I filmed the creatures below this August in the Mediterranean (specifically, the port of Fornells in Minorca of the Balleares). The images were taken from a boat moored just outside the harbor and flowting above a field of posidonia in case that is relevant.
There were many of them and they were whizzing about at high speed:
This is to give you an idea of the scale. The jellyfish on the right was about the size of half a tennis ball.
They were a very strange sight since there were so many of them and moving so fast. You can see a video of them in motion here:
The image quality is not very good but you can see how fast the things were moving. So, any idea what these are? I am assuming they are larvae of something or other but what?