This is a shot of the Mnemiopsis leidyi from the Greek Aegean.
This comb jelly is one of the worst invasive species in our region. A decade ago the Mnemiopsis was brought to the Black sea with ballast tank waters and it found itself in a heaven with no existing predators. An opportunistic predator feeding on zooplankton and fish larvae, the mnemiopsis infestation grew to disastrous proportions and is pointed out as the main reason for the dramatic decline of fish stock in the Black sea.
By pure chance its main predator Beroe ovata - another comb jelly - was brought to the Black sea the same way. The Beroe managed to withstand the low salinity levels of the Black sea and began regulating the Mnemiopsis numbers. But it took at least 5 years for the things to start returning to normal...
I was very sad to find many Mnemiopsis in the Northern Aegean and not a single Beroe. This proves the infestation is already spread to the Medterranean.
The Mnemiopsis leidyi feeds at night and when hunting it spreads in something that looks a lot like the Imperial fighters from the Star wars - (-0-) . On every dive my focus light attracted a cloud of zooplankton and small crustaceans that spoiled my shots. But in this case it helped me to photograph a Mnemiopsis with full 'belly'.