When the sea temperature warms up, the jellyfish population seems to expand, which is not good if you like splashing around in the sea. Now I have yet to see a jellyfish since arriving here in 2011, despite spending many hours in the water - mostly off the rocky beaches around Sliema - but I am told that 2011 was an unusually thin year for jellyfish and so you can't always count on avoiding them. In fact sightings have already shown a marked increase by early summer 2012.
There are a wide variety of these little critters around, and the chart on the left (from the 'spot the jellyfish' initiative) shows a selection of the more common ones. The most common of all is the Mauve Stinger, which will give you a pretty painful sting but no more than that. The most dangerous types, however, are the Box Jellyfish and Portuguese Man o' War. If you see something that looks like either of these, get moving in the opposite direction.
