I was climbing a steep logging road on a late spring morning when I came upon a puddle. There was nothing particularly notable about it, nor about the surrounding forest, which featured neither the mossy complexity of an old forest nor the tangled, prickly chaos of a recent timber harvest. Lying in a shallow depression in the tread of a former skid trail, now rewilding, the puddle’s waters were murky from recent rains. By all accounts, it was an ordinary puddle in an ordinary, middle-aged northern hardwood forest.

But as field naturalists are prone to do, I stopped anyway. Notebook in hand, I jotted down a few of the puddle’s dimensions while swatting away mosquitoes. I was looking for evidence of amphibian breeding—a diagnostic feature for determining whether this was a bona fide vernal pool or merely a glorified mud puddle. A barred owl feather floated on the surface, wood frog tadpoles darted among the detritus, mosquito larvae wriggled about, and near the edge of the puddle, attached to a small stick, was a cluster of eggs coated in green slime.

Upon closer inspection, I realized that the green slime wasn’t merely coating the egg mass—it was growing within the mass, in the gelatinous material in which the eggs sit. Standing in ankle-deep muck beside an old Adirondack logging road, I was looking at one of the strangest partnerships in the natural world.

Strange—but Symbiotic—Bedfellows

The eggs belonged to the spotted salamander (Ambystoma maculatum), and the mass’s green hue came from an alga (the singular form of “algae”) called Oophila amblystomatis, a name that translates fittingly to “lover of salamander eggs” or “a fondness for salamander eggs.”

The spotted salamander is a mysterious creature in its own right. For nearly the entire year, the species remains hidden beneath the forest floor, emerging only once during the first warm rains of spring to migrate by the thousands to nearby vernal pools. There, females deposit in the water gelatinous masses containing dozens to hundreds of fertilized eggs, identical to the one I had found, before returning below ground for the next 364 days. As the embryos develop through the spring, the “lover of salamander eggs” algae proliferate within the egg masses, turning their hosts a distinctive shade of green.

This association between algae and spotted salamander eggs has likely been observed for as long as humans have been peering into vernal pools, but the true extent of the relationship did not come into focus until quite recently. In 2011, researchers discovered that the algae were not merely growing within the egg masses’ gelatinous material—they were growing within the future salamanders themselves! It was the first known example of a photosynthesizing microorganism infiltrating a vertebrate’s actual cells.

The finding challenged long-held assumptions about the limits of symbiosis between life’s kingdoms. “Photosymbiosis,” whereby a photosynthesizing microorganism lives within a host, is a well-documented form of symbiosis, but usually involves and benefits a host that cannot produce or acquire its own food (also known as a “heterotroph”). Since spotted salamanders have no trouble finding their own food, they are not heterotrophs, making the relationship I witnessed all the more remarkable. (While not entirely accurate, the relationship is often referred to as the only known example of an animal “photosynthesizing,” though the algae are the ones doing the photosynthesizing.)

A Mutually Beneficial Partnership

Like most of nature’s enduring partnerships, both participants appear to gain something from the arrangement. The algae help oxygenate the egg mass and simultaneously absorb waste products, while the salamander embryos provide nutrients and carbon dioxide that fuel algal growth. It may seem like a fairly modest advantage, but when considering that only 1 in 200 eggs reaches the larval stage and only a small fraction of those larvae ever reach adulthood, even marginal benefits may be enough to tip the scales for natural selection to preserve the partnership.

This relationship is hardly the only oddity lurking in our wetlands: Wood frogs survive winter by freezing solid—their hearts stop until spring. Gray treefrogs, we now know, may actually be two nearly identical species, one carrying twice the number of chromosomes as the other. Mole salamanders have developed hybrid lineages that challenge our understanding of speciation. These are all evolutionary experiments still unfolding in the wet corners of our woods, in countless pools and puddles scattered across the Northeast.

And these fascinating puzzles are just a few of those among vertebrates, which make up only a tiny fraction of life’s diversity. The vast majority of organisms are small, inconspicuous, and poorly understood. Entire chapters of many of their natural histories remain unwritten, hidden in plain sight. Nature’s greatest mysteries are not always found in remote jungles or the old-growth cathedrals of temperate rainforests; sometimes the most remarkable stories unfold in the most ordinary places.

If such a relationship could remain overlooked for so long in one of the Northeast’s well-known amphibians, it raises an obvious question: What other evolutionary wonders have yet to be discovered throughout our rolling, rewilding landscape?

As an ecologist at Northeast Wilderness Trust (NEWT), I spend most days studying wild lands. As terrestrial creatures ourselves, things often feel more familiar on land—the sights, smells, and colors are welcoming, and the wildlife we encounter is recognizable and often charismatic. But the boundaries of a wilderness preserve do not stop at the river’s edge or the pondshore; they often extend into or encompass these freshwater systems. So every now and then my Wildlands Ecology colleagues and I turn our attention to the creatures that lurk in murky depths—those perhaps less photogenic but equally crucial and intrinsically valuable counterparts to the terrestrial species we know so well.

A Slippery Mystery

The American eel (Anguilla rostrata) is among the most fascinating and yet mystifying of these creatures. Once nearly ubiquitous in freshwater streams and rivers throughout Atlantic watersheds, the American eel is now largely confined to coastal waters and undammed inland waterways. Yet their story is bigger than the familiar tale of abundance severely diminished by human intervention. The American eel has puzzled scientists and naturalists for centuries, with critical details of the species’ biology still shrouded in mystery.

 

The American eel is North America’s only catadromous fish. They live much of their lives in freshwater (or brackish) environments but must return to the ocean to reproduce. This life cycle inverts that of anadromous fish like salmon, which live primarily in the ocean and return to rivers to spawn.

No one has ever observed eel spawning in the wild, but based on clues from their migration, we know that their life begins in the warm depths of the Sargasso Sea off the U.S. East Coast. There, adult eels spawn, producing tens of millions of eggs, and then perish. From those eggs hatch larvae (leptocephali) shaped like willow leaves, which drift along on ocean currents for months. As the larvae approach the continental shelf, they metamorphose into “glass eels”—transparent, androgynous juveniles—and begin the journey inland toward estuaries and rivers.

Once in fresh or brackish waters, they transform again, this time into elvers, and later into yellow eels, spending years to decades feeding and growing in streams, lakes, marshes, and rivers. In the yellow eel phase, they are nocturnal, hiding in sediment, root tangles, or under logs, nourishing themselves with a wide diet of macroinvertebrates, crustaceans, small fish, and amphibians.

When yellow eels reach maturity, they change once more, now into silver eels—their bodies develop sexual organs, their eyes enlarge, and their digestion shuts down. Fat reserves sustain them on the final, long migration back to the Sargasso Sea, during which they will not consume a single meal. There, the adults spawn and die, leaving their offspring to begin the cycle anew. Since they only develop sexual organs at this final, brief stage in life, leaving them little time to breed, scientists have struggled for centuries to figure out how reproduction occurs among eels.

American Eels as an Indicator Species

This catadromous life cycle, dependent on both healthy fresh- and saltwater systems, makes the American eel a reliable indicator of wild, connected aquatic ecosystems. They move nutrients and energy from the sea far into inland waters, reaching headwater ponds and streams that other migratory fish cannot. Eels are both predator and prey, a food source for herons, otters, larger fish, and people, while also regulating populations of invertebrates and small vertebrates.

Their ability to live in salt, brackish, and freshwater—and even to climb small waterfalls and cross damp ground between waterbodies—places them among the most adaptable fish on the continent. That such an adaptable species has declined so markedly demonstrates the severe disruption that barriers like dams and culvert wreak on aquatic ecosystems. These obstructions block eels’ upstream journey as juveniles and their downstream migration as adults. Global culinary demand for glass eels puts added pressure on already stressed runs, and shifting ocean currents caused by climate change may disrupt the timing of their larval drift.

In some places, like the Penobscot River, removing obsolete dams or replacing undersized culverts has already begun to restore eel runs and the health of entire watersheds. Conserving riparian forests, wetlands, and estuaries, and allowing natural processes to restore ecological integrity through passive rewilding, also benefits these migratory fish and countless other species that depend on clean, connected, and resilient waterways. When we conserve forever-wild land with an equal focus on both water and earth, imagining not only old, intact forests but also unbroken aquatic pathways, we give the American eel—and the watersheds it sustains—a chance to recover.

On a cool and breezy summer afternoon, a team of seasoned ecologists and I made our way across a steep talus slope in a remote section of Vermont’s Groton State Forest: a 25-acre stand of ancient hardwoods known as Lords Hill. Tucked into the hills of the town of Marshfield, this forest is a rare remnant of an older age—one of the few places in the state where towering sugar maple, yellow birch, white ash, hemlock, and basswood have been left to grow, die, decay, and regenerate largely undisturbed for centuries.

We were there to revisit a long-term monitoring plot established decades ago and to measure the diameter of trees tagged as early as 1977. With me were ecologist and naturalist Charlie Cogbill, our leader, NEWT board member Brett Engstrom, NEWT’s Wildlands Ecology Director Shelby Perry, and Rose Paul, former director of Science and Freshwater Programs for The Nature Conservancy. We slowly and systematically worked our way across 50-by-20 meter grids, calling out data while Charlie stood at each grid’s center, clipboard in hand, scribbling notes, and confirming each measurement with familiarity and enthusiasm.

“Yellow birch. Tag number 451. Diameter 72 centimeters!” I called out across the talus slope, my voice bouncing off of moss-covered granite boulders.

Charlie, perched atop one such boulder, flips through his notes. “Yellow birch… 451… YES!” he exclaimed triumphantly. “That one put on four centimeters since 2002,” he added, grinning.

 

What happens when a forest is given the freedom to rewild after a long history of logging? This might seem like a question that science and ecological research have already answered; that each step of the rewilding process has been mapped out with great precision, telling us exactly what happens in the absence of active management.

After all, we know very well what happens under active management. Many land managers work hard to produce the outcomes they want, constantly tweaking variables to shift species composition, adjust light levels, or thin out tight tree stands. This management leads to fairly predictable results—why shouldn’t the opposite, passive rewilding, entail the same predictability?