They have not been seen and they have not been missed.
The Malta office of the International Ocean Institute is investigating a sharp drop in jellyfish sightings this summer.
"We are investigating what is going on, it could be a variety of factors, notably sea water temperature" scientist Alan Deidun said when contacted. We are in the process of comparing the sea temperature this month with that of last year."
Currents, he added, could also be a factor.
He confirmed that sightings of the mauve stinger (Pelagia noctiluca), the most common type of jellyfish in Maltese waters, were down by between 80% and 90%.
"We are seeing far fewer jellyfish, even though the variety is wider," Dr Deidun said.
He warned, however, that a swarm of mauve stinger could still show up within weeks.
"The sea warmed up quickly in the past few days," he said.
Blooms normally start towards late autumn in the warmer waters off north Africa, making their appearance further north in Maltese waters later in the year.
Thousands of purple jellyfish bloomed earlier than usual in late December and swarmed the coastal area around Marsalforn and Xwejni bay in Gozo.
Their early presence had been seen at the time as a possible indication of the unstoppable warming of the Mediterranean Sea, whose temperature has risen by as much as 0.5˚C since 2002 at least in the western half of the basin.
The mauve stinger is a highly versatile species and can tolerate sea temperatures of between eight and 22 degrees, delaying its release of miniature jellyfish until optimal environmental conditions are found.
Until 1998, Pelagia blooms occurred every 12 years and had an average duration of four years. However, they became more common due to climate change, overfishing and coastal urbanisation and discharges.
Jellyfish sightings can be reported at www.ioikids.net/jellyfish, via SMS on 7922 2278 or through ioi-moc@um.edu.mt.
In April, the IOI reported the first sightings in Malta of the moon, or saucer, jellyfish – Aurelia aurita – which is distinguished by its quartet of horseshoe-shaped gonads (reproductive organs) found just beneath the surface of its bell.
This species is translucent and non-stinging and is normally much more abundant in the Atlantic and in the western parts of the Mediterranean, since it prefers temperate conditions. It is consumed by a number of predators, which include the leatherback sea turtle and many seabirds. The species rarely survives for more than six months.
Also reported near Malta is the minute but beautiful Cladonema radiatum species, characterized by its bright orange and highly visible eyespots at the base of their tentacles.
The polyp stage of this jellyfish is normally found attached to seaweeds or to seagrasses, like Posidonia oceanica. The species is a cosmopolitan one, being known from the Mediterranean, North Sea, Atlantic and even as far afield as Japan. It has a hopping way of swimming, then it suddenly folds its tentacles and lets itself descend.