<p>Chemical defense using secretion of ink containing purple pigments derived from red algae is common in sea hares. Aplysia juliana, however, prefers green algae, and thus, its ink is white, not purple. Since opaline, the sea hares' other defensive secrwetion, is also white, ink and opaline were not distinguished from each other in previous experiments. Thus, deterrence of this white ink alone towards predators has never been tested. In this study, we tested the deterrence of the white ink of A. juliana as well as the extract of their skin, using the Japanese spiny lobster Panulirus japonicus as a model predator. Parallel experiments on Aplysia gigantea, a sea hare with purple ink, were performed to test whether P. japonicus is deterred by purple ink. The skin extract, but not the white ink, of A. juliana was deterrent. In contrast, purple ink, but not the skin extract, of A. gigantea was deterrent. These results show that the skin of A. juliana contains defensive chemicals against P. japonicus, whereas the white ink itself does not.</p>