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