Thecosomates

Thecosomates, commonly called sea butterflies, are a group of small, pelagic marine gastropods within the larger clade of pteropods. They spend their entire life cycle in the open ocean, using their wing-like extensions of the foot, or parapodia, to swim and maneuver in the water column.

Morphology and classification

Thecosomates typically possess a delicate external shell that is calcareous, transparent, and often conical or teardrop-shaped. The shell is mainly composed of aragonite and is readily dissolved in more acidic waters, making these animals particularly vulnerable to changes in ocean chemistry. The body is streamlined, with the foot modified into locomotory fins that aid buoyancy and propulsion. They are one of the two major groups of pteropods, the other being Gymnosomata (the sea angels), with both groups commonly referred to as pteropods.

Habitat and ecology

Thecosomates inhabit pelagic, open-ocean environments worldwide, from tropical to polar regions, and are especially common in productive epipelagic zones. They feed primarily on microalgae and other small plankton, acting as important grazers in marine food webs. Their translucent shells and small size make them a key link between primary production and higher trophic levels. When they die, their calcareous shells contribute to carbonate flux in the deep sea.

Reproduction and significance

Reproduction is sexual, with eggs released into the water column and developing into free-swimming larval stages. Thecosomates are important in paleoceanography because their aragonitic shells fossilize in marine sediments, providing clues about past ocean conditions. They are also considered indicators of ocean health, as declining populations can reflect changes in ocean acidity and temperature. Notable genera include Limacina and Cavolinia.