"Duck, Duck, Clam" or "Awesome Adaptations"

We were sitting underneath a tree in the mountain view cemetery on a sunny day in Oakland. The bay and the golden gate were laid out in front of us in all their glory and the dead bodies were laid out below us in all their sarcophagi.  Suddenly, a cute duck couple landed a few feet away from us.  They looked pretty much like this:

Cute duck couple: top left, male - bottom right, female.

We thought about giving them some food, but all we had were these disgusting circus peanuts in the shape of bunnies. We didn't want to poison the ducks with the same stuff with which we poison ourselves, so we kept the bunnies hidden.

Magic Eye of bunny-shaped circus peanuts.  Cross your eyes and a new image will appear (j/k).

The female duck was quacking and waddling around like a duck should, but the male duck was hobbling along pitifully a few feet behind.  Upon closer inspection it seemed that the male duck's foot had become totally rotten, as a dark mass could be seen on the bottom of its leg. It looked like this, except really gross:

Imagine this, except black and rotten.

After a few minutes of feeling bad for the poor rotten-foot duck, we realized that this dark mass was actually some sort of clam or mussel attached to the duck's foot!  Miles away from the nearest lake, a clam!

Aside: both clams and mussels are bivalves, a class of animals which wriggled its way into existence around ~500 million years ago. This is twice as many years ago as when dinosaurs first appeared on earth. They have been around a loooooong time.

The obvious next move was to chase the bird down and free it's foot, which we tried to do, but with no success.  It soon occurred to us however, that if we were to catch the bird and remove the clam, what would we do with the clam??  It was then that we realized that the clam stood a much greater chance of survival if we left it alone, as the duck would likely return to some body of water sooner or later and the clam could then let go and fall to the bottom with the rest of its clam and fish friends...

...and someday maybe even retire as Ariel's bra.

For real though, what a triumph for the clam, attaching itself to that duck's foot!  Think about it, that tiny little thing that usually has to move around by sticking a big foot/tongue thing out of its shell or clapping its shell together in a weird back-stroke move...

...it had managed to attach itself to an animal that had learned how to fly, and in so doing was attempting to spread his/her offspring further than it ever could have hoped to do otherwise. The clam had done it!!! His ancestors would be so proud!

We later discovered that Darwin had noticed this very phenomenon in 1882 (1):

"There can, I think, be no doubt that living bivalve shells must often be carried from pond to pond, and by the aid of birds occasionally even to great distances."

FINALE:  As noted above, bivalves came onto the scene ~500 million years ago, but the first birds didn't arrive until ~150 million years ago.  In the ~350 million intervening years, bivalves honed their shell-clamping-shut abilities to adapt to life in the sea.  However, when birds and their easily-clamp-able feet came along, they put this shell-clamping adaptation to use as a mechanism to spread their offspring to far off places via the sky!  Clams learned how to fly too!

This re-purposing of an adaptation is known as an exaptation.  Another example of an exaptation is the use of feathered-wings for flight, as the feathers were likely first evolved for heat-regulation and only later used in flight.

Life is so insane.

(1) Darwin, C. (1882) On the dispersal of freshwater bivalves. Nature 25:529-530.