"Sea butterflies" refers to a type of marine animal called pteropods. They are small creatures that live in the ocean and have a translucent body shaped like a butterfly. Full definition
And just as for sea butterflies, the carbonate shortage that comes with ocean acidification means trouble for coral reefs. (sciencenewsforstudents.org)
Some key prey species like sea butterflies may be harmed. (climatestate.com)