If you go to the pool this summer, watch out for False Caustics. They look like the dancing lights at the bottom of the pool, but they stay mostly still.
False Caustics (Praya dulcis) are in fact siphonophores that colonize pools with poor filtration systems. If you swim through them, they will burn your skin. Their sting is rarely fatal, but it causes excruciating pain that can last several years.
These siphonophores are colonies of tiny organisms related to jellyfish. They are the only known species in their group to live in fresh water, and likely imitate caustics as a form of camouflage.