Researchers from the University of Malta have just published a scientific report in the international journal Marine Biodiversity Records on a very unusual jellyfish that suddenly appeared in local waters for the first time last year.
This species, called Cassiopea andromeda, and popularly known as the "Upside-down Jellyfish", is actually a circumtropical species found in warmer parts of the Atlantic and Pacific, including also the Red Sea. (This type of jellyfish has nothing to do with the thousands of jellyfish seen in Maltese waters yesterday - see separate story)
The University said Cassiopea andromeda is not a native of the Mediterranean but as have done hundreds of other marine organisms, it entered the Mediterranean through the Suez Canal and established populations in the Levantine Sea and Aegean Sea, where it is now quite a common species.
The almost one-way influx of Red Sea species into the Mediterranean through the Suez Canal has been going on since the Canal was opened and is a phenomenon termed ‘Lessepsian migration', named after the French diplomat Ferdinand de Lesseps who was instrumental in the development of the Canal.
Until now, no populations of Cassiopea andromeda were known from further west than the south-eastern Aegean Sea. It was therefore quite surprising when the team of researchers: Prof. Patrick J. Schembri from the Department of Biology, Dr Alan Deidun, and Mr Patrick Vella, both previously postgraduate students at the Department of Biology and now with the Physical Oceanography Unit of the University and with Salpa Ltd, respectively, came across an aggregation of at least 50 individuals close to the mouth of Marsamxett Harbour in March of last year.
The jellyfish, ranging in size from an umbrella diameter of 3cm to 11cm, were carpeting the muddy-sand bottom at a depth of 3.5-6m. Remnants of this aggregation were still present at the end of June.
Although they sting, these jellyfish are not dangerous because their sting is very mild and most people do not feel it at all.
Moreover, these jellyfish do not normally swim in the water but lie on the bottom with their umbrella facing and touching the substratum and their arms, which are short and shrubby, pointing towards the surface - hence the common name of Upside-down Jellyfish. They adopt this peculiar posture for a reason: within their tissues these jellyfish house large numbers of symbiotic microscopic photosynthetic microorganisms that need light to manufacture their food. T
he jellyfish accommodate their guests by exposing the arms, where the bulk of these symbionts are found, to the light; in turn these symbionts provide the jellyfish with nutrients. While the Upside-down Jellyfish gets a substantial proportion of its dietary requirements from this partnership, the animals also capture and feed on minute animals of the plankton.
How these alien jellyfish arrived in Malta is unknown. Given that there are no known populations of this species between the Aegean and the central Mediterranean, the most likely explanation is via shipping, either in ballast water or on the hull of vessels.
The Upside-down Jellyfish is the latest of a number of warm water alien species that are expanding their range towards the central Mediterranean. Many Mediterranean marine biologists suspect that this is related to the warming trend observed in Mediterranean waters over the past few decades.