Thecosomata

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

Pteropods – Swimming Mollusks from Parafilms on Vimeo.

 

The thecosomata are pelagic, swimmer and holoplanktonic opisthobranchs that spend their entire lives in planktonic phase. Their name refers to having the body protected by a cover or theca (case in Greek) that could be calcified or a cartilage-like structure. As an adaptation to pelagic life, the thecosomata have foot expansions that allows them to use as swimming fins. For this reason they are commonly called Sea butterflies.

In species with calcified shell, it could be spirallled such as Limacina or shaped like a conical or pyramidal tube, as in the species of the genera Cavolinia and Clio, but the shell is always fragile, with low consistency, to reduce the weight of the animal and facilitate its suspension in water. Species with calcareous shell may also have a horny operculum, usually with spines, used to completely enclose and protect the animal inside the shell. Other species, such as genus Cymbulia, have no calcified shell but have a cartilaginous consistency structure instead, the pseudoconch, which is internal because it is covered by a thin epithelium; this pseudoconch is semitransparent and has a curled shape that allows to accommodate the animal’s visceral mass. Some species of the Desmopteridae family lack any kind of shell.

Corolla spectabilis, a typical Thecosomata

The head of thecosomata has a pair of tentacles, called rhinophores, for olfactory purposes. The mantle cavity is well developed in this group but only a few species, such as genus Peracle, have a gill. Other species can breathe through the mantle epithelium. The radula has 3 to 5 teeth per row and digestive tract has some sort of masticatory plates to grind the eaten food.

Biology
The thecosomata are great swimmers and it has been calculated that they can swim at a speed of up to 45 cm per second. Daily vertical migrations are common in most species of thecosomata. Some epipelagic species as Clyo pyramidata perform daily vertical migrations in the water column of up to 1,000 m, so during the day they live at approximately this depth while every night climb to more superficial levels for feeding. In other species it has been observed that while adults are found in deep waters, juveniles tend to live in shallow waters.

The thecosomata are microphages that feed on microscopic plankton (dinoflagellates, diatoms, radiolarians, foraminifera, etc.) that is captured after being directed to the mouth. In the Euthecosomata swimming fins beat and cilia on the ciliated epithelium directs the water into the mantle cavity where phytoplankton food is retained by mucus, forming a cord that goes into the mouth. In Pseudothecosomata plankton capture is different: epithelia and mantle fins secrete large amount of mucus forming an extensive mucous network up to 2 m long floating around the animal, food particles are retained in this network and the animal simply collects and swallows the mucous network from time to time.

Thecosomata are hermaphrodites and when two individuals come close together they have a mutual intercourse in which both fertilize each other. After fertilization the eggs are released into the water immersed in gelatinous or mucous substance. The ovigerous capsules hatching can produce shelled or unshelled veliger larvae. The larval stage can last from 45 to 90 days and the adults live between 1 to 2 years.

The progressive acidification of marine waters as a result of increased athmospheric CO2 levels affect very negatively the survival rate of shelled thecosomata and there is a theory that because of this reason, if the present trend continues, the southern seas will soon be uninhabitable for these testacean species.

Distribution
Thecosomata, as many marine plankton species, usually have a wide geographic distribution that depends largely on ocean currents. They can be found from polar waters to tropical seas but there are frequent changes in size and morphology of many species related to their geographical location, which has led to consider forms or subspecies.

Thecosomata of the Catalan coast are mainly cited from empty shells or pseudoconchs stranded on beaches after storms or occasionally collected by commercial trawlers.

Classification
There are about 80 thecosomata species known worldwide. They are classified mainly by the shape of the shells, while other traits such as the shape of the fins, mantle or oral appendages are secondary. Thecosomata are divided in two groups, Euthecosomata that have a calcified shell and Pseudothecosomata who lack a calcified shell, so they secrete a cartilaginous pseudoconch, or simply have the body exposed. There are 37 thecosomata species cited in the Iberian Peninsula and Balearic Islands while 16 have been cited in the Catalan coast.

The taxonomy of Thecosomata by WORMS is:

  • Order Thecosomata Blainville, 1824
    • Suborder Euthecosomata Meisenheimer, 1905
      • Family Cavoliniidae Gray, 1850 (1815)
        • Genus Cavolinia Abildgaard, 1791
        • Genus Diacavolinia van der Spoel, 1987
        • Genus Diacria J.E. Gray, 1840
      • Family Cliidae Jeffreys, 1869
        • Genus Clio Linnaeus, 1767
      • Family Cuvierinidae van der Spoel, 1967
        • Genus Cuvierina Boas, 1886
      • Family Creseidae Rampal, 1973
        • Genus Creseis Rang, 1828
        • Genus Hyalocylis Fol, 1875
        • Genus Styliola Gray, 1847
      • Family Limacinidae Gray, 1840
        • Genus Heliconoides d’Orbigny, 1835
        • Genus Limacina Bosc, 1817
    • Suborder Pseudothecosomata Meisenheimer, 1905
      • Family Cymbuliidae Gray, 1840
        • Genus Cymbulia Peron & Lesueur, 1810
        • Genus Corolla Dall, 1871
        • Genus Gleba Forskal, 1776
      • Family Desmopteridae Chun, 1889
        • Genus Desmopterus Chun, 1889
      • Family Peraclidae Tesch, 1913
        • Genus Peracle Forbes, 1844

Some thecosomata species cited in the Mediterranean Sea or in the Iberian Peninsula:

Cavolinia gibbosa

Cavolinia gibbosa

Cavolinia-globulosa 5mm @Kaula Rock, Hawaii, Vanderbilt Pac.Equatorial Exp. 1951 by Cory Pittman - CAS collection (www.seaslugsofhawaii.com)

Cavolinia globulosa

Cavolinia inflexa by Christian Sardet (www.planktonchronicles.org)

Cavolinia inflexa

Cavolinia tridentata 12 mm (Pacific population) by Cory Pittman seaslugsofhawaii.com - source CAS collection

Cavolinia tridentata

Cavolina uncinata by Russell Hopcroft UAF-CoML reproduced with permission (www.cmarz.org)

Cavolinia uncinata

Clio cuspidata with hydroids by Cheryl Clarke UAF-CMarZ reproduced with permission (www.cmarz.org)

Clio cuspidata

Clio pyramidata 7 mm (Pacific population) by Cory Pittman seaslugsofhawaii.com

Clio pyramidata

Corolla spectabilis

Corolla spectabilis

Creseis clava (C.acicula) 3.9 mm (Pacific population) by Cory Pittman seaslugsofhawaii.com

Creseis clava

Creseis conica @ Desterro, Santa Catarina, Brasil by Joop Trausel and Frans Slieker (Natural Museum Rotterdam)

Creseis conica

Creseis virgula by Russell Hopcroft UAF-CoML reproduced with permission from www.cmarz.org

Creseis virgula

Cuvierina columnella (Rang, 1827) by Russell Hopcroft UAF-CoML reproduced with permission (www.cmarz.org)

Cuvierina columnella

Cymbulia peronii by Gilles Cavignaux

Cymbulia peronii

Diacavolinia limbata

Diacavolinia limbata

Diacavolinia longirostris 8 mm (Pacific population) by Pauline Fiene seaslugsofhawaii.com

Diacavolinia longirostris

Diacria trispinosa by Russell Hopcroft UAF-NOAA-CoML reproduced with permission (www.cmarz.org)

Diacria trispinosa

Heliconoides inflatus (Embolus inflatus-Limacina inflata) 0.9 mm (Pacific population) by Cory Pittman seaslugsofhawaii.com

Heliconoides inflatus

Limacina bulimoides by Russell Hopcroft UAF-NOAA-CoML reproduced with permission (www.cmarz.org)

Limacina bulimoides

Limacina helicina by Alexander Semenov

Limacina helicina

Limacina leseuri by Russell Hopcroft UAF-NOAA-CoML reproduced with permission (www.cmarz.org)

Limacina lesueurii

Limacina retroversa by Russell Hopcroft UAF-CoML

Limacina retroversa

Peracle reticulata

Peracle reticulata

Styliola subula 7 mm (Pacific population) by Pauline Fiene seaslugsofhawaii.com

Styliola subula

Bibliography