The blunthead puffer sphoeroides pachygaster, a circumglobal species that extended its range from the Atlantic into the Mediterranean in the early 1980s, and has since spread throughout the entire Mediterranean Sea. It was first recorded from Maltese waters in 1999 and nowadays is often caught by trawl fishing.

The sea-slug meliba viridis, an Indo-Pacific species introduced into the Mediterranean via shipping in 1970, and was first recorded in the Maltese Islands in 2008. The animal in the photograph, taken off western Comino, is about 16 cm long. Photo: Sarah Gauci Carlton

A total of 61 authenticated alien species and another five unconfirmed ones were recorded in Maltese waters by the end of last year, according to an extensive survey Julian Evans, Jacqueline Barbara and Patrick J Schembri, from the university’s Department of Biology.

Analysis of the known or probable mode of arrival of these species indicated that the most common mode of introduction was through boating and shipping.

Other species were first introduced elsewhere in the Mediterranean and then managed to spread to the Maltese Islands under their own steam.

Thirty of these records were made since the turn of the century, clearly indicating that the rate of new records was at an all-time high. This was likely due to the present day warming trend of Mediterranean surface water, which favoured the occurrence, establishment and range extension of warm-water species in the central Mediterranean.

The blue-spotted cornetfish fistularia commersonii, an Indo-Pacific species that entered the Mediterranean through the Suez Canal in 2000 and which reached the Maltese Islands in 2005. The fish in the photograph is some 80cm in length and was photographed in Gozo. Photo: Julian Evans

The researchers also documented another phenomenon – the spread of Atlantic warm-water species to the central Mediterranean – which was almost certainly related to this warming trend.  To date, seven such species have been recorded in the Maltese Islands, so the total number of new species (aliens + Atlantic range extenders) now stood at 73.

Overall, the most represented groups were molluscs (21 species), fish (15 species), crustaceans (eight species) and red algae (seven species).

More than half of the newcomers (38 species) established breeding populations, while a further eight species were considered to be invasive.

These species were the seaweeds lophocladia lallemandii, womersleyella setacea and caulerpa cylindracea, the bivalve brachidontes pharaonis, the crab percnon gibbesi, and the fish fistularia commersonii, siganus luridus and sphoeroides pachygaster.

The green seaweed caulerpa taxifolia var. distichophylla which was first recorded from Malta in 2013 after having been introduced into the Mediterranean in 2006. The small size of the fronds (the ones in the photo are some five centimetres long) and the fact that it tends to grow embedded among taller-growing algae makes it very difficult to spot, but ongoing surveys for this species are revealing that it has spread to various localities around the Maltese Islands. Photo: Julian Evans

The latter species, a pufferfish, was particularly interesting because it was one of the Atlantic species that extended their range to reach the central Mediterranean independent of any human involvement, and was therefore not considered to be an alien species.

Although recognition of the threats posed by invasive species resulted in the inclusion of management of such species in a number of recent policy actions, including local and EU legislation, these legal documents referred exclusively to “alien” species.

The Maltese researchers argued that all newcomer species had the propensity to disrupt native ecosystems, irrespective of whether they were considered to be “alien” or “naturally range-expanding” species.

Therefore, although humans were not responsible for the introduction of range-expanding species, for management considerations, assessment and monitoring of such species was as important as for invasive alien ones.

The Marine Ecology Research Group welcomes reports from sea-users, and others, of unusual or new marine organisms.

Contact group leader Prof. Schembri at the Department of Biology, tel. 2340 2272, e-mail: patrick.j.schembri@um.edu.mt .