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March, 2013

Plant Eats Toad

by Valerie

In the grand spirit of tabloid sensationalism, I couldn't resist using such a silly sounding title. It is, however, descriptive of a real phenomenon, neither exotic nor the stuff of email-perpetuated urban legends. The setting for this curious event is our backyard pond. We have a number of different plants that live in the water, including submerged water weeds and floating species such as Duckweed and Water Fern. One kind of plant, Bladderwort (Utricularia sp.), has a remarkable adaptation for obtaining the nutrients it needs.

Usually found growing in acidic water such as swamps, Bladderwort has evolved to get supplemental nitrogen by trapping animals. Some other plants do this, including the Pitcher Plants, Sundew, and the famous Venus Flytrap. They all have modified leaves capable of catching small creatures, such as insects, and then digesting them with enzyme-rich fluids. What is really interesting is that all four of these plants have a different way of accomplishing the same result. Pitcher Plants have leaves that are filled with liquid; insects are lured to the lip of the pitcher and then fall in, usually because the surface is slick, and drown. Sundews have leaves covered with little projections, each bearing a droplet of sticky liquid. This substance acts like glue to capture the prey. The Venus Flytrap has a more active approach: its leaves are hinged, with prongs on the edges. When an insect touches one of the sensitive hairs on the inside of the leaf, the two parts snap shut and trap the victim. The Bladderwort also has the ability to move faster than is decent for a plant.

Bladderworts are small but sometimes extensive plants, with fine stems and slender leaves. The kind we have in our pond simply float near the surface of the water. They grow in messily tangled masses that look like green hairballs with little nodules on them. Under close inspection, the nodules can be seen to be small sac-like "bladders" with a branched projection near the opening. When tiny creatures, such as daphnia, water bugs, or other aquatic arthropods, touch the triggers, the lid of the bladder snaps open with such force that the suction causes the unfortunate critter to be sucked into the pod, where it is then trapped and digested.

Every spring, the Gulf Coast Toads that live in our gardens make use of the pond to mate and lay their eggs. The large adult toads have absolutely no need to worry about the Bladderwort. They spend the night busily laying long strings of eggs, the male clutching the female tightly as he fertilizes them. By morning, there are long strands of gelatin-coated eggs crisscrossing the pond, tangled amidst the vegetation in every direction. Unlike toad eggs in northern areas, which may take up to a couple of weeks to develop and hatch, those of the Gulf Coast Toad hatch in 24 hours. I was amazed to see this when I first moved here. The first morning after the toads' orgy, I could see the first cell divisions of the eggs. The second morning, they had hatched! It is obvious that the ephemeral nature of many water sources around here means that the toads must develop quickly, just in case their little pond dries up sooner rather than later.

Two tadpoles stuck on the pods of Bladderwort

Although the tadpoles have broken out of their egg casing, they are still merely fetuses just hanging on the strands of slime, completely unable to swim yet. However, a day later, they are wiggling and starting to move about. By the third day, they are swimming around. They've used up the yolk from their egg and must start to feed. But at that age, they are still small, weak and vulnerable. This is when the Bladderworts are a menace. As the tiny tadpoles slide over every submerged surface, grazing on the algae coating the plants, some of them inadvertently touch the triggers of the Bladderworts. The plant responds with a suction so quick and strong that it is difficult to catch even with high-speed photography. The frail little tadpoles are no match and, although they cannot fit into the bladder, they end up stuck to it, with a little of their body pulled inside. At first I thought they would eventually pull loose, but that doesn't happen. They die and decompose, remaining stuck to the Bladderwort for several days.

A tadpole is not yet a full-fledged toad, but it is still rather disconcerting to know that a plant, especially one so small and innocuous looking, can kill and consume a vertebrate. The stuff of science fiction is often not all that distant. Sometimes it is right in our own backyard.

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