HUNGRY UTRICS

by William H. Amos, Vermont, US

 

For each natural wonder there are enthusiasts whose special interests set them apart from others. Some people collect fossilized dinosaur dung, some raise earthworms for pleasure and profit, and around this time of year some examine roadside puddles and ponds to see if tiny carnivorous weeds have appeared. In summer and fall these curious plants remain inconspicuously submerged; only in spring do their slender stalks rise into the air, each bearing a delicate, colorful flower. The plants are different kinds of bladderwort, or Utricularia, and those who hunt bladderworts call their discoveries "utrics." For this article I propose to nickname the collectors "utrickers."

There are about 250 species of Utricularia around the world, from Arctic regions to Australia, South and North America, Europe, Asia, and Africa. We have several kinds living here in northern New England, but it's a sure bet that few people look at puddles, and most visitors to ponds and small lakes either have never seen the little plants, or dismiss them as just one more bit of weedy swamp vegetation.

Why should a utricker bother to seek these delicate aquatic plants? Because of their flowers and their curious way of life. I've noticed small bright yellow or blue and purple flowers of a few nearby varieties, but I know from color photographs that other utric species produce spectacular, sometimes crimson, flowers. With petals forming a pair of lips, the flowers are usually only about half an inch in diameter, so it takes a magnifier or a highly enlarged photograph to appreciate their beauty. Utric flowers from around the world are as colorful and varied as miniature orchids.

Bladderworts can be so attractive that avid utrickers grow and propagate as many species as they are able to find, then swap them or buy other varieties from fellow enthusiasts. It's a convenient pastime, for an entire utric garden may occupy only a countertop. For the true enthusiast, there are catalogs of available species and entire books devoted to utric growing. The hobby is well-ensconced, but only among Utricularia cognoscenti.

Better known carnivorous plants include sundews, the Venus flytrap, and pitcher plants, but even the most common bladderwort is as effective a trap as any of these. To appreciate a utric's "behavior," you must first get accustomed to its small size. Many common bladderworts float freely in the water because they have no roots. Not all utrics are aquatic, however, for some of the showy varieties elsewhere live in marshy soil and produce larger flowers, to the delight of collectors. The distinction between the two kinds is not always clear, for some terrestrial utrics can produce aquatic stages, and the water utrics may occasionally live in wet soil.

Aquatic bladderwort foliage is delicate and fern-like, but when hundreds of strands are bunched together, they form dense, compact mats in shallow water. The true nature of bladderwort is apparent only with very close examination, and here the reason for their English name is clear. The slender leaves bear large numbers of small pear-shaped bladders, each about an eighth of an inch long. Even the plant's name is derived from the Latin utriculus, meaning "little bag," and biologists call the bladders "utricles."

When empty, each bladder is indented from side to side and flattened on the bottom, with an opening at the end away from its supporting stalk. This aperture is a sort of mouth that is guarded by a hanging flap or door. Four bristles stick out from the lower edge of the door and serve as triggers. When a small animal—a rotifer perhaps—swims by and touches these sensory hairs, they twist and the shape of the unstable door-flap suddenly changes. It flies open and water rushes in to fill the compressed bladder. One instant a little animal is outside in the pond, the next it is inside the bladder and the door has snapped shut again, tightly sealed by a special collar of soft tissue. Soon much of the water is pumped out of the bladder, which begins to resume is partly flattened shape. The pump mechanism consists of clusters of small hairlike cells arranged in bunches of four that line the inside of a bladder. They absorb water, then conduct it outside the bladder to other special cells where it is passed off to the surrounding pond. This done, the trap is reset and within fifteen or twenty minutes it is ready for the next victim.

Captured prey swims around inside its prison for as long as a full day, unable to force the door open because it only swings inward. How effective are bladderwort traps? One observer found that a single bladder caught a dozen water fleas (small crustaceans known as cladocerans) in ninety minutes, possibly several at one time. Another plant with ninety bladders held a total of 270 captured crustaceans. If different species of Utricularia live in the same pond, they may trap distinctly different prey. This is due partly to a difference in average bladder size, partly to one species living close to the surface and another near the bottom.

It may be that planktonic animals, such as crustaceans, larval insects, and one-celled protozoans, as well as creeping worms and molluscs, are not caught simply by blundering into the sensory hairs. The trap is baited. The hairs produce an edible mucus that seems to attract those small creatures equipped with senses capable of sniffing or tasting it. Furthermore, a large colony of bladderwort creates a miniature aquatic jungle, so even without the enticement of dissolved scent, the tangled mass of plant stems is filled with small animals going about their everyday activities.

Bladderwort does not kill its victims. Within a day they die of confinement, lack of food, or perhaps from other natural causes. Bacterial action results in the victims' decay that releases nutritious proteins and other substances needed by the plant. Special digestive cells line the inside of a utricle and absorb food for the entire plant, and they also may produce enzymes to hasten the process. Once a victim's soft tissues have been absorbed, the left-over skeletal parts can't be expelled and remain inside the bladder. Utricles, therefore, contain a permanent history of past feasts that can be analyzed by the curious microscopist.

I've been only an occasional utricker. But when I'm out walking and notice bladderwort in pools and ponds, I admit whatever else I've been doing gets sidetracked as I bend down to examine what goes on in the utric world. To get really involved, I'll take a strand or two back to the lab to study and photograph. When you see all that this small carnivorous plant does with its construction of flawless mechanical traps made from common foliage, you realize again that nature truly is most wonderful in little things.

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.

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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 keyword to locate them.

 

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Published in March 1999 Micscape Magazine.

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