The Lake Where You Live

If you look down into the water from your pier, you might see tadpoles lying on the bottom. As a kid I haunted what we called the Pollywog Pond, a marshy puddle in the middle of a residential neighborhood where tadpoles abounded.

I would catch a few with a little scoop net, take them home, put them in an aquarium, and see if they would change into frogs. A couple of times a tadpole developed front and hind legs, but I never produced a fully formed frog.

What amazes me now about tadpoles, but didn’t occur to me then, is how their metamorphosis includes a complete change in the manner of breathing. Going from tadpole to frog means switch from gills to lungs, and those ways of conveying oxygen into the bloodstream are very different.

Lungs draw in atmospheric air, which is 21 percent oxygen. The oxygen exchange takes place in tiny sacs called alveoli that are surrounded by capillaries – micro-scale blood vessels. Oxygen diffuses into the capillaries, and carbon dioxide diffuses out.

The basic principle of gills is the same – they are incredibly rich in capillaries where the gas transfer happens. But the concentration of oxygen in the water is extremely low compared to the air above. In a well-aerated lake, the oxygen in the water is about eight parts per million, or 0.0008 percent. That means the gills must be extremely efficient,

So how can a tadpole that hatches from an egg with gills make the conversion to lungs? It means growing an entirely new set of organs and shedding the old one. It turns out that the lungs start to develop just a few weeks into the metamorphosis.

And the gills remain until the transition to lungs is almost complete. So for a time, the tadpole has two ways to acquire oxygen: There’s a period in which they can test drive their new respiratory system.

You may have seen a tadpole come up to the surface and then dart back down. What it’s doing is taking a sip of air. Ultimately the gills disappear and the newly minted frog is ready for life lying in the water, eyes and snout exposed, breathing contentedly. Or, in the case of species like tree frogs, living nearly full-time on land.

On our property, the grandsons enjoy chasing leopard frogs that make their way up our long, steep lakefront hill to hide out in our “lawn” no-mow grass, wildflowers and weeds.

Frogs aren’t the only creatures that change how they breathe as they go through their lifecycles. Dragonflies, for example, spend time in the water and breathe through gills. The adults don’t have lungs; they take in air through small openings, called spiracles, in the sides of their abdomen. From there, oxygen travels through branching tubes that get progressively smaller and feed oxygen directly to the cells. 

I can’t help but be amazed at all this, but I must say I am perfectly content with a simple life of breathing through lungs.

Ted Rulseh, a writer, author and advocate for lake protection, lives on Birch Lake in Oneida County. Visit him and his blog at https://thelakeguy.net.