An interesting history of carnivorous plants

In 1875, Charles Darwin published an interesting treatise called Insectivorous Plants. The subject was new for its time: a group of plants that eat insects. Less than three years later, the subject became popular in popular culture thanks to a story circulated from Madagascar. A naturalist claimed to have seen a large carnivorous tree devouring a young woman.

No cannibal plants have been discovered so far, or at least the discoverers have not returned. Nor do we know exactly what will happen on Earth during an unexpected eclipse. However, there are many plants that 'eat' meat.

Natural conditions for carnivorous plants

Plant carnivory is primarily an adaptation to low nutrient levels. Carnivorous plants are usually terribly competitive with other plant species and can only survive in nutrient-poor areas where other plants cannot. Thus, the vast majority of carnivorous plants are found in bogs or similar areas. The aquatic environment in such places is often quite nutrient poor, and several of these plants can be found here, some of which may make an interesting addition to a home aquarium.

Strictly speaking, the word "carnivorous" does not really apply to any of these plants. Carnivorous implies eating meat, and eating is a process of energy production. Carnivorous plants are entirely photosynthetic and therefore get all their energy from sunlight and carbon dioxide. At any rate, we can say that they "eat" carbon dioxide.

Carnivorous plants do not rely on their prey for energy, but for nutrients. They only get fertilizer from their prey, so they cannot be said to eat anything. However, many carnivorous plant species do produce special enzymes that break down their prey.

Carnivorous plants in the aquatic environment

Aquatic environments can be extremely nutrient poor, especially compared to terrestrial habitats. Thus, at least two genera of aquatic predators have emerged. Neither of these species is capable of preying on most aquatic insects or larger prey, but instead preys on zooplankton and organisms of similar size. In general, you don't have to worry about these plants eating aquarium fish.

Some of these plants may eat small numbers of fry, but even this is unlikely. They can be successfully fed with frozen or live daphnia, bloodworms, mosquito larvae, etc.

Bubbly

The best known of the aquatic predators are the bubbles of the genus Utricularia. This is a large genus with at least 200 species, distributed all over the world, with the exception of Antarctica. The vast majority of species are actually terrestrial, living in wet soils and swamps. However, there are quite a few aquatic species.

Bubbleworms get their name from small traps placed on stems that resemble bubbles or seed capsules. These traps are generally considered the most sophisticated of all predatory plants. The bladder starts with a tiny bag of water. Active transport from the bladder through the cell walls results in a vacuum. The bladder has a tiny door on the end, and when the prey swims into it, the internal vacuum sucks in the prey and a small amount of water. This reaction is almost instantaneous. Digestive enzymes can break down small prey in as little as an hour. The trap can be ready again within half an hour after digestion.

Remarkably, traps appear to have the ability to ingest food in stages. Large traps of some species are capable of catching items such as mosquito larvae or young tadpoles.

Bubbles are known for their stunning colours. In fact, if you've ever visited a silted pond, you've noticed a small flowering aquatic plant, it was probably a bubbly. The flowers can be bright yellow, purple or white (as well as other colours) and, although small, are quite beautiful, especially in large numbers. These plants also bloom easily in aquariums: the bright flowers are held above the water. The colour and appearance of the flowers vary depending on the species.

Bubbles can be grouped into three informal groups - those resembling standard stem plants, free-floating plants and creeping plants. All types are suitable for keeping in an aquarium. Keep nutrient levels low in the aquarium. Algae can quickly suffocate these plants and should be avoided. They do well in moderate to bright light.

• The best known of the vesicularia is Utricularia vulgaris, found in Europe and Asia. A very similar species is known from North America, U. macrorhiza, and until recently the two species were one. This plant remotely resembles the cabomba with its fan-shaped, filamentous leaves. There are many small bladders around the leaf area. The stems can reach up to 90 cm in length. The flowers are small, about 2.5 cm, bright yellow. The traps are quite small.

Utricularia gibba, known as floating or humpback vesicularia, is a free-floating species that does not root. In the wild it can form large floating mats. It has an almost cosmopolitan distribution as it is found in the United States, South America, Europe, Asia and Australia. The bright yellow flowers are held on stems above the water. It is a remarkably easy plant to grow. It can be grown in a clear glass container by the window. This is the bubbly that many aquarists are most familiar with - it appears as a pest in planted aquariums and remotely resembles floating string algae. It does look quite beautiful in culture, though.

Utricularia aurea is the most common of the pinnate bubba plants you are likely to encounter in the trade. Often sold simply as 'vesicularia', it is native to South East Asia where many of our aquatic plants are grown. It may not produce bubbles in the aquarium. It can be grown free-floating or rooted. The flowers are yellow with a touch of red.

Utricularia graminifolia is a small creeping foreground plant that has become popular for its occasional use in Takashi Amano's magnificent aquariums. It has small, lanceolate leaves that form a lawn at the bottom of the aquarium. This plant is rather difficult to grow in an aquarium.

Utricularia sandersonii is a lesser known semi-aquatic plant. The leaves are similar in size and shape to caddis with small flecks of bubbles. It is much easier to grow than U. graminifolia and forms a beautiful dense carpet. Similarly, both U. praelonga and U. livida are easier to grow than U. graminifolia, and they form beautiful thick mats in the aquarium.

Utricularia purpurea, vesicularia purpurea, is one of the most interesting species in the genus. It occurs throughout North and Central America. The bladders are very numerous. The flowers are beautiful and, as you can guess from the name, purple. Despite the abundance of bladders, this plant may be in the process of evolution, giving up its predation! The bladders contain a mixture of algae, bacteria and even zooplankton that feed on debris in the bladder. Purple bladder bladders do function, and can catch prey.

Water wheel (Aldrovanda vesiculosa)

Bubbles are the best known aquatic predators, but they are not the only ones. The waterwheel (Aldrovanda vesiculosa) is a fascinating predatory plant. It is found in Europe, Africa, Asia and Australia. Sellers of carnivorous plants often offer it for sale.

The waterwheel is the only representative of its genus, although several extinct species are known. It has no roots and is a floating aquatic plant, although it can be anchored in an aquarium. The leaves surround the stem with six to nine whorls, each containing a small trap. As the name implies, the plant is shaped like spokes of a wheel with multiple wheels on each stem. These traps are only a couple of millimetres in size but resemble miniature Venus fly traps. It is a sort of shell trap made from a specialised sheet.

Each half of the trap contains several small trigger hairs. When the microorganism touches these trigger hairs, the leaf closes in just a quarter of a second - quite impressive for a plant, especially as it closes under water pressure. These traps can catch small zooplankton such as daphnia, mosquito larvae and the like. Although the idea that bubbles eat anything other than the smallest fry is absurd, care should be taken when keeping a water wheel in the aquarium. Large traps are capable of eating tiny fish, perhaps the size of a newborn guppy. When kept in a spawning tank, the water wheel may eat small numbers of fry.

This plant is particularly delicate. It is also endangered in most of its habitat. It requires mild, acidic water with a low nutrient content.

Carnivorous plants can make interesting additions to aquariums. Besides predation, these plants are beautiful and their uniqueness cannot be discounted.