Actually, Mesodinium chamaeleon is both. This single-celled organism definitely eats other creatures, which makes it an animal. But it also absorbs algae cells that can then give it extra energy through photosynthesis. So what on earth is this strange creature?
Found off the Scandinavian coast, Mesodinium chamaeleon is an animal that forms a symbiotic relationship with the algal plants called cryptonomads. The resulting organism is just one of several hybrids that muddy the supposedly ironclad boundaries between the plant and animal kingdoms. And, as New Scientist explains, M. chamaeleon actually holds onto its symbiotic partners for so long that it's questionable whether it's even really a hybrid anymore:
M. chamaeleon takes in algal cells, just like M. rubrum, but it doesn't keep them permanently. Nor does it digest them immediately, as a hungry animal-like organism might. Instead, the cells remain intact for several weeks before being broken down, during which time they keep producing sugar by photosynthesis. M. chamaeleon also changes colour depending on whether it is hosting red or green algae or both. Other Mesodinium species either retain their captured cells for ages or digest them immediately.
For more on this, check out New Scientist's Zoologger page.
Original paper at The Journal of Eukaryotic Microbiology. Image by Øjvind Moestrup/The Journal of Eukaryotic Microbiology.