Scientists unravel the colorful sea slug's solar-energy theory

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1 Scientists unravel the colorful sea slug's solar-energy theory By Scientific American, adapted by Newsela staff on Word Count 945 Pictured is the emerald sea slug, Elysia chlorotica. Photo: EOL Learning and Education Group, via Flickr Nature is full of thieves. Instead of collecting pollen and nectar from flowers, robber bees raid the hives of other pollinators and steal their honey. Some ant species routinely kidnap and enslave members of neighboring colonies. Massive-winged frigate birds also called pirate birds swoop down on smaller seabirds and snatch the fish right out of their beaks. The sea slugs known as sacoglossans are some of the most remarkable biological burglars on the planet. The animals are about the length of a postage stamp or two. They feed on simple underwater plants known as algae, which have no stems, roots or leaves, but do contain chlorophyll, a green pigment or coloring matter. Chlorophyll is used in photosynthesis, a process by which plants capture the sun's energy and combine it with carbon dioxide and water to make food. Oxygen is produced as a byproduct of photosynthesis. The part of the algae that contains chlorophyll is a jelly bean shaped structure known as a chloroplast. Most sea slugs digest the chloroplasts right away, but some species store them for weeks to months in large transparent digestive glands, turning the animals brilliant shades of green.

2 Shedding New Light On Hoarding For the past several decades, scientists thought that the slugs hoarded chloroplasts in order to become solar-powered animal-plant combinations. The theory made a lot of sense. Experiments confirmed that, when placed in the light, the slugs actively absorbed carbon dioxide, just like leaves. Furthermore, some of the slugs could stay alive in a well-lit laboratory for months with no food. Besides, why else would the animals go to the trouble of preserving the chloroplasts, if not as a long-term energy source? A recently published study is forcing scientists to completely rethink solar-powered sea slugs, however. Sven Gould of the University of Dusseldorf and his team deprived six sacoglossans of food for 55 days: two enjoyed 12 hours of light followed by 12 hours of darkness each day; two lived with the same cycle, but received a drug that disrupted photosynthesis; and the remaining two slugs lived in continuous darkness. At the end of the starvation period, all the slugs were alive, equally pale and all had lost about the same amount of weight. It was clear that their health and survival did not depend on whether or not they could photosynthesize. It s very appealing to think that here you have an animal slowly turning into plant, Gould says. But now we re back at square one. Gould has proposed that, rather than becoming part vegetable, the slugs are doing something lots of critters do: saving some of the food they collect for times of hardship. Sexy Sea Slugs? Some sacoglossans hibernate in the winter, burrowing into debris at the bottom of the ocean floor. Sunlight would not help them endure this subterranean slumber, but a digestive sac full of chloroplasts would do the trick. That would also explain how the slugs in his experiment made it through nearly two months of perpetual night without any food and why all the slugs were pale and shrunken after the ordeal. Even if these slugs are not relying primarily on photosynthesis for nutrition, they might be using some of the oxygen produced by the process. If so, they would resemble the spotted salamander, which has evolved a symbiotic relationship with algae that live in its eggs: inside the eggs, developing salamanders breathe in the oxygen the algae release while the plants enjoy the animal s exhaled carbon dioxide. There are several other possible explanations for the slugs green coloration. Perhaps some of the slugs are camouflaging themselves after all, they spend an awful lot of time hanging out on green plants. Or maybe green is to these slugs as pink is to flamingos: sex

3 appeal. Flamingos get their red and pink colors from the carotenoid proteins in the plankton they eat. Healthier, well-nourished flamingos are pinker and more attractive to mates. Perhaps sea slugs display chloroplasts in a similar flirtatious way. A Tangled Relationship In addition, the relationship between sacoglossans and chloroplasts may not be as straightforward a case of theft as it once seemed. Perhaps once swallowed, the chloroplasts continue to take care of themselves the way they would inside an alga or reflexively trigger certain defenses that help them survive in a hostile environment. The slugs could be taking advantage of this self-sufficiency to enjoy a few weeks or months of free oxygen. Alternatively, the chloroplasts might consent to keep busy because they are getting carbon dioxide or something else out of the deal, like the algae living in the salamanders' eggs until the slugs double-cross them and digest the chloroplasts properly. Ongoing research by Mary Rumpho of the University of Connecticut raises further questions about just what is going on. E. chlorotica, the emerald-green slug species Rumpho studies, seems to be so dependent on algae that if baby slugs are not exposed to it for at least seven days, they stop growing and die. It is almost as if these sea slugs have evolved to self-destruct if there is no algae around in their youth, because without the plants and their chloroplasts they simply would not make it. The chloroplasts E. chlorotica captures from its algae primarily make lipids, which provide a source of energy as well as building materials for cells. A young slug needs lipids to grow rapidly, and in most species the slug's own body has to produce these lipids. What if E. chlorotica has given over its lipid production to chloroplasts? If so, captive chloroplasts would be so much more than a postponed meal for the slugs they would be more like a continually renewed vital organ.

4 Quiz 1 A recent study is prompting scientists to rethink a common theory on sea Which excerpt BEST supports the central idea above? The animals are about the length of a postage stamp or two. They feed on simple underwater plants known as algae, which have no stems, roots or leaves, but do contain chlorophyll, a green pigment or coloring matter. Experiments confirmed that, when placed in the light, the slugs actively absorbed carbon dioxide, just like leaves. Furthermore, some of the slugs could stay alive in a well-lit laboratory for months with no food. At the end of the starvation period, all the slugs were alive, equally pale and all had lost about the same amount of weight. It was clear that their health and survival did not depend on whether or not they could photosynthesize. There are several other possible explanations for the slugs green coloration. Perhaps some of the slugs are camouflaging themselves after all, they spend an awful lot of time hanging out on green plants. 2 Which of the following statements BEST represents Gould's perspective on his discovery? It will likely eliminate the need for further research into the biology of sea It demonstrates that the earlier theory about sea slugs was appealing, but unproductive. It is a starting point for new discoveries about sea slugs and chloroplasts. It suggests that scientists currently have a very limited understanding of sea 3 How does the article develop the idea that nature is full of "thieves"? by thoroughly describing a species of slug that "steals" and uses a part from other species by providing a variety of examples of animal species that "steal" from other species by describing a relationship in which two species "steal" from each other in a battle for survival by presenting a perspective on nature that reveals its harsh realities, including "stealing" and death

5 4 Read the sentence from the section "A Tangled Relationship." Alternatively, the chloroplasts might consent to keep busy because they are getting carbon dioxide or something else out of the deal, like the algae living in the salamanders' eggs until the slugs doublecross them and digest the chloroplasts properly. Does this particular sentence support a CENTRAL idea of the article? Why or why not? Yes; it develops the idea that the relationship between sea slugs and chloroplasts is more complex than scientists had previously thought. Yes; it explains the primary experiment and recent discovery that caused scientists to rethink their theories about sea slugs and chloroplasts. No; it explains a theory that is interesting, but only indirectly related to the experiment regarding sea slugs and chloroplasts. No; it develops an alternate theory that seems to contradict the main theory about sea slugs and chloroplasts discussed in the article.

6 Answer Key 1 A recent study is prompting scientists to rethink a common theory on sea Which excerpt BEST supports the central idea above? The animals are about the length of a postage stamp or two. They feed on simple underwater plants known as algae, which have no stems, roots or leaves, but do contain chlorophyll, a green pigment or coloring matter. Experiments confirmed that, when placed in the light, the slugs actively absorbed carbon dioxide, just like leaves. Furthermore, some of the slugs could stay alive in a well-lit laboratory for months with no food. At the end of the starvation period, all the slugs were alive, equally pale and all had lost about the same amount of weight. It was clear that their health and survival did not depend on whether or not they could photosynthesize. There are several other possible explanations for the slugs green coloration. Perhaps some of the slugs are camouflaging themselves after all, they spend an awful lot of time hanging out on green plants. 2 Which of the following statements BEST represents Gould's perspective on his discovery? It will likely eliminate the need for further research into the biology of sea It demonstrates that the earlier theory about sea slugs was appealing, but unproductive. It is a starting point for new discoveries about sea slugs and chloroplasts. It suggests that scientists currently have a very limited understanding of sea 3 How does the article develop the idea that nature is full of "thieves"? by thoroughly describing a species of slug that "steals" and uses a part from other species by providing a variety of examples of animal species that "steal" from other species by describing a relationship in which two species "steal" from each other in a battle for survival by presenting a perspective on nature that reveals its harsh realities, including "stealing" and death

7 4 Read the sentence from the section "A Tangled Relationship." Alternatively, the chloroplasts might consent to keep busy because they are getting carbon dioxide or something else out of the deal, like the algae living in the salamanders' eggs until the slugs doublecross them and digest the chloroplasts properly. Does this particular sentence support a CENTRAL idea of the article? Why or why not? Yes; it develops the idea that the relationship between sea slugs and chloroplasts is more complex than scientists had previously thought. Yes; it explains the primary experiment and recent discovery that caused scientists to rethink their theories about sea slugs and chloroplasts. No; it explains a theory that is interesting, but only indirectly related to the experiment regarding sea slugs and chloroplasts. No; it develops an alternate theory that seems to contradict the main theory about sea slugs and chloroplasts discussed in the article.