Pteropoda, also called pteropods, are a group of small, free-swimming sea snails and sea slugs that live in the open ocean. Their name comes from the Greek words pterón (meaning "wing") and poús (meaning "foot"). Most pteropods live near the ocean's surface, within the top 10 meters, and are less than 1 centimeter long. Scientists are still discussing whether all pteropods share a single common ancestor. Some studies suggest they might be related to cephalopods, like squids and octopuses, but this remains debated. Recent research using DNA analysis supports the idea that pteropods are all descended from one ancestor.
Pteropoda includes two main groups: Thecosomata, also known as sea butterflies, and Gymnosomata, also called sea angels. Thecosomata have a shell, while Gymnosomata do not. These two groups may or may not be closely related. If they are not closely related, their similarities—such as being small, transparent, and using wing-like flaps (called parapodia) to swim—may have developed independently as adaptations to their shared lifestyle in the open ocean.
Taxonomy
The group Pteropoda was first created by Cuvier in 1804, and he called it "ptéropodes." Later, François Péron and Charles Alexandre Lesueur believed the group was larger and added other types of organisms, including some from the opisthobranch, heteropoda, and ctenophora groups. In 1810, these scientists split the group into two categories: those with shells and those without shells.
In 1824, H.M.D. de Blainville renamed the two groups Gymnosomata and Thecosomata. He also gave the combined group a new name, Aporobranchia, instead of Pteropoda. He removed most of the added groups but kept Phyllirhoë, placing it in a new group called Psilosomata. Later, Phyllirhoë was classified within the order Nudibranchia.
Other scientists tried to classify Pteropoda differently. J.E. Gray divided it into Dactylobranchia (with only the genus Cavolinia) and Pterobranchia (including all other genera). Cuvier and his followers did not agree with de Blainville’s classification and preferred the original system from Le Règne Animal.
Rang (1829) used Cuvier’s system but added a feature about whether organisms had a distinct head. L. Oken wanted each order to have four families and each family to have four genera for symmetry. P.A. (1829) divided Pteropoda based on the size of their fins, naming "Macroptérygiens" (only Pneumonoderma) and "Microptérygiens" (all others). W.B. Clark (1829) classified Pteropoda as a family and changed its name to Pteropodidae, a name later reused for a family of fruit bats.
Eventually, all these classifications were abandoned. As more species were discovered through scientific expeditions, the division of Pteropoda into Thecosomata and Gymnosomata became widely accepted.
The relationship between these two groups is not fully clear, but they are likely sister groups, meaning they share a common ancestor.
Evolutionary history
Pteropods are believed to have started living during the Early Cretaceous period, about 133 million years ago. They spread into different groups during the middle to late Cretaceous period. The oldest pteropod fossil found so far belongs to the Limacinidae family and was discovered in early-middle Campanian deposits on the San Juan Islands.
Threats
A study was done on the West Coast of the United States to examine how ocean acidification affects pteropods. Scientists tested Limacina helicina, a type of pteropod, to see how sensitive it is to lower pH levels. This pteropod has a shell made of calcium carbonate, which may be damaged by the more acidic ocean water caused by ocean acidification. Researchers placed a pteropod shell in ocean water with the pH level expected by the year 2100. After about 1.5 months in the water, the shell had almost fully dissolved.
Distribution
Pteropods are found in all major oceans, typically between 0–10 meters (0–33 feet) below the ocean surface and across all latitudes. They can be found deeper than 10 meters, but their numbers decrease as depth increases. However, data shows that pteropods from tropical regions are more spread out in deeper areas. Pteropods are rarely found in the deep sea, as few live below 500 meters. They are most common in areas with high nutrient availability, such as continental shelves, where they are more numerous. Spring is a time when pteropod populations increase, though data indicates that pteropods south of the equator are less abundant during certain seasons. According to current data, 93% of the world’s pteropods belong to the Thecosomata suborder, while 7% belong to the Gymnosomata suborder.