The Mediterranean Sea is losing its biological distinctiveness, and the same phenomenon is occurring in other seas. It gives urgency to a better understanding of the factors that affect marine biological invasions. A chemoecological approach is proposed here to define biotic conditions that promote biological invasions in terms of enemy escape and resource opportunities. Research has focused on the secondary metabolite composition of three exotic sea slugs found in Greece that have most probably entered the Mediterranean basin by Lessepsian migration, an exchange that contributes significantly to Mediterranean biodiversity. We have found toxic compounds with significant activity as feeding deterrents both in the cephalaspidean Haminoea cyanomarginata and in the nudibranch Melibe viridis . These findings led us to propose aposematism in the former and dietary autonomy in producing defensive metabolites in the latter case, as predisposing factors to the migration. In the third mollusk investigated, the anaspidean Syphonota geographica , the topic of marine invasions has been approached through a study of its feeding biology. The identification of the same compounds from both the viscera of each individual, separately analyzed, and their food, the seagrass Halophila stipulacea , implies a dietary dependency. The survival of S. geographica in the Mediterranean seems to be related to the presence of H. stipulacea . The initial invasion of this exotic pest would seem to have paved the way for the subsequent invasion of a trophic specialist that takes advantage of niche opportunities.

The striking color patterns of chromodorid (and other) nudibranchs appear to be indicative of aposematism. In Müllerian mimicry, all the mimic species have a defense mechanism. It has been proposed that a group of blue, white, and yellow Mediterranean and northeastern Atlantic species of the genus Hypselodoris form a Müllerian mimetic circle. One of these, H. fontandraui, lacks the mantle dermal formations (repugnatorial glands) that are typically found in other members of this circle and are reservoirs of feeding deterrent compounds. It therefore seemed possible that this animal lacks chemical defense and acts like a Batesian mimic. Within this study, we found that this nudibranch contains the furanosesquiterpenoid tavacpallescensin, most probably derived from sponges of the genus Dysidea, upon which it probably feeds. The metabolite concentrations were measured from samples of the mantle rim, other external parts, and internal organs. Concentrations were about 4 times higher in the mantle rim than in the other external parts, and more than 20 times higher in the mantle rim than in the internal organs, considerably exceeding the threshold value of concentration showing the maximum dose effect as feeding deterrent against the crustacean Palaemon elegans. In conclusion, the reported data clearly demonstrate that H. fontandraui is chemically defended in much the same way as its aposematic, co-occurring, and blue-colored congeners within the Müllerian mimetic circle and is not a Batesian mimic.