Marine Alien Species

Marine Alien Species


Marine Alien Species
Selene Dorsalis

In-situ long-term measurements of temperature and salinity recorded at the Straits of Gibraltar have shown that the deep water leaving the Mediterranean for the Atlantic through the same straits is warmer (ca. 0.30C) and saltier (ca. 0.06 units) than ten years ago. Other studies seem to suggest that the Mediterranean Sea is warming up at a faster rate than other basins.

Native thermophilic (‘warm temperatureloving’) species, usually restricted to the southern and eastern, warmer sectors of the Mediterranean Sea are now moving northwards and westwards. This phenomenon (meridionalization) is particularly evident in fish, where over 30 native species havealready spread in the northern areas of the Basin. Lobotes surinamensis (the Atlantic tripletail) is a case in point. This cosmopolitan fish species was previously only known from the Mediterranean from the eastern half of the basin, very rarely being recorded from central areas. Up to 2005, just one individual for the species had been recorded from Maltese waters.

As documented by Deidun et al. (2010), the species was reported on numerous occasions during 2009 and 2010, in shoals and with juveniles, in Maltese nearshore waters, suggesting that the species has established populations in this part of the Mediterranean. Meridionalization has been documented for a number of species including the peacock wrasse (Thalassoma pavo), previously only abundant in the Eastern Mediterranean and now well established in the Western Basin as well.

Similarly, climate warming facilitates the establishment and spread of tropical, exotic species that are introduced via the Suez Canal or the Straits of Gibraltar, mainly through maritime transport (in ballast water or as fouling organisms on the hull of ships) or via aquaculture This process (tropicalization) is fast advancing and, to date, almost 1000 exotic (non-indigenous) species have been recorded from the Mediterranean Sea, being native of tropical or temperate Atlantic regions (e.g. the African moonfish – Selene dorsalis – first recorded in Malta in 2008) or of Erythrean (East African) or Indo-Pacific areas (e.g. the upsidedown jellyfish – Cassiopea andromeda – first recorded in Malta in 2009). The fish familyTetraodontidae (Pufferfish family) constitute a striking example of the tropicalization of the Mediterranean fish fauna, with the number ofpufferfish species recorded for the Mediterranean waters rising from three (E. guttiferum, L. Lagocephalus and L. spadiceus) to 10 species, with seven novel tetraodontids of Lessepsian or tropical-Atlantic origin. In view of its toxicity, fishermen were until recently being paid 1 euro for every specimen of L. lagocephalus they caught!

Exotic species coming in through the Suez Canal are aptly called Lessepsian migrants, as a tribute to the French diplomat Ferdinand de Lesseps who steered the signing of the agreement endorsing the development of the Suez Canal. The opening of the canal (which hosts ca. 6% of the global maritime traffic, equivalent to almost 20,000 large vessels per year) in 1869 is independent of climate change but it has certainly provided us with a snapshot of the ramifications of the warming sea by providing a corridor and paving the way for the tropical migrants to enter the warmer Mediterranean.


Marine Alien Species
Cassiopea andromeda

German professors visiting the Suez Canal area in the late nineteenth century commented on the marked dissimilarity between the marine species on both sides of the canal – the warming of the Mediterranean is bridging such a divide, with mostly, as yet, unknown impacts. The first record of a Lessepsian fish migrant dates back to 1902 – nowadays, the count of such species hovers around the 150 mark, or ca. 20% of the total fish species in the Mediterranean.

Some exotic marine species have exhibited a spectacular range expansion within the Mediterranean, ever since their first entry. Notable examples include the blue-spotted cornet fish (Fistularia commersonii), a Lessepsian migrant which was first recorded in the Mediterranean in 2000 but which is nowadays regularly caught and observed as far west as Sardinia and the Ligurian Sea; conversely, the Sally lightfoot crab (Percnon gibbesi), an Atlantic migrant this time, was first recorded in the Mediterranean in 1999 and has spread as far eastwards as the Turkish coast!

Whilst the ecological impact of most exotic marine species eludes us so far, for some species such an impact is only too glaring. For instance, the alien nomadic jellyfish species (Rhopilema nomadica – so far unrecorded from Maltese waters) has showed up along Levantine coastlines every summer since the mid 1980’s, forming large swarms which depreciate the touristic amenity of the coastal areas and clogging fishing nets. In 2001, the Israel Electric company was forced to remove tons of biomass of the voracious exotic jellyfish from its seawater intake pipes, for an estimated cost of 50,000 US dollars!

The thematic of marine alien species is being taken very seriously by different organizations and legislative bodies, not least by the European Commission, who funded a large-scale, webbased project named DAISIE (Delivering Alien Invasive Species Inventories for Europe) which provides updated information on different marine alien species online ( In addition, the International Commission for the Scientific Exploration of the Mediterranean Sea (CIESM) has published a number of atlases of exotic species, which can also be downloaded for free (

The author is actively researching the thematic of marine alien species in local waters and thus any records of such species, substantiated with photos or video footage or the specimens themselves, would be greatly appreciated (


Alan Deidun

written for Bubbles April 2011