Clionidae

Clionoidae is a family of molluscs known as "sea angels". They are in the subclass Heterobranchia, class Gastropoda. They are small pelagic gastropods. 
Habitat.
Sea angels are pelagic, meaning that they live in the open ocean. They can be found all over the world, but are most common in cold waters, especially around the poles. 
Description.
Sea angels are usually around 1-2 centimeters in length, with average sizes varying between species. The largest species, "Clione limacina," can reach lengths of up to 8.5 centimeters. 
Diet.
Sea angels are carnivores. Their main food source is "Thecosomata," or "sea butterflies", a clade of sea snails that are related to sea angels. When sea butterflies are not available, they can survive by feeding on small crustaceans. "Clione limacina" can survive for a year without food. 
Threats.
Carbon dioxide released by human activity reacts with ocean water in such a way that the ocean becomes more acidic. This process is known as ocean acidification. More acidic oceans make it harder for some shell-producing animals to survive. This includes sea butterflies, which are sea snails with very thin shells. Since sea angels are dependent on sea butterflies as a major food source, ocean acidification may impact sea angels as well.