Pteropoda

Pteropoda, meaning "wing", and πούς are specialized free-swimming pelagic sea snails and sea slugs, marine opisthobranch gastropods. Most live in the top 10 m of the ocean and are less than 1 cm long. The monophyly of Pteropoda is the subject of a lengthy debate; they have even been considered as paraphyletic with respect to cephalopods. Current consensus, guided by molecular studies, leans towards interpreting the group as monophyletic.
Pteropoda encompasses the two clades Thecosomata, the sea butterflies, and Gymnosomata, the sea angels. The Thecosomata have a shell, while the Gymnosomata do not. The two clades may or may not be sister taxa; if not, their similarity may reflect convergent adaptation to their particular lifestyle.

Taxonomy

The group Pteropoda was established by Cuvier as "ptéropodes" in 1804. François Péron and Charles Alexandre Lesueur thought the group to be larger, and so they also included the opisthobranch taxa, the heteropoda taxa, and even the Ctenophora. In 1810, these authors divided the whole group in two separate groups: Those with a shell and those without a shell.
In 1824, H.M.D. de Blainville named these two groups Gymnosomata and Thecosomata and named the combining order Aporobranchia instead of Pteropoda. He rejected the additional genera, except Phyllirhoë which he upgraded to a third group that he called Psilosomata. Only much later was Phyllirhoë classified within the order Nudibranchia.
Other attempts were made to describe the Pteropoda: J.E. Gray divided the Pteropoda into Dactylobranchia and Pterobranchia. Cuvier did not accept the classification by de Blainville; they preferred the original classification as described in Le Règne Animal.
Rang followed the Cuvierian classification but tried to include the character of having a distinct head or not. The German naturalist L. Oken went one step further and, for the sake of symmetry, wanted each order to contain four families and each family to contain four genera. P.A. 1829, divided the Pteropoda according to the size of their fins: "Macroptérygiens" and "Microptérygiens". W.B. Clark treated the Pteropoda as a family and emended the spelling to Pteropodidae
Finally, all these attempts were abandoned and, as more and more species were described as a result of several scientific expeditions, the classification of the Pteropoda into Thecosomata and Gymnosomata was generally adopted.
The relationship between these two clades is not unequivocally established, but it seems that they are sister taxa.

Evolutionary history

Pteropods are estimated to have originated during the Early Cretaceous, around 133 million years ago, with the diversification into the major lineages occurring during the mid-late Cretaceous. The oldest known fossil pteropod is a member of Limacinidae from the early-middle Campanian deposits of the San Juan Islands.

Phylogeny

Threats

Vulnerability to ocean acidification

A study was conducted on the West Coast of the United States to see ocean acidification's effects on pteropods. Limacina helicina was used to test the sensitivity to decreasing pH. This species of pteropod is potentially vulnerable to the corrosive waters associated with ocean acidification due to their calcium carbonate shell. The shell of a pteropod was immersed in ocean water with the projected pH level that the water will reach by the year 2100. After a month and a half in the water, the shell had almost completely dissolved.

Distribution

Pteropods are found in all major oceans, usually below the ocean surface and in all levels of latitude. Pteropods can be found lower than 10 meters, but in less amounts in terms of biomass, however, pteropod distribution is more spread out deeper based on findings. This can be explained as Pteropods from tropical areas become more common in deeper areas. They are not found commonly in the deep sea, in fact, few live lower than 500 meters below sea level. Continental shelves, areas containing many opportunities for nutrients, and productivity are locations in which Pteropods are usually populous, according to patterns in data. Springtime is a peak season for Pteropoda, as they reach higher populations, though data shows that Pteropoda south of the equator are less abundant seasonally. In addition, current data suggests that 93% of the world's pteropods are part of the Thecosomata suborder, while the 7% are Gymnosomata.