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

These are some species cited in the Mediterranean Sea or around the Iberian Peninsula:

Clione limacina eating Limacina by Alexander Semenov

Clione limacina

Cliopsis krohnii @ Lake Worth Lagoon, Florida, USA 2-10-2011 by Anne DuPont

Cliopsis krohnii

Pneumoderma mediterraneum

Pneumoderma mediterraneum

Pneumoderma violaceum

Pneumoderma violaceum

The last species is very common in polar seas.

Bibliography

    Ballesteros M. 2007. Lista actualizada de los opistobranquios (Mollusca: Gastropoda: Opisthobranchia) de las costas catalanas. Spira. 2(3):163–188.
    Barnich R, Uthe D. 1998. The Gymnosomata (Gastropoda : Opisthobranchia) in the plankton of the French Mediterranean coast. Vie Et Milieu-Life and Environment. 48(1):15–24. Available from ://WOS:000074390300003.
    Bouchet P, Rocroi JP. 2005. Classification and nomenclator of gastropod families. Malacologia. 47(1–2):1–397.
    Bouchet P, Rocroi JP, Hausdorf B, et al. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia. 61(1–2):1–526.
    Chaban EM. 2008. Opisthobranchiate Mollusca of the orders Cephalaspidea, Thecosomata and Gymnosomata (Mollusca, Opisthobranchia) of the Chukchi Sea and the Bering Strait. IN: B.I. Sirenko & S.V. Vassilenko (eds), Fauna and zoogeography of benthos of the Chukchi Sea. Issledovaniia Fauny Morei [Explorations of the fauna of the seas] 61(69): 149-162. [in Russian].
    Dadon JR, Chauvin SF. 1998. Distribution and abundance of gymnosomata (Gastropoda : Opisthobranchia) in the southwest Atlantic. Journal of Molluscan Studies. 64:345–354. https://doi.org/10.1093/mollus/64.3.345.
    Fahrner A, Haszprunar G. 2000. Microanatomy and ultrastructure of the excretory system of two pelagic opisthobranch species (Gastropoda: Gymnosomata and Thecosomata). Journal of submicroscopic cytology and pathology. 32(2):185–94. Available from http://www.ncbi.nlm.nih.gov/pubmed/11085207.
    Klussmann-Kolb A, Dinapoli A. 2006. Systematic position of the pelagic Thecosomata and Gymnosomata within Opisthobranchia (Mollusca, Gastropoda) - revival of the Pteropoda. Journal of Zoological Systematics and Evolutionary Research. 44(2):118–129. https://doi.org/10.1111/j.1439-0469.2006.00351.x.
    Lalli CM, Conover RJ. 1973. Reproduction and development of Paedoclione doliiformis, and a comparison with Clione limacina (Opisthobranchia, Gymnosomata). Marine Biology, 19: 13-22, 7 figs.
    Massy AL. 1920. Eupteropoda (Pteropoda Thecosomata) and Pterota (Pteropoda Gymnosomata). British Antarctic ('Terra Nova’) Expedition 1910, Nat. Hist. rep, Zoology, 2(9). Mollusca, 3, 203-232, 9 figs.
    Massy AL. 1932. Mollusca: Gastropoda Thecosomata and Gymnosomata. Disc. Reports, 3: 267296, pl 39.
    McGowan JA. 1968. Thecosomata and Gymnosomata. Veliger 3(suppl):103-135, pls 12-20.
    Mifsud C. 2001. A mysterious, living, “giant” Gymnosomata species near the Maltese Islands (Gastropoda, Opisthobranchia). Basteria 65(1-3):57-60.
    Minichev YS. 1963. Anatomy of Anopsia gaudichaudi (Souleyet) and systematic position of Gymnosomata (Opisthobranchia). Zool. Zhurn., 42(9): 1317-1328, 6 figs.
    Morton JE. 1957. Opisthobranch Mollusca, Gymnosomata fam Clionidae. Fiches; Cons. intern. Explor. Mer, Zooplankton-sheet, 80: 1-4, 7 figs.
    Newman LJ. 1989. Pneumodermopsis spoeli, a New Pteropod (Opisthobranchia: Gymnosomata) from Australian Waters. Bull. Zool. Mus. Univ. Van Amsterdam 12(4):81-88.
    Pelseneer P. 1887. Report on the Pteropoda, 1. The Gymnosomata. Report of the H M.S. Challenger, 19(4): .
    Pruvot Fol A. 1934. Faune et flore de la Méditerranée. GastropodaOpisthobranchiaGymnosomata. Comm Intern. Expl Sc. Mer Méditerr.
    Starobogatov JI. 1971. Ordo Thecosomata; ordo Gymnosomata; ordo Heteropoda, In: Polevoy oprodeliteli planktona. Moscou (Zool. Inst. Akad): (without page or figure numbers, not bound, mimeographed).
    Tesch JJ. 1950. The Gymnosomata, 2. Dana Rep, 6(36): 1-55, 37 figs.
    Tesch JJ. 1903. Vorläufige Mitteilung über die Thecosomata und Gymnosomata der Siboga-Expedition. Tijdschrift der Nederlandsche Dierkundige Vereeniging (2) 9 (2): 111-117.
    Tesch JJ. 1904. The Thecosomata and Gymnosomata of the Siboga Expedition. Siboga- Expeditie, Leiden, E.J. Brill, 52: 1-92.
    Thollesson M. 1999. Phylogenetic analysis of Euthyneura (Gastropoda) by means of the 16S rRNA gene: use of a “fast” gene for “higher-level” phylogenies. Proceedings of the Royal Society B-Biological Sciences. 266(1414):75–83. https://doi.org/10.1098/rspb.1999.0606.
    van der Spoel TS. 1972. A taxonomical outline of the Gymnosomata (Mollusca). Basteria, 36(25): 7588.
    van der Spoel S. 1976. Pseudothecosomata, Gymnosomata and Heteropoda (Gastropoda). Utrecht (Bohn, Scheltema & Holkema): 484 pp, 246 figs.
    Vortsepneva EV, Tzetlin AB. 2014. NEW DATA ON THE FINE STRUCTURE OF HOOKS IN CLIONE LIMACINA (GASTROPODA, OPISTOBRANCHIA) AND DIVERSITY OF THE JAW APPARATUS IN GASTROPODS. Zoologichesky Zhurnal. 93(3):466–478. https://doi.org/10.7868/s0044513414030179.

    Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.