The blue runner (Caranx crysos) is a controversial fish. Many think it is an “alien” species, that is, one which only recently entered the Med; others just keep calling it jackfish. In fact it is endemic to the Mediterranean, where it was described as early as in the ’60s. Yet it’s true that in the last years the encounters have increased drastically, although restricted to the southernmost regions.
Be it the global warming, be it other factors, let’s just state that this fish shows perfectly how the sea is a continuously changing system.
It is a pelagic species, meaning it lives in the open sea, except during the mating period between April and October, when it moves near the coast.
Here it is always in a frenzy: it hunts by diving into the crowds of small fishes, like in this wonderful video from the Secchitella dive site in Linosa.
As a small kid the blue runner likes to call any floating object home.
The most remarkable example is the jellyfish Cotylorhiza tubercolata, under whose umbrella you’ll often discover minuscule juveniles looking for protection against the predators (like in the picture).
Other artificial objects do work as well, especially because under them grow barnacles and dwell small molluscs and crustaceans that they can easily feed upon.
A curiosity. In the mating period the blue runner has a marked sexual dimorphism: the males use to “dress” in black (like in the video), which evidently females don’t dislike… You then see them hunting together: a… modern couple!