Life in the Torrent

by William H. Amos, Vermont, US

 

The sweltering heat was enough to send Charlie the dog and me to our hillside brook where we waded in the deliciously cold water. He stood there, cooling off, day-dreaming. A biologist's eye soon got the best of me and I began looking at one of our area's most interesting small plants.

Many years ago, not knowing what it was, I first noticed a special kind of moss in a mountain flume. Where the water rushed over the brink of a large rocky ledge, a blackish carpet of plant life hugged the granite. While ignorant of its nature at the time, a sample taken to the lab told me it was Fontinalis, or torrent moss. Some years later, while preparing an article on the ecology of upland brooks for National Geographic, I spent frustrating hours standing in our own Stark Brook, trying to get underwater close-up photographs of the torrent moss community. Eventually I succeeded and in doing so, learned a little about this remarkable plant community and the miniature wildlife it shelters.

Fontinalis is one of only a few completely aquatic mosses in North America. More familiar sphagnum moss flourishes under wet conditions, but the few varieties of torrent moss live underwater and nowhere else. Looked at closely, structure of Fontinalis is not very different from ordinary moss found on rocks and logs, although its life cycle—alternating between sexual gametophyte and asexual sporophyte—is weighted heavily in favor of nonsexual reproduction, which means it either grows in vegetative fashion horizontally across a rock face, or produces spores that are whirled downstream to start some distant, future colony.

Only when flowing water is shallow and relatively gentle do sexual individuals, male and female, develop. Like any moss, the males produce swimming sperm that seek a female's egg, so if the brook they live in is deep and very swift, there is little chance of fertilization. Spores, on the other hand, develop directly within a special organ, the sporangium, that when mature releases countless thousands of protected cells into the water. If only a few eventually lodge in cracks in a faraway swift brook, they split open and tiny alga-like green threads (protonema) emerge to take hold, eventually growing into small leafy plants. Occasionally you'll find torrent moss in a clear, well-aerated pond, but never in stagnant water. Don't try to take this moss back to a home aquarium or to your lily pond, for the strands will soon turn brown and die.

There are very few plants anywhere in the world capable of withstanding the power of swiftly flowing water. To inspect a colony of torrent moss, use a glass-bottomed viewer and you'll see how tough these little plants can be. Their long stems bow downstream, tiny slender leaves trembling with the force of the water. That state of affairs is when my interest picks up.

I have been in tropical rain forests when major storms pass overhead. Treetops whip violently in the fierce wind, sometimes losing branches that are broken off and blown far away. Under these conditions, animals of the upper canopy must descend at least midway on the trunks where, despite the buffeting roar overhead, birds and butterflies fly safely through gently moving air of the lower forest spaces. Except for the sound above, the world is quiet even further down on the forest floor. Ants and lizards and other animals go about their business as if nothing odd is happening high in the treetops. Experiences of this sort always come to mind when I examine Fontinalis under a river's torrent.

To test this out, I made a tiny pitot tube, which is nothing more than a thin glass tube curved at right angles, one side short, the other long. (A piece of flexible transparent tubing would do as well.) I placed the short end facing upstream in the current, then watched how far the water rose in the long vertical tube. Out in the center of a rapid brook, the water reached high in the vertical portion. But while lowering the bent end slowly into the moss, I watched the water level drop in the upright tube, finally coming to rest exactly at surface level. In other words, all was quiet at the bottom of the torrent moss world, despite the storm of rushing water overhead.

With that, I had to see firsthand what was going on at that lowest protected level. Little by little experimental apparatus was devised until eventually I was able not only to watch tiny animal life existing within the torrent moss, but photograph it under magnification with a modified endoscope, the sort of instrument surgeons often use.

It turned out that the animal population in a Fontinalis "forest" is very large and composed of a wide variety of extremely small creatures and plants. Most would be unfamiliar to a non-biologist, but for those of us who use microscopes and strong magnifiers, they included round worms, segmented worms, rotifers, gastrotrichs, seed shrimp, water fleas, copepods, scuds, larval insects of many kinds, tiny adult aquatic beetles, and one-celled creatures without end. The plant world was represented by diatoms, desmids, and short strands of filamentous algae. Not one of these organisms would flourish, or even survive, for a moment outside its mossy home in a swift brook. The more I watched, the more I realized these plants and animals were zoned vertically in their tiny forest in exactly the same way birds and insects and mammals are vertically zoned among great trees during a storm. The analogies are exact; only the scale differs.

It is possible to carry the comparison even further, for in a tropical forest you eventually come to its edge, after which savanna and grassland are exposed to the full fury of a storm. Animals out there aren't going to fly about or walk safely upon the ground. They either huddle in a protected spot, or seek shelter inside a previously built burrow or nest.

A mat of torrent moss also stops somewhere along a brook's rock ledge, beyond which only a very few small animals are capable of existing without protection. One, an extremely flat zebra-striped stonefly nymph, hugs the rough granite with sharp claws at the end of wide-splayed legs, surviving in a boundary layer where friction slows the current a little. If it stood up, it would be plucked off and hurled downstream. Nearby may be little humped enclosures constructed from cemented sand grains, each containing its builder, the larva of a caddisfly. Near where torrent moss stops along the edge of a brook's riffle, tiny black "twigs" bending and being whipped by the current seem to mimic the moss strands. These are the remarkable larvae of the not-so-appreciated blackfly, the subject of another story. And where the current is swiftest, a tiny mountain midge larva inches its way along, held in place by the only true piston-operated suction cups in the animal kingdom, even more efficient than the suction disks of squid and octopus in the ocean.

It is difficult to enter the world of torrent moss, so different from ours in scale and force of energy. Study and even a free-wheeling imagination fail to unlock all the secrets of survival in such a place. As much as any subject worthy of study, it is a truly hidden world. But it is there—an unseen part of our larger everyday existence.

Comments to the author Comments to the author sent via our contacts page quoting page url plus : ('wamos','')">Bill Amos welcomed.

1999 William H. Amos

Bill Amos, a retired biologist and frequent contributor to Micscape, is an active microscopist and author. He lives in northern Vermont's forested hill country colloquially known as the Northeast Kingdom, and takes delight in studying the several ponds on his land.

Editor's note: Other articles by Bill Amos are in the Micscape library (link below). Use the Library search button with the author's surname as a keyword to locate them.

 

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