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Big view of patch of water shield. Photo credits: Emily Stone

We could not have asked for a more perfect morning. The black, tannin-stained waters of the Bark Bay Slough were perfectly calm under a warming sun as we launched my trusty Old Town Penobscot 16 canoe from the dock. We paddled easily, blissfully, into the morning, savoring the magic of floating through a beautiful place.   Bark Bay opens to the northeast, and nestles between two ridges of erosion-resistant sandstone on the western face of the Bayfield Peninsula near Herbster. It is the estuary of the Bark River, and the back of the bay is almost entirely cut off from Lake Superior by a giant sandbar. Boggy little islands dot the enclosed slough—each with their own arrangement of weather-sculpted bonsai trees. Exhibiting both scrappiness and grace, the stunted white pines, black spruces, and tamaracks twist toward the sky. Around their bases crowd thickets of leatherleaf and sweet gale dotted with wild roses.  

As my cousin Alyssa and I paddled slowly eastward through the island maze, mats of bog and fen vegetation crept out from the shore and narrowed our passageways. We never knew if our next turn would bring us to a cul-de-sac or a channel. That slight element of mystery was delightfully thrilling, and brought back memories of playing pretend with Alyssa in my grandpa’s shrubbery. Add to that a shy painted turtle just poking his head out from under a water lily leaf, and we were completely entertained. I don’t often have a day where my curiosities so perfectly match that of my adventure buddy. On this day, I savored every breath.  

Water shield fall colors

Following our interest, and beckoned by their beauty, we spent at least ten minutes in a patch of water shield, Brasenia schreberi. The palm-sized oval leaves of this aquatic plant float on the surface of calm waters. A thin, flexible stem attaches to the middle of each leaf’s underside and tethers it to a horizontal stem that runs along the muddy bottom. Clear, slippery jelly coats the undersides of the leaves and protects them from grazing snails and harmful microbes. When it grows in thick patches, which is often, water shield creates a spectacular mosaic of color, especially in late summer when brilliant bursts of red and yellow paint the dying leaves.   Other patterns add interest to a patch of water shield, too. Most of the glossy leaves are riddled with holes or dissected by squiqqly lines, which all look black against the tannin-stained water.  

Top of the leaf, showing gelatinous mines

Backlit, showing contents of mines

What would make those marks? And was it still around? One by one, I held a few different leaves up to the sun, and tried to capture this living example of stained-glass beauty. To my surprise, these trails differed from other leaf mines I’ve seen. Leaf miners are a group of insects (including moths, sawflies, flies, and beetles) whose larvae feed on the insides of a leaf between its “skins.” These weren’t mines, though they were channels, in some cases now worn clear through the leaf, with only a few of the tougher veins keeping the sides of the seam from ripping apart.   Another, more subtle difference, was that although not all the trails were the same thickness, each trail was the same width from beginning to end. What I’ve enjoyed about most other leaf mines is that they start small and get bigger, allowing me to imagine the little larvae growing up.  

Perhaps the only person who’s really investigated the origin of water shield’s leaf marks was a fisheries biologist named Adelbert L. Leathers in 1922. From his writing, I learned that the squiggles are actually channels and not mines. In a mine, the larva burrows between the upper and lower skins of the leaf, eating the innards, but leaving both of the outer layers intact. In contrast, these channels in the water shield are created by removing the upper epidermis entirely.  

With weak mouthparts, the midge larva Polypedilum braseniae, cuts off a strip of the epidermis, scrapes it clean of nutritious green cells, and then uses silk to fasten the strip into a roof above its channel. This setup allows water to continuously flood the channel. The larvae breathe through blood gills and will die if they dry out. Even submerging too deeply will kill the larvae, presumably because they need the super-oxygenated water you find near the surface and in contact with living plant cells.   Mr. Leathers watched as green plant cells moved through the digestive tract of the pale yellow larvae. He noticed that when the food exits the other end as frass (poop), it’s still useful. The larva places frass where it can help hold a strip of epidermis to make it easier to scrape, and it eventually becomes part of the channel roof. After seven to ten days, the larva spins a silken pupal case and metamorphoses into a tiny midge with shimmering wings and an ethereal green abdomen.  

Still, much mystery remains. Although Mr. Leathers was able to find the midge’s eggs on submerged water lily leaves and successfully hatch them, he could not get the tiny larvae to grow or survive. They only arrive at the water shield leaves to begin channeling after they are mid-way through their larvalhood. The strength it takes to cut through the epidermis limits the size of larvae that can make a channel, and the larvae would rather start in an old channel and branch off than go to the work of sawing through fresh material.  

I love discovering that there are unsolved mysteries in science. When I start investigating something new to me, I never know if my next question will open up to a cul-de-sac or a clear channel. That element of mystery means that observing nature is always delightfully thrilling.  

Special Note: Emily’s book, Natural Connections: Exploring Northwoods Nature through Science and Your Senses is here! Order your copy at http://cablemuseum.org/natural-connections-book/.  Listen to the podcast at www.cablemusum.org!  

For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: "Better Together--Celebrating a Natural Community" is now open!