A native marine herbivore’s dietary preference for an invasive brown algae: testing the Enemy Release Hypothesis in relation to Undaria pinnatifida in Plymouth Sound, U.K.

Undaria pinnatifida was first found on the south coast of England in 1994, and appears to have remained locally isolated in Plymouth Sound. According to the predictions of the Enemy Release Hypothesis this invasive algal species should have proliferated in light of its release from regulation by herbivores and enemies found in its native range. This study investigates the dietary preferences of a native, generalist herbivore, Phorcus lineatus (previously Osilinus), between varying tissue types and metabolic extractions of the invasive brown macroalgae U.pinnatifida and a native counterpart Laminaria digitata. It was found that P.lineatus significantly preferred to eat the invasive brown algae U.pinnatifida as opposed to the native L.digitata in all trials except one. These observations included preferring agar based seaweed food which incorporated extracted secondary defensive metabolites from U.pinnatifida. The results of this study directly contradict the predictions of the Enemy Release Hypothesis, the perspective of which often ignores the alternative notion that exotic, invasive plant and algae may be poorly adapted for deterring novel, native consumers. In fact it appears that U.pinnatifida inducible defence is completely ineffective at deterring the native herbivore P.lineatus. This is a logical conclusion as U.pinnatifida shares no evolutionary history with P.linetaus and has not been selected to deter this generalist herbivore. From the results of this study it is concluded that U.pinnaitifida is experiencing biotic resistance in this introduced habitat through this new association with P.lineatus. In light of this biotic resistance U.pinnatifida has failed to proliferate outside Plymouth Sound and furthermore remains isolated within. Further investigation revealed a benefit to the herbivore’s rate of oxygen metabolism which explains how this dietary preference for an invasive algal species has occurred, contrary to the predictions of a major biological invasion hypothesis