Remember photosynthesis: How plants capture energy from the sun. Chloroplasts (the green parts of leaves) are where the sun’s energy is captured.
Andrew, Elise. “These Cute Sea Slugs Are The Sheep Of The Sea.” IFLScience, IFLScience, 15 Aug. 2016, www.iflscience.com/plants-and-animals/these-cute-sea-slugs-are-sheep-sea/.
Händeler, Katharina, et al. “Functional chloroplasts in metazoan cells – a unique evolutionary strategy in animal life.” Frontiers in Zoology, vol. 6, no. 1, 2009, p. 28., doi:10.1186/1742-9994-6-28.