The Lessepsian fishes Siganus rivulatus and S. luridus are common herbivores in the eastern Mediterranean, where they adapted themselves to the algal resources of the new environment. It appears that in the eastern Mediterranean S. rivulatus grazes on the majority of available macrophytes, with a preference for certain taxa such as Sphacelaria spp., Polysiphonia spp., Ulva spp., Jania spp. and Halopteris spp. Siganus luridus seems to select some of most common macrophytes found in the eastern Mediterranean (e.g,. Halopteris spp., Padina spp., Sphacelaria spp., Polysiphonia spp., Ulva spp., Sargassum spp.). Siganus luridus occasionally ingests the toxic exotic macrophyte Caulerpa racemosa. Both Siganidae feed non-selectively during the cold season. Grazing of macrophytes varies in proportion with seasons, reflecting the seasonal variations of macrophyte populations. The proportions also appear to change with fish size, probably in relation to energetic needs as well as changing grazing capabilities with relation to age. In addition, small invertebrates can be ingested accidentally, while the ingestion of sand might have a function in digestion. In the Mediterranean, both Siganidae have considerably modified their diet to adapt themselves to new algal resources, which differ significantly from those found in the Red Sea.