Pteropods are tiny marine mollusks separated into two groups, the sea butterflies (clade Thecosomata) and the sea angels (clade Gymnosomata). Sea butterflies develop shells, but sea angels do not. These tiny gastropods are such a major dietary staple of larger marine species, they are affectionately deemed the potato chips of the sea.
Photo courtesy of NOAA
Pteropods are small winged snails that float in midwater. When they die, their internal shells of aragonite (calcium carbonate) sink to the seabed, contributing to biogenic oozes. The presence of pteropod remains in samples collected from deep in the ooze reveal changes over millennia in water temperatures and sea levels.