The Carnivorous Waterwheel


Bladderworts (Utricularia spp.) aren't the only carnivorous plants stalking prey below the water surface. Meet the waterwheel (Aldrovanda vesiculosa). At first glance it looks rather unassuming but closer inspection will reveal that this carnivore is well equipped for capturing unsuspecting prey. 

The waterwheel never bothers with roots. Instead, it lives out its life as a free floating sprig, its stem it covered in whorls of filamentous leaves, each tipped with a tiny trap. The trapping mechanism is a bit different from its bladderwort neighbors. Instead of bladders, the waterwheel produces snap traps that closely resemble those of the Venus fly trap (Dionaea muscipula). These traps function in a similar way. When zooplankton or even a small fish trigger the bristles along the rim, the trap snaps shut and begins the digestion process.


This similarity to the Venus fly trap is more than superficial. DNA analysis reveals that they are in fact close cousins. Together with the sundews, these plants make up the family Droseraceae. The evolutionary history of this clade is a bit confusing thanks to a limited fossil record. Today, the waterwheel is the only extant member of the genus Aldrovanda but fossilized seeds and pollen reveal that this group was once a bit more diverse during the Eocene. Whenever these genera diverged, it happened a long time ago and little evidence of it remains.


At one point in time, the waterwheel could be found growing in wetland habitats throughout Africa, Europe, Asia, and even Australia. Today it is considered at risk of extinction. Its numbers have been severely reduced thanks to wetland degradation and destruction. Of the 379 known historical populations, only about 50 remain in tact today and many of these are in rough shape. Agricultural and industrial runoff are exacting a significant toll on its long term survival. To make matters worse, sexual reproduction in the waterwheel is a rare event. Most often this plant reproduces vegetatively, reducing genetic diversity. What's more, natural dispersal into new habitats is extremely limited. 


Oddly enough, populations of this plant have popped up in a few locations in eastern North America. These introductions were not a mistake either. Carnivorous plant enthusiasts concerned with the plight of this species in its native habitat began introducing it into water ways in New Jersey, New York, and Virginia where it is now established. Oddly enough, these introductions have performed far better than any of the reintroduction attempts made in its native range in Europe. Of course, this is always cause for concern. Endangered or not, the introduction of a species into new habitat is always risky. Still, there is hope yet for this species. Its popularity among plant growers has led to an increase in numbers in cultivation. At least folks have learned how to cultivate it until more comprehensive and effective conservation measures can be put into place. 

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4] [5]

A Primer on Trigger Plants

I would like to introduce you to another group of plants capable of abrupt movements. Whereas many species have evolved moving parts as a means of capturing prey or deterring herbivores, the following genus moves as means of achieving pollination. Meet the genus Stylidium a.k.a. the trigger plants.

Native to parts of Asia and Australia, these beautiful little herbs are quite diverse, making generalizations difficult. Still, there is one thing they all share, a fused set of reproductive organs that lash out at unsuspecting pollinators. When a visiting insects of sufficient size lands on a flower, its weight causes a rapid change in turgor pressure within the column's tissues.

Stylidium debile     [SOURCE]

Stylidium debile  [SOURCE]

The rapid change in pressure sends the column flying. The position of this reproductive hammer varies from species to species. Some bash their pollinators on the back whereas others strike them under the abdomen. When the flowers first mature, only the male portions are ripe. Thus, the initial visit dusts the insect with pollen. Once the pollen is gone, the column resets itself and the female portions start to mature. The next time an insect visits the bloom, the stigma will do the bashing. With any luck, the visiting insect will have already been dusted with pollen from a previous plant. In this way, the plant avoids self pollination.

Another morphological aspect shared among member of this genus are the production of glandular trichomes. These minute hairs cover the body of the plant and produce sticky mucilage that ensnares tiny insects. It was originally thought that this was a merely a defense mechanism that may represent a form of proto-carnivory.

However, analysis of the mucilage revealed that plant is also producing digestive enzymes capable of breaking down insects unfortunate enough to have been caught. It remains to see whether or not the plants absorb nutrients in the same way as sundews but the fact that these plants share the same nutrient-poor habitats as many other Australian carnivores lends some credibility to asking that question.

Photo Credit: and Francis Nge

Further Reading: [1] [2]

A Carnivorous Plant and its Bug

Carnivory and symbiosis are two topics within the field of botany that are endlessly fascinating. Because they are static entities, plant evolution has gone through some very interesting pathways for survival. Recently, a group of plants found only on the southern tip of Africa have shone a light on yet another interesting plant/insect relationship that is unlike any other yet known to science.

The genus Roridula contains two species that, for all intents and purposes, look like carnivorous plants. They closely resemble sundews in having leaves packed full of sticky hairs that ensnare hapless insects. However, they are neither closely related to sundews nor do they have any sort of digestive enzyme for breaking down their insect victims. Why then would these plants go through the trouble of producing glandular traps? There must be some adaptive benefit to make up for the cost of production. A closer look at these plants revealed that indeed there is.

Living on Roridula plants are tiny capsid bugs that are covered in a special waxy substance that keeps them from getting stuck in the sticky traps. The bugs move about the sticky leaves, looking for trapped insects. When the insects are found, the capsid bugs impale them with their proboscis and suck them dry. As the capsid bugs feed, their droppings end up littering the Roridula leaves. This is how Roridula gets the added nutrients it needs to survive. Although the plants are not capable of actively digesting the full insects, they are capable of absorbing the components of the capsid bug feces. They are literally getting a little bit of fertilizer every time a capsid bug goes to the bathroom. By offering the capsid bugs a place to live and plenty of free, immobilized prey, the plant is able to get nitrogen-rich meals in return!

Photo Credit: Alex Lomas, CARNIVORASLAND, and Darwiniana

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Zoophagous Liverworts?

Mention the word "liverwort" to most folks and you are going to get some funny looks. However, mention it to the right person and you will inevitably be drawn into a world of deep appreciation for this overlooked branch of the plant kingdom. The world of liverworts is best appreciated with a hand lens or microscope.

A complete lack of vascular tissue means this ancient lineage is often consigned to humid nooks and crannies. Look closely, however, and you are in for lots of surprises. For instance, did you know that there are liverworts that may be utilizing animal traps?

Right out of the gates I need to say that the most current research does not have this labelled as carnivorous behavior. Nonetheless, the presence of such derived morphological features in liverworts is quite sensational. These "traps" have been identified in at least two species of liverwort, Colura zoophaga, which is native to the highlands of Africa, and Pleurozia purpurea, which has a much wider distribution throughout the peatlands of the world.

The traps are incredibly small and likely derived from water storage organs. What is different about these traps is that they have a moveable lid that only opens inward. In the wild it is not uncommon to find these traps full of protozoans as well as other small microfauna. Researchers aimed to find out whether or not this is due to chance or if there is some active capture going on.

Using feeding experiments it was found that some protozoans are actually attracted to these plants. What's more these traps do indeed function in a similar way to the bladders of the known carnivorous genus Utricularia. Despite these observations, no digestive enzymes have been detected to date. For now researchers are suggesting that this is a form of "zoophagy" in which animals lured inside the traps die and are broken down by bacterial communities. In this way, these liverworts may be indirectly benefiting from the work of the bacteria.

This is not unheard of in the plant world. In fact, there are many species of pitcher plants that utilize similar methods of obtaining valuable nutrients. Certainly the lack of nutrients in the preferred habitats of these liverworts mean any supplement would be beneficial.

Photo Credits: Matt von Konrat Ph.D - Biblioteca Digital Mundial (, HESS ET AL. 2005 (, and Sebastian Hess (

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A Plant With Lobster-Pots

Pitfall, pitcher, urn, snap, bladder, sticky - all of these words have been used to describe the various means by which carnivorous plants capture their prey. But what about "lobster pot?" Believe it or not, there is a genus of plants that has evolved a strategy for catching prey that would make lobster fishers proud.

That genus is Genlisea. It comprises roughly 30 species of what are common called "corkscrew plants." They are native to both Central and South America as well as Africa. These plants are small and can be found growing in saturated, nutrient-poor soils, conditions that select for any trait that can supplement what the plant can't get from its soil environment. Unlike more charismatic carnivorous plants, the meat-eating habit of this group would not be readily discernible to the casual observer.

Above ground they resemble their cousins the bladderworts (Utricularia). The flowers are quite showy and most species present them in either yellow or purple. At ground level sits a dense rosette of leaves. These are only part of the foliar picture. The corkscrew plants produce an entirely different set of leaves that take care of their nutrient needs. To find these, however, one must look underground.

Genlisea have no roots. Instead, they are anchored into the soil by truly bizarre, highly modified leaves. These leaves produce no chlorophyll and look absolutely nothing like what we expect leaves to look like. Instead, they form a hollow cylinder that corkscrews down into the permanently saturated soils in which it lives. This is where its carnivorous habits take place.

Along the length of each corkscrewed leaf runs a slit-like opening. Lining the mouth and inside of the chamber are backwards pointing hairs. Like a lobster pot trap, animals can enter these slits with ease. Getting back out, however, is nearly impossible. The only option trapped critters have is to continue onward to their doom. Towards the end of the traps sits a chamber where most of the digestion takes place. A quick caveat here: to say animals is a bit misleading. Most of what these plants are feeding on are small, soil-dwelling protozoans.

Regardless, the traps are quite efficient. It was only recently discovered that this was a true form of carnivory. Darwin himself had suggested it after careful examination but it wasn't until the 1990's that any digestive enzymes were detected. Still, it is a bit of a mystery exactly how or even if these plants actually attract their prey. Some researchers have found substances within the cylinders that are hypothesized to act as chemical attractants, however, more work needs to be done on this.

The traps don't spell certain death for all life. In an interesting study, researchers identified 29 different kinds of algae living inside the traps. Since dissolved oxygen is quite low inside, most of these algae are specialized for anoxic environments. The nature of the relationship between the algae and the corkscrew plants is not certain at this point. Some think it might be commensal whereas others feel that the algae may compete with the plant for phosphorus. Again, more work is needed.

The carnivorous nature of this genus isn't the only interesting aspect of their evolutionary history either. Some member of this genus, specifically Genlisea aurea, exhibit some of the smallest genomes of any flowering plant. This is not an ancestral state for this group meaning that at one time, the common ancestor had a much larger genomes but subsequent pruning has gotten rid of most of the "non-coding" sequences. Though there is plenty of speculation as to why this has happened, it is still anyone's guess at this point.

Photo Credits: NoahElhardt (assumed-Wikimedia Commons), Scott Zona (, and B Mlry (

Further Reading:

Going Veg With Nepenthes ampullaria

Carnivory in the plant kingdom is an interesting evolutionary adaptation to living in nutrient poor environments. It has arisen in only a handful of different plant families and indeed, the genera that exhibit it are considered highly derived. There is something to be said about a sessile organism that can take down mobile prey at the rate that most carnivorous plants do.

Perhaps part of our fascination with these botanical wonders stems from their move towards dietary habits not unlike our own. The reason for their predatory behavior is to acquire nutrients like nitrogen and phosphorus. Without these essential nutrients, life as we know it would not exist. It is no wonder then that carnivorous plants have evolved some very interesting ways of getting them into their tissues and to me, there is nothing more peculiar than the way in which Nepenthes ampullaria gets its much needed nitrogen fix.

A rather widespread species, N. ampullaria is at home in the understory of the rain forests of the southeast Asian islands. It differs from its carnivorous cousins in a multitude of ways. For starters, the pitchers of N. ampullaria are oddly shaped. Resembling an urn, they sit in dense clusters all over the jungle floor, below the rest of the plant. Unlike other Nepenthes, the pitchers have only a small, vestigial lid with no nectar glands. Finally, the slippery, waxy surface that normally coats the inside of most Nepenthes pitchers is absent in the pitchers of N. ampullaria. All of these traits are clues to the unique way in which this species has evolved to acquire nitrogen.

N. ampullaria doesn't lure and digest insects. Instead, it relies on leaf litter from the forest canopy above for its nutritional needs. The urn-like shape, lack of a hood, and clustered growth enable the pitchers to accumulate considerable amounts of leaf litter in the pitchers. Because the pitchers are relatively long lived for a Nepenthes, lasting upwards of 6 months, they offer up a nice microhabitat for a multitude of insect and even frog larvae. The collective group of organisms living within the pitchers are referred to as an inquiline community.

Photo by: Benjamin Tapley

Photo by: Benjamin Tapley

Over time, an inquiline community develops in each of the pitchers. This is the key to the success of N. ampullaria. As the inquiline organisms breakdown the leaf litter, they release copious amounts of nitrogen-rich waste. The pitchers can then absorb this waste and begin to utilize it. At least one study found that an individual plant can obtain 35.7% of its foliar nitrogen in this manner. It has also been demonstrated that the pitchers actively manipulate the pumping of hydrogen ions into the fluid within to keep it less acidic than that of other Nepenthes.

I don't know if I would consider this a case of herbivory as the nitrogen is still coming from an animal source but it is nonetheless an interesting adaptation. Instead of using valuable resources on actively digesting its own prey, N. ampullaria is getting other organisms to do the work for it. Not too shabby.

Photo Credit: Jonathan A. Moran, Charles M. Clarke, and Barbara J. Hawkins

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