to the optimal spawning date, they will move rapidly. But if the rain comes early they may take their time, stopping to eat and drink on their way to the coast."

I can't help it: now I'm picturing a tiny roadside diner just for crabs. Maybe some enterprising red crab posted one up at the top of the overpass. You've gotta root for the crabs to find a bridge and continue on their journey for a hedonistic spring break. YOLO, little guys. 

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