Animals Using Solar Energy for Photosynthesis or Electric Power
Plants and Animals That Use Light Energy
Most people consider plants to be simpler creatures than animals, but green plants have one big advantage that animals lack. They have the wonderful ability to make food inside their bodies by photosynthesis, using simple chemicals that they absorb from their environment and the energy of sunlight. Photosynthesis takes place inside the chloroplasts of plant cells.
Despite their more advanced structure and functions, the bodies of humans and most animals can’t use the sun’s energy (except in reactions such as the production of vitamin D in human skin) and can't produce food. Their cells have no chloroplasts, so they are dependent on plants for their survival, either directly or indirectly.
Researchers have discovered that some animals can use the sun’s energy, however. Some have incorporated plant chloroplasts into their bodies and even genes from the plant cell nucleus into their DNA. The chloroplasts carry out photosynthesis inside the animal, producing a carbohydrate and oxygen. The animal uses the carbohydrate for food. At least one animal has developed a solar cell—one that converts solar energy into electricity.
Four amazing, solar-powered animals are a green sea slug named the eastern emerald elysia, an animal known as the mint-sauce worm, an insect named the oriental hornet, and the embryos of the spotted salamander.
Solar-Powered Sea Slugs: Elysia chlorotica
The Eastern Emerald Elysia
The beautiful eastern emerald elysia (Elysia chlorotica) is a type of sea slug. It's found along the east coast of the United States and Canada in shallow water. The slug is about an inch long and is green in colour. Its body is often decorated with small white spots.
Elysia chlorotica has wide, wing-like structures called parapodia that extend from the sides of its body as it floats. The parapodia undulate and contain vein-like structures, making the slug look like a leaf that has fallen into the water. This appearance may help to camouflage the animal. The parapodia are folded over the body when the animal is crawling over a solid surface.
Algae in the Eastern Emerald Elysia
The eastern emerald elysia feeds on a filamentous green alga called Vaucheria litoria that lives in the intertidal zone. When it takes a filament into its mouth, the slug pierces it with its radula (a band covered with tiny chitinous teeth) and sucks the contents out. Due to a process that is not completely understood, the chloroplasts in the filament are not digested and are retained. The process of acquiring chloroplasts from the alga is known as kleptoplasty.
The chloroplasts collect in the branches of the slug's digestive tract, where they absorb sunlight and carry out photosynthesis. The branches of the digestive tract extend throughout the animal's body, including the parapodia. The slug's expanded "wings" provide a greater surface area for the chloroplasts to absorb light.
Young slugs that haven't collected chloroplasts are brown in color and have red spots. The chloroplasts build up as the animal feeds. Eventually they become so numerous that the slug no longer needs to eat. The chloroplasts make glucose, which the slug's body absorbs. Researchers have discovered that the slugs can survive as long as nine months without eating.
Gene Transfer for Photosynthesis
Chloroplasts contain DNA, which in turn contains genes. Scientists have discovered that a chloroplast doesn't contain all the genes needed to direct the process of photosynthesis. The other required genes are present in the DNA of a plant cell's nucleus. Scientists have found that at least one of the required algal genes is also present in the DNA of the eastern emerald elysia's cells, however. At some point in time, the algal gene became incorporated into the slug's DNA.
The fact that the chloroplast—a plant organelle—can survive and function in an animal's body is amazing enough, but even more amazing is the fact that the sea slug's genome (genetic material) is made of both its own DNA and algal DNA.
Mint sauce is made from mint leaves, vinegar, and sugar. It's a popular addition to lamb dishes in Britain. The name of the sauce is used for a tiny beach worm found in Europe. A collection of mint-sauce worms looks very much like culinary mint sauce under certain lighting conditions.
The Mint-Sauce Worm
A green worm (Symsagittifera roscoffensis) can be found on certain beaches on the Atlantic coast of Europe. The animal is only a few millimetres long and is often known as the mint-sauce worm. Its colour comes from the photosynthetic algae living in its tissues. The adult worms rely entirely on substances made by photosynthesis for their nutrition. They are found in shallow water, where their algae can absorb sunlight.
The worms collect to form a circular group when their population is sufficiently dense. Furthermore, the circle rotates—almost always in a clockwise direction. At lower densities the worms move in a linear mat, as shown in the video below. Researchers are very interested in the reasons why the worms move as a group and in the factors that control this movement.
The Mint-Sauce Worm Moving Over a Beach in Portugal
The Oriental Hornet
The oriental hornet, or Vespa orientalis, is a red-brown insect with yellow markings. There are two wide, yellow stripes next to each other near the end of the insect's abdomen. The hornet also has a narrow yellow stripe near the start of its abdomen and a yellow patch on its face.
Oriental hornets are found in southern Europe, southwest Asia, northeast Africa, and Madagascar. They have also been introduced to part of South America.
The hornets live in colonies and usually build their nest underground. The nests are occasionally constructed above ground in a sheltered area, however. Like bees, the hornet colony consists of one queen and many workers, which are all females. The queen is the only hornet in the colony that reproduces. The workers take care of the nest and colony. The male hornets, or drones, die after fertilizing the queens.
The hard outer covering of an insect is called an exoskeleton or cuticle. Scientists have discovered that the exoskeleton of the oriental hornet produces electricity from sunlight and acts as a solar cell.
Oriental Hornets Cooling Their Nest Down on a Hot Day
How Does the Oriental Hornet Produce Electricity?
By examining the hornet's exoskeleton under very high magnification and investigating its composition and properties, scientists have discovered the following facts.
- The brown areas of the exoskeleton contain grooves that split incoming sunlight into diverging beams.
- The yellow areas are covered by oval protrusions which each have a tiny depression that resembles a pinhole.
- The grooves and holes are thought to reduce the amount of sunlight that bounces off the exoskeleton.
- Lab results have shown that the surface of the hornet absorbs most of the light that strikes it.
- The yellow areas contain a pigment called xanthopterin, which can turn light energy into electrical energy.
- Scientists think that the brown areas pass light to the yellow areas, which then produce electricity.
- In the lab, shining light on the oriental hornet's exoskeleton generates a small voltage, showing that it can act as a solar cell.
Inside an Oriental Hornet Nest
Why Does the Oriental Hornet Need Electrical Energy?
It's not yet known why the oriental hornet needs electrical energy, although researchers have made some suggestions. The electricity might give the insect's muscles extra energy or it might increase the activity of certain enzymes.
Unlike most insects, the oriental hornet is most active in the middle of the day and early afternoon when the sunlight is most intense. Its exoskeleton is thought to provide a boost in energy as sunlight is absorbed and converted into electrical energy.
The Spotted Salamander
The spotted salamander (Ambystoma maculatum) lives in the eastern United States and Canada, where it's a widespread amphibian. The adults are black, dark brown, or dark grey in colour and have yellow spots. Researchers have discovered that the embryos of the spotted salamander contain chloroplasts. The discovery is exciting because the salamander is the only vertebrate known to incorporate chloroplasts into its body.
Spotted salamanders live in deciduous forests. They are rarely seen because they spend most of their time under logs or rocks or in burrows. They emerge at night to feed under the cover of darkness. The salamanders are carnivores and eat invertebrates such as insects, worms, and slugs.
Spotted salamanders also emerge from their hiding place in order to mate. The female generally finds a vernal (temporary) pool in which to lay her eggs. The advantage of a pool of water compared to many ponds is that the pool doesn't contain fish that would eat the eggs.
Adult Spotted Salamanders
How Do Spotted Salamander Embryos Obtain Chloroplasts?
Once the salamander's eggs are laid in a pool, a single-celled green alga called Oophila amblystomatis enters them within a few hours. The relationship between the developing embryo and the alga is mutually beneficial. The alga uses the wastes made by the embryos and the embryos use oxygen produced by the alga during photosynthesis. Researchers have found that in eggs with algae, embryos grow faster and have a better survival rate.
It used to be thought that the algae entered the salamander eggs but not the embryos inside the eggs. Now scientists know that some of the algae do enter the embryo's body, and some even enter the embryo's cells. The algae survive and continue to photosynthesize, producing food for the embryo as well as oxygen. Embryos without the algae can survive, but they grow more slowly and their survival rate is lower.
Spotted Salamander Eggs and Embryos
Animals and Photosynthesis
Now that one vertebrate has been found to carry out photosynthesis, scientists are on the lookout for more. They feel that it's more likely in vertebrates that reproduce by releasing eggs into water, where the eggs can be penetrated by algae. The young of mammals and birds are well protected and aren't likely to absorb algae.
The idea that animals can use solar energy vía isolated chloroplasts or algae or entirely on their own is a fascinating one. It will be interesting to see if more animals with these abilities are discovered.
© 2013 Linda Crampton