The first carnivorous plant you have probably come across was the infamous Dionaea muscipula, more commonly known as the Venus flytrap. Over 800 species of carnivorous plants are currently known to science, according to Hans Lambers’ book A Jewel in the Crown of a Global Biodiversity Hotspot.
The Venus flytrap has six trigger hairs that cause the two hinged lobes to snap shut when prey comes into touch with them (ThoughtCo).
What makes these plants carnivorous? Aaron M. Ellison and Lubomír Adamec say there are 19 genera of these plants that comprise at least ten evolutionary distinct lineages linked by a group of shared, functional traits that allow plants to get the majority of their nutrients through the attracting, catching, and digesting of prey. Even though these functional traits or physiological processes can occur, usually individually, in noncarnivorous plants, they are usually seen together in carnivorous plants, where they are combined in series. Carnivorous plants possess these coordinated sets of characteristics known as the “carnivorous syndrome”.
Biogeography
Charles Darwin, the British naturalist and evolutionary theory pioneer credited for the theory of natural selection, first predicted that botanical carnivory is most advantageous in nutrient-depleted environments. Researchers Juniper et al. have also discovered other significant elements, such as calcium concentration of soils, soil moisture, shade, and competition with noncarnivorous plants.
According to the San Diego Zoo Wildlife Alliance, carnivorous plants are found worldwide, with environments ranging from acid bogs and alkaline pine barrens to frigid streams of melting snow and steamy tropical rain forests. However, the habitats of all carnivorous plants share a common characteristic: nutrient-poor, almost sterile soil, in which many plants would struggle to survive, much less grow. Many carnivorous plants are found in North America, Southeastern Asia, and Australia, according to Encyclopedia.com. “Hotspots”, geographic areas where a large number of species are located, include the Nepenthes in Southeast Asia, the Drosera in South Africa and Australia, and the Sarracenia in the Southeast United States.
Drosera capensis, commonly known as the Cape sundew, traps prey using its sticky hairs (Wikipedia).
How Did Carnivorous Plants Evolve?
Carnivorous plants are found across the flowering-plant family tree, specifically in four of the significant angiosperm lineages (the Monocots, Core Eudicots, Rosids, and Asterids) and in five orders (Poales, Caryophyllales, Oxalidales, Ericales, and Lamiales). According to researchers Aaron M. Ellison and Nicholas J. Gotelli, carnivory has developed at least six times in five angiosperm orders independently.
Although there is hardly any fossil record of carnivorous plants, a Nature article studies the genome of Cephalotus follicularis, the Australian pitcher plant, by comparing its digestive fluids to other plants. They have discovered that the genes used to make digestive enzymes in all pitcher plants have a common evolutionary origin, despite millions of years of evolution between them. The researchers found that the Australian pitcher plant is closer to the starfruit, Averrhoa carambola, than to other pitcher plant species native to the Americas and Southeast Asia, implying that carnivory has evolved repeatedly in plants because they probably had to deal with the nutrient-scarce soils in which they grow. Victor Albert, a plant-genome scientist at the University of Buffalo, who co-led the study, says what the plants are “trying to do is capture nitrogen and phosphorus from their prey.”
In another Current biology study, an international team of botanists and biologists found a three-step process toward carnivory after studying three modern carnivorous plants, including the Dionaea muscipula, Aldrovanda vesiculosa, and Drosera spatulata. An early non-carnivorous ancestor of these three plants underwent a whole-genome duplication approximately 70 million years ago, creating a second copy of its whole DNA. This duplication allowed leaf and root genes to evolve and perform other roles, which was the first step in the evolution of plant carnivory. For instance, some leaf genes became genes for traps, and those that typically serve roots seeking soil nutrition became carnivory-specific mechanisms. The second step was using a new nutrient source (prey), lowering the selective pressure on genes involved in non-carnivorous nutrition. This resulted in significant gene losses. The last step was forming clade-specific, independent hunting techniques depending on the environment of the plants.
Positions of carnivorous plant families in the current overall angiosperm phylogeny (Journal of Experimental Botany).
Anatomy of Carnivorous Traps
Carnivorous plants lure prey in a variety of ways. According to carnivorousplants.org, there are six basic trapping mechanisms. In general, a trap is either active or passive. Active traps use movement mechanics to capture prey, while passive traps depend on morphological structure.
1. Adhesive: The adhesive trap is the simplest trapping mechanism.
Flypaper: The Pinguicula has unique short-stalked glands on its flypaper-like leaves that produce a sticky mucilage to capture small insects. The glands are stalked to keep the leaf from touching the slime while it attracts its prey.
Fixed tentacles: Plants with fixed tentacles work similarly to the flypaper-like leaves, with the exception that they have a longer grasp to catch any passing insects. Drosophyllum, Roridula, Triphyophyllum, and Byblis have tentacles that do not move and serve only as a trapping mechanism.
Mobile tentacles: Drosera tentacles have multiple functions. They move to envelop the prey in slime with as many tentacles as possible and then release digestive enzymes. The nutrients from the prey are absorbed by the tentacles and glands on the leaf surface.
2. Pitfall: These traps have leaves shaped like pits.
Open with a pool of water: The simplest pitfall traps are Bromeliad carnivores (Brocchinia and Catopsis), which have the base of the whorl of leaves seal to form a cup to gather water. Other carnivores, such as the Nepenthes, Heliamphora, and Cephalotus, have extensively modified leaves where each leaf is a separate trap. The traps may also have nectaries, bright colours, or a flower-like scent to lure prey. The lip of the trap is often slippery, while the inside of the trap is waxy.
Covered or no pool of water: Most Sarracenia and Darlingtonia contain very little water, if any at all, and starve their prey to death. Many of these traps have hoods to keep rain out. The plant determines how much water is in the trap and pumps it in or out as needed. In addition, the leaves can produce digestive enzymes and alter the pH of the water.
3. Lobster pot: These traps are similar to lobster traps, hence the name. A lobster pot features an entrance that prey can easily locate and enter from the outside, but the entrance is difficult to find or exit from the inside. Once inside, the prey may simply exit if they discover the entrance or figure out that they can easily leave. The entrance of Darlingtonia and Sarracenia psittacina is dark, while the rest of the trap has light coming in through translucent cells called areoles. Prey that is light oriented is more likely to miss the entrance.
4. Pigeon trap: Genlisea plants can be found in water or water-saturated soils. Its main prey are protozoans, and they enter the trap by pushing through hairs that point inward. They cannot get back out past the hairs after they have entered the trap. Pigeon traps work similarly, except instead of hairs, thin metal hairs are used. Sarracenia psittacina also uses a similar trapping mechanism, with hairs in the neck of the trap that allow prey to travel in only one direction.
5. Snap: When a prey moves into the trap of the Dionaea muscipula or Aldrovanda vesiculosa, it brushes against the trigger hairs and the trap swiftly encloses it. Unlike human-made hinged snap traps, this snap trap bends the halves of the trap to capture the insect. The trap closes by immediately expanding the cells on the outer surface of the leaf. Each side of the leaf is flat before the trap is activated. The halves are then cupped after being triggered.
6. Suction: The Utricularia has suction traps that move too quickly to see what is going on. It is said to be the most complicated plant leaf on earth. The traps prepare themselves by pushing water out of the sealed trap, creating what would be considered a vacuum if air were involved. The trap functions so rapidly when triggered that the highest speed video cameras show the prey outside the trap in one frame and already pulled into the trap on the next.
Leaves and bladder-like traps of the Scanning electron micrograph of the "bladder"
bladderwort, or Utricularia (Brittanica). of Utricularia gibba, colour added (Sci News).
Prey Digestion
A PeerJ article on carnivorous plants explains that enzymes are required to digest prey, which could be linked to distinct morphological trapping mechanisms. Several studies have found that prey capture significantly increases the production of digestive enzymes. Certain digestive enzymes, on the other hand, are readily secreted in the absence of prey, suggesting that this is plant regulation of enzyme secretion because energy is used to produce and secrete enzymes.
Nitrogen is the most important mineral nutrient that plants need. However, its availability in many terrestrial ecosystems is very limited. To adapt to these unfavourable conditions, the plants have developed leaves that are able to attract, trap, and digest prey into simpler mineral compounds, which eventually get absorbed for plant growth and reproduction at the cost of reduced photosynthesis.
Coordination of trap closure, secretion, and NH4+ uptake in the Venus flytrap (Science Direct).
The Future of Carnivorous Plants
Researchers Ellison and Adamec mention that although we know the basic biology of carnivorous plants, there is still a lot left to learn. Addressing future research opportunities in specific areas will result in hundreds of theses and dissertations, as well as decades of interesting work for researchers. Genomic analysis of carnivorous plants is still new, so further studies can help us discover patterns of convergent evolution and the underlying molecular and physiological processes that determine these observed patterns. In addition to that, genomic data can reveal pathways with potential technological applications. Furthermore, comparing carnivorous plants to noncarnivorous plants can help researchers answer many questions on the physiology, ecology, and evolutionary biology of all plants.
Unfortunately, a quarter of the world’s carnivorous plant species are at risk of extinction due to the impacts of clearing, logging, and climate change on many wetlands, according to a recent Science Direct study. These scientists say that “Without urgent action, we stand to lose some of the most ecologically unique, evolutionary interesting, and horticulturally-celebrated species on the planet.” In an article published in BBC, poachers are drawn to these plants because of their beauty and unique prey-catching abilities. Many of them are already critically endangered, so botanists are racing against the clock to study these species before they become extinct.
However, it is not all doom and gloom. Scientists from Science Alert say that the future of carnivorous plants is all in our hands. We can rescue at least some of these species if we change our attitudes and behaviours as a global community, such as shutting down black markets from illegally selling carnivorous plants and enacting better development restrictions.
Article Author: Tanya Kor
Article Editors: Valerie Shirobokov, Stephanie Sahadeo