Many species of sacoglossan sea slugs are oligophagous, only feeding a few species of closely related algae. Some sacoglossans are also kleptoplastic and can photosynthesize by incorporating chloroplasts taken from their algal food into specialized digestive cells. The duration of photosynthesis varies among species and provides nutritional benefits to the slugs from several days up to many months. Elysia papillosa is a small species of sacoglossan ranging through the Caribbean and Gulf of Mexico and can photosynthesize for a short duration after feeding. This slug is associated with algae in the genus Penicillus; however, the slug does not use all species of Penicillus equally. They are significantly more likely to found on P. capitatus than P. lamourouxii in mixed algae beds, although they do feed on both algae in the field. Interestingly, slugs feeding exclusively on P. lamourouxii grow 1.5-2x larger than slugs feeding on P. capitatus. Here we found that slugs were able to maintain active photosynthesis for twice as long when feeding on P. lamourouxii compared to P. capitatus. We also found that E. papillosa collected from P. capitatus in the field were significantly more likely to travel towards to chemical signals from P. capitatus than P. lamourouxii, even though they could grow larger and photosynthesize longer if they fed on P. lamourouxii. It is unclear why E. papillosa chooses algae that provide lesser benefits to growth and photosynthesis. This preference may be due to structural or biochemical defenses provided by the algae against potential predators or the environment. However, additional studies will be required to determine why slugs prefer P. capitatus.