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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:

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This is to give you an idea of the scale. The jellyfish on the right was about the size of half a tennis ball.

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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?

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closed as too broad by Devashish Das, leonardo, J. Musser, The Last Word, MattDMo Jul 27 '14 at 1:13

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.