Thursday , 24 October 2019
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Plants or animals? Or both?

Maneka Sanjay Gandhi

Mesodinium chamaeleon, a strange green creature found in Denmark that lives at the bottom of the sea, is causing researchers to rethink traditional ways of classifying living organisms.

Traditionally there is a difference between animals and plants:

• Animals cannot produce their own energy and must eat other animals or plants to get energy for their survival.

• Plants use photosynthesis to get energy from the sun for their survival. Chlorophyll is the green pigment in plant cells that enables photosynthesis to happen, and is one of the defining traits of plants.

Mesodinium chamaeleon does both.

Using its thousands of hairs it moves rapidly through water, finding plants to eat – like an animal. And when it eats the plant, it becomes a plant:

By keeping the chlorophyll granules active in its stomach, Mesodinium chamaeleon uses their ability to convert sunlight into energy. This photosynthesis makes Mesodinium chamaeleon a plant.

After a while, it digests the plant and then turns back into an animal, which then goes hunting for a new plant to consume.

The sea slug Elysia chlorotica was once described as “a leaf that crawls”. They use straws to suck chloroplasts (sacs that contain chlorophyll) from algae and keep those chloroplasts for months, living off the energy of photosynthesis.

Till the 1970s, scientists classified living organisms as either animals or plants. Since then specialists have added three kingdoms: fungi, protozoans, and algae.

Just as there are creatures that are male that turn into female, or become both to self fertilise, this complex web of life has a category of beings that are neither animals or plants or, in another way, both animals and plants.

Some animals look like plants. Others are animals that turn into plants, or vice versa! There is no combination that nature has not thought of first!

The bizarre “sea lilies” are animals, despite their plant-like appearance. The sea lily stalk is fastened to the sea bottom by a stalk and has a bulbous body with frond-like tentacles. But it has a mouth, a gut and an anus (near the mouth!) and feeds on microscopic plants and animals.

The amazingly coloured sea anemone is not a flower. It spends its life attached to rocks on the sea bottom, or on coral reefs, waiting for fish to pass close enough to ensnare them within its venom-filled tentacles.

There are over 1,000 species of anemones. Sea anemones are animals, but they look so much like plants that they are named after a group of flowers. They can move and feed on other unsuspecting organisms that get trapped in their tentacles. In fact, sea anemones belong to a group of animals called cnidarians that also includes jellyfish. Interestingly, there are even components of their nervous system that are the same as humans’. Their bodies are composed of an adhesive pedal disc, or foot, a cylindrical body, and an array of tentacles surrounding a central mouth. The tentacles are triggered by the slightest touch, firing a harpoon-like filament into their victim and injecting a paralysing neurotoxin. The helpless prey is then guided into the mouth by the tentacles. But they get their oxygen and sugar through green algae whom they live with. There is a cnidarian called the “Venus flytrap sea anemone” that completely looks the plant Venus Flytrap which eats insects. It is an animal that looks like a plant that imitates a carnivorous plant that feeds like an animal (Ooph!) The Venus flytrap, despite being a plant, feeds on other organisms—and some of its parts move faster than its animal prey.

Corals are not plants. They are animals. Many groups of animals do not move, and live attached like plants to a surface for most of their life, including sponges, corals, mussels, and barnacles.

Seaweeds are not plants. They are protists – organisms that belong to the kingdom that includes protozoans, bacteria, and single-celled algae and fungi. Seaweeds may have been the ancestors of all animals and plants. Protists are able to use an animal-like behaviour (eating other organisms) to acquire plant-like traits (photosynthesis).

Mushrooms are not plants. They are fungi and there is a huge variety. Other fungi are rust, yeasts (used to make bread and beer), and slime moulds (like those that grow on old fruit). Mushrooms are often treated like vegetables, but fungi are actually closer to animals than plants. Like plants, they do not move, but they cannot use the energy of sunlight directly through photosynthesis. Their source of energy are other organisms. But instead of “hunting” them, they grow on top of them (soil, trees, human feet), or on top of decaying dead organisms (dead bark, dead animals, your bread). Eating a mushroom is much closer to eating a hamburger than other veggie substitutes.

Algae are aquatic single-celled life forms that appear as a kind of growth, or slime, on top of bodies of water. They look like plants without roots, or leaves, but they are not. Nori seaweed which wraps sushi, red dulse (a snack in Ireland and Iceland that some claim tastes like bacon when fried), kelp (which is a key ingredient in many Asian meals) – all of them are unrelated to plants. The fronds of kelp have three parts: a stem, leaf-like blades and gas-filled spheres, or bladders, which float the kelp toward the surface where the penetration of sunlight is at its highest. The fronds are anchored to the seafloor by a part of the algae known as the holdfast, which is a tangled ball but has no roots, as is the case with the roots of flowering plants. And, unlike flowering plants that absorb nutrients through their roots, kelp absorbs nutrients through all parts of its tissues. Kelp reproduces both sexually and asexually.

Euglena are not plants, animal, or fungi. They are green freshwater organisms with a red eyespot and a tail, found in still waters where they may bloom in numbers sufficient to colour the surface of ponds and ditches green or red. Euglena is a plant, because it is able to produce its own food by photosynthesis when light is available. But, like an animal, it moves and feeds on food, whenever it needs to feed, by engulfing the food with its body when sunlight is not available. Its eyespot is light sensitive and can be compared to our eyes. So far scientists have refused to put it into any category!

As scientists become better at their jobs they will find that all beings are the same. We knew decades ago (‘The Secret Life of Plants’) that trees hear, have emotions, react to negativity. One day, with more sophisticated technology, we will realise that they are us – animals – who simply live in a different way.

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