Gymnosomata

The gymnosomata form a pelagic opisthobranchs order that, together with thecosomata, are known under the name of pteropods (Pteropoda = wing shaped foot) due to expansions of their bodies functioning as swimming fins. The name gymnosomata refers to the naked (gymnos) body (soma). The gymnosomata are commonly called sea angels.

Description
The gymnosomata generally have a translucent body and use to be small sized, as few species reach 5 cm in length as Clione limacina. They lack both shell and mantle cavity, have a fusiform or cylindrical body and bilateral symmetry. The head is well developed and has a pair of oral tentacles and a pair of rhinophores. The mouth, which can be evaginated forming a proboscis or trunk, is armed with tentacles equipped with suckers, hooks and a radula with a median tooth and several side teeth. The digestive tube lacks the horny plates, present in other pteropods, used to grind food. Some species have gills. The foot lobes are divided into two anterior propodial lobes and two lateral epipodial lobes, these highly developed and provided with a musculature that allows functioning as swimming fins.

Clione limacina by Manuel Ballesteros by Manuel Ballesteros

Biology
The gymnosomata are planktonic (mainly epi and mesopelagic) and active swimmers but, unlike Thecosomata, they are voracious predators and carnivores. They actively seek their prey, which are usually thecosomata, that are being captured using the proboscis and the buccal tentacles. The gymnosomata swim faster than thecosomata and also perform vertical migrations in the water column at night, ascending to shallow levels at sunset and descending back at dawn. The gymnosomata are proterandric hermaphrodites, they perform internal fertilization when two individuals mate and both exchange sperm. Shortly after fertilization, they spawn to the water, the eggs embedded in a gelatinous structure as a string or blob. Larvae veligers have a thimble-shaped shell that is lost in the metamorphosis.

Distribution
About 40 gymnosomata species are known worldwide. Many of them have a wide geographical distribution, from polar to equatorial seas. In some species there are different forms or subspecies depending on the latitude of the sea in which they live and others like Clione limacina have a mainly polar distribution. In this species, individuals living in low warmer latitudes are smaller than those in polar waters.

Taxonomy
According to WoRMS the presently accepted taxonomy for Gymnosomata is this:

Suborder Gymnosomata
- Superfamily Clionoidea Rafinesque, 1815
  - Family Clionidae Rafinesque, 1815
    - Genus Cephalobrachia Bonnevie, 1913
    - Genus Clione Pallas, 1774
    - Genus Fowlerina Pelseneer, 1906
    - Genus Massya Pruvot-Fol, 1924
    - Genus Paedoclione Danforth, 1907
    - Genus Paraclione Tesch, 1903
    - Genus Thalassopterus Kwietniewski, 1910
    - Genus Thliptodon Boas, 1886
  - Family Cliopsidae O. G. Costa, 1873
    - Genus Cliopsis Troschel, 1854
    - Genus Pruvotella Pruvot-Fol, 1932
  - Family Notobranchaeidae Pelseneer, 1886
    - Genus Notobranchaea Pelseneer, 1886
  - Family Pneumodermatidae Latreille, 1825
    - Genus Abranchaea F.-S. Zhang, 1964
    - Genus Platybrachium Minichev, 1976
    - Genus Pneumoderma de Roissy, 1805
    - Genus Pneumodermopsis Keferstein, 1862
    - Genus Schizobrachium Meisenheimer, 1903
    - Genus Spongiobranchaea d’Orbigny, 1836
- Superfamily Hydromyloidea Pruvot-Fol, 1942 (1862)
  - Family Hydromylidae Pruvot-Fol, 1942 (1862)
    - Genus Hydromyles Gistel, 1848
  - Family Laginiopsidae Pruvot-Fol, 1922
    - Genus Laginiopsis Pruvot-Fol, 1922