Horror, Sleaze and Trash.

When underwater crop circles were first discovered, people were astonished. But soon the culprit was found to be pufferfish. Males create these underwater crop circles to attract females.
Females base their decision about whether or not to mate with a male upon his construction skills, although the researchers still don’t understand what it is, exactly, that females are looking for in their ideal circular pattern.

If things go well, however, the female will lay her eggs in the center of the circle, and then, like most fishes, the males will fertilize those eggs externally. (Source)