Sea butterflies (also called pteropods) are sea snails aptly named: they are shelled marine snails, each with a foot like a wing, that swim in the water column like butterflies. This one, Atlanta peronii, is very small: the biggest specimen on record was less than half an inch (11 millimeters) long! They are completely transparent, so you can see all their organs, and have shiny black eyes.
This A. peronii was observed in the Sargasso Sea by the Census of Marine Zooplankton.
See more photos of cool zooplankton collected by the researchers in this zooplankton biodiversity slideshow.