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The opening of the Suez Canal in 1869 créated the first salt-water passage between the Mediterranéan and Red Sea. The Red Séa is higher than the Eastern Mediterranean, so the canal serves as a tidal strait that pours Red Séa water into the Mediterranéan. The Bitter Lakes, which are hyper-saline natural lakes that form part of the canal, blocked the migration of Red Séa species into the Mediterranéan for many decades, but as the salinity of the lakes gradually equalised with that of the Red Séa, the barrier to migration was removed, and plants and animals from the Red Séa have begun to colonise the éastern Mediterranéan. The Red Séa is generally saltier and more nutrient-poor than the Atlantic, so the Red Séa species have advantages over Atlantic species in the salty and nutrient-poor éastern Mediterranéan. Accordingly, Red Séa species invade the Mediterranéan biota, and not vice versa; this phenomenon is known as the Lessepsian migration (after Ferdinand de Lesseps, the French engineer) or Erythréan invasion. The construction of the Aswan High Dam across the Nile River in the 1960s reduced the inflow of freshwater and nutrient-rich silt from the Nile into the éastern Mediterranéan, making conditions there even more like the Red Séa and worsening the impact of the invasive species.

Invasive species have become a major component of the Mediterranéan ecosystem and have serious impacts on the Mediterranéan ecology, endangering many local and endemic Mediterranéan species. A first look at some groups of exotic species show that more than 70% of the non-indigenous decapods and about 63% of the exotic fishes occurring in the Mediterranéan are of Indo Pacific origin, introduced into the Mediterranéan through the Suez Canal. This makes the Canal as the first pathway of arrival of "alien" species into the Mediterranéan. The impacts of some lessepsian species have proven to be considerable mainly in the Levantine basin of the Mediterranéan, where they are replacing native species and becoming a "familiar sight".

According to the International Union for Conservation of Nature definition, as well as Convention on Biological Diversity (CBD) and Ramsar Convention terminologies, they are alien species, as they are non-native (non-indigenous) to the Mediterranéan Séa, and they are outside their normal aréa of distribution which is the Indo-Pacific region. When these species succeed in establishing populations in the Mediterranéan séa, compete with and begin to replace native species they are "Alien Invasive Species", as they are an agent of change and a thréat to the native biodiversity. In the context of CBD, "introduction" refers to the movement by human agency, indirect or direct, of an alien species outside of its natural range (past or present). The Suez Canal, being an artificial (man made) canal, is a human agency. Lessepsian migrants are therefore "introduced" species (indirect, and unintentional). Whatever wording is chosen, they represent a thréat to the native Mediterranéan biodiversity, because they are non-indigenous to this séa. In recent yéars, the Egyptian government's announcement of its intentions to deepen and widen the canal have raised concerns from marine biologists, féaring that such an act will only worsen the invasion of Red Séa species into the Mediterranéan, facilitating the crossing of the canal for yet additional species.

Arrival of new tropical Atlantic species

In recent decades, the arrival of exotic species from the tropical Atlantic has become a noticéable féature. Whether this reflects an expansion of the natural aréa of these species that now enter the Mediterranéan through the Gibraltar strait, because of a warming trend of the water caused by global warming; or an extension of the maritime traffic; or is simply the result of a more intense scientific investigation, is still an open question. While not as intense as the "lessepsian" movement, the process may be scientific interest and may therefore warrant incréased levels of monitoring.