It's pretty common knowledge that plants use chlorophyll to turn sunlight into energy. It's what gives them their green color. Animals typically don't have chlorophyll, and so need to consume plants or other animals to gain the energy necessary to survive. (A few single cell plankton produce chlorophyll, but they barely meet the definition of an animal) A newly discovered sea slug has learned how to cut out the middleman. According to Sidney Pierce, the biologist who discovered these creatures, they can steal the chloroplasts from algae, as well as the genetic material needed to make chlorophyll.
This is possibly one of the coolest discoveries I've seen in marine biology, along with the tool-using octopus. Scientists knew that organisms could swap genetic material; bacteria do it all the time. However, the fact that this sea slug can steal the genes to make chlorophyll so successfully is remarkable. Baby sea slugs are born with the ability to produce chlorophyll, although not the ability to produce chloroplast that allow them to use the chlorophyll. It's a very neat evolutionary adaptation. The sea slugs have integrated one of the traits that make plants so successful. How, scientists have no idea. There is no known mechanism that lets the DNA transfer like this. It's another area to be further researched.
Source: Live Science- Surprising Sea Slug is Half-Plant, Half-Animal