WEIRD CARNIVORES

First posted here on 2023-07-30.

If I say 'carnivore', you're probably picturing some kind of big cat or feral dog chowing down on a deer carcass. It looks up, head covered in blood and eyes sparking in the light of dusk, or perhaps it's chasing its prey terrifyingly fast, finishing the deal with a crack of the neck... Although you're all science fans. You know damn well that most carnivores are lazier than that, spending the majority of their time digesting their hard-earned food and conserving energy. But, well, a carnivore doesn't have to be an animal, or even have a huge, jawed mouth.

CARNIVOROUS PLANTS

Most people are familiar with the charismatic venus fly trap, and perhaps know of pitcher plants or sundews as well. But why bother? Plants make their own food using photosynthesis, so it doesn’t make much sense for them to eat other food as well. But much like animals, plants need a range of nutrients. Photosynthesis only produces carbon chains for plants to turn into sugary things like cellulose. Plants also need, for example, nitrogen so they can make proteins and other vital molecules. Ordinarily, bacteria and archaea in the soil fix nitrogen from the air, which plants can then take up. Sometimes this even takes place in a symbiotic relationship in structures called root nodules - see my explanatory poster for more information!

However, a lot of plants live in areas which are either deficient in nitrogen fixing bacteria or have such a high competition for nitrogen that very little remains in the soil. This kind of environment can be found in rainforests or swamps where there is a high density and diversity in plants, and little high quality soil. In order to avoid competition, plants have evolved carnivory to acquire nitrogen and other nutrients from a different source. That source is insects.

As you can see in this beautiful phylogeny by Ellison et. al, carnivory has evolved at least six times independently across the flowering plants. They also show up in some more ancient types of plant, but I’ll get onto that later. It’s possible to trace the ancestry of these plants from each other but for the sake of the length of this article I’ll restrain myself. For now, we can separate the different plants into five broad trap-types: flypapers, pitfalls, snap traps, bladder traps and lobster pots.

Flypaper traps are probably the most ancestral of the flowering carnivorous plants. These plants work with leaves modified to secrete a sticky glue. The plants then attract insect prey, catch them and digest them using externally secreted enzymes, absorbing the resulting goop through their leaves. A familiar one, at least in the UK houseplant industry, is the sundew, which boasts long hairs on its leaves to hold the glue balls and further entangle its prey. Some species are even able to quickly curl in their leaves to prevent the escape of either their prey or their prey’s digested carcass.

Pitfall traps, commonly known as pitcher plants, are by far my favourite and as such I will be careful to keep this part in proportion with the others. They have evolved three times - in Australia, North America and the Malay archipelago - with a remarkable convergence. They all feature a rolled-up leaf to act as the pitcher, which secretes digestive enzymes on the inside. You can see how this evolved from flypaper traps! The pitcher then has a lip on top which has a ribbed structure to channel insects inside the trap. The lip, or peristome, has microscopic hydrophobic structures to make it more slippery. They can have hairs and ridges on their outside to help insects climb them, and can be either grounded or epiphytic, living in trees. Furthermore, some go further than eating insects - some species of Nepenthes have evolved to be perfectly shaped like shrew and bat toilets, and collect their faeces to digest! My Sarracenia hybrid is called Patricia and has less disgusting eating habits, but is very convenient to stick next to an open window in summer to stop the flies coming in.

Here is a picture of Patricia by the way - note her lovely red trumpets! This is another attraction trick - she mimics meat as much as she can to attract her preferred prey, flies. A lot of other carnivorous plants do this too!

Moving on, our next type of trap is the snap trap, A.K.A. the venus fly trap. These bad boys are some of the speediest movers in the entire plant kingdom, capable of snapping shut fast enough to trap the slickest of insects before it can even react. While you shouldn’t trigger these traps unnecessarily as the huge energy expenditure with no reward can kill them, you can find plenty of slow motion footage of them shutting online. As Attenborough explains in this extremely dramatic clip, if a visiting insect causes two hair triggers to go off in quick succession, the trap will snap shut and digestive juices will release, killing it. I’m not sure that flies make that kind of sad squeaking as they die though, or that the fly trap burps as it finishes its meal.

Now we’re moving on to those which you may not have known about previously. Or at least, like me, you may have been unaware they were carnivorous. Bladderworts, the genus Utricularia, contain bladders as the name suggests. For many years botanists thought they were flotation devices as many of these plants have an aquatic lifestyle. However, the traps are actually poorly-understood (by everyone, not just me) vacuum chambers which upon triggering suck their prey into the digestion chamber. Bladderworts defy normal organs like shoots and roots, instead preferring to blend characteristics for their own purposes while eating prey varying in size anywhere from rotifers to baby fish. Crazy little guys!

So far, our plants have predominantly been preying on arthropods. However, Genlisea bucks the trend and is the first identified genus of plants to prey on protozoa. What may appear to be roots are actually subterranean leaves for this semi-aquatic plant. They form a winding, hairy maze which proves attractive to protozoans and, as they are shepherded towards the main body of the plant, digests and absorbs them as well. While they are less spectacular than some species I have mentioned so far, I reckon they’re some of the cooler ones if only because of how long they’ve hidden their true nature!

So, that’s a quick tour of the different types of carnivorous plants. I was also going to include some other strange carnivores in this article but it’s already way too long as-is so I may turn those into a supplemental at some other time. For now, I hope you’ve enjoyed these strange little friends. This certainly won’t be my last post about them!

Sources:

Barthlott, W., Porembski, S., Fischer, E. & Gemmel, B. (1998). First protozoa-trapping plant found. Nature, 392. doi: 10.1038/33037

Ellison, A.M. & Gotelli, N.J. (2009). Energetics and the evolution of carnivorous plants—Darwin’s ‘most wonderful plants in the world’. Journal of Experimental Botany, 60. doi: 10.1093/jxb/ern179

Miranda, V.F.O, Silva, S.R., Reut, M.S., Dolsan, H., Stolarczyk, P., Rutishauser, R. & Płachno, B.J. (2021). A historical perspective of bladderworts (Utricularia): Traps, carnivory and body architecture. Plants, 10. doi: 10.3390/plants10122656

Many thanks to Dr Chris Thorogood for several fascinating discussions as well as his excellent teaching.