Posidonia oceanica (L.) Delile is a flowering plant not algae. It lives according to water clarity in sandy bottom, at depths from 1 to 40 m of the Mediterranean. The lessepsian fish, Siganus luridus (Ruppell, 1828) arc common herbivores in the eastern Mediterranean, where they adapted themselves to the new environment. This member of Siganidae have considerably modified their diet to adapt themselves to new plant and algal resources. It appears that in the Eastern Mediterranean of Turkey (Gazipasa) S. luridus seems to select P oceanica as an major food and they gradually destroy P oceanica meadows.