Sacoglossa

This group comprises about 300 opisthobranch species characterized, among other things, by an strictly herbivorous diet (only 2 or 3 species are known to feed on other opisthobranch eggs) and the presence of a radula with a single row of teeth (uniseriate). Radular teeth are pointed and are specialized in piercing the algae cells they feed on. Once worn, these teeth are not expelled to the environment as other opisthobranchs do, but they remain in a ventral sack of the buccal bulb, the ascus (a word derived from the Greek word “ἀσκός” for a skin bag used as a bottle, hence the name Ascoglossans or Sacoglossans used for this group). Within this sac or “ascus” the worn teeth are reabsorbed. The pharynx of the sacoglossans can use its muscles to suck the juices from the algae cells perforated by the radular teeth.

Elysia timida by Enric Madrenas

At the morphological level, there are sacoglossans with an external shell, known as testacean sacoglossans, like Oxynoe and shell lacking sacoglossans or naked sacoglossans like Bosellia. The naked sacoglossans can be confused with true sea slugs or nudibranchs. The sacoglossans are usually well cephalized, the often rolled rhinophores stand out on the head, have a crawling foot and the mantle can have papillae or cerata like Hermaea and Placida, lack any expansions like Limapontia or can even have a modified mantle forming a pair of parapodia like Elysia and Thuridilla.

Biology
Many species of sacoglossans, especially those without shell, are able to retain plasts (chloroplasts, rhodoplasts or other) in their tissues, obtained from the algal cells that they feed on, and maintain these functional plasts performing photosynthesis for hours, days, weeks and even months. This ability is called kleptoplasty (literally “steal plasts”) and is considered a peculiar kind of symbiosis between the sacoglossan and the algae plasts. It is not yet known the mechanism by which ingested algal plasts are not digested in the sacoglossan digestive tract. To explain the plasts operation (that require the expression of some nuclear algal genes) inside the body of the sacoglossans, it has been postulated that a long sacoglossan-algae coevolution originated an horizontal gene transfer from the algae to the sacoglossan so its plasts are somewhat “recognized” and kept functional. This retention capacity of plasts in the sacoglossan tissues has two beneficial effects on the animal. On one hand, the sacoglossan tissues where the plasts are accumulated acquire the plast color, green for chloroplasts, red for rhodoplasts, so the sacoglossan achieves a near perfect homochromy and crypsis on the alga it feeds on. Furthermore, the symbiont plasts, thanks to water, CO2 and the other bioelements supplied by the sacoglossan, allow for photosynthesis to take place, and thereby provide the animal with organic material which serves as food. Many species of sacoglossans, when food is scarce in the environment, are able to survive on the photosynthesized organic matter supply by their symbiont plasts. When this happens, the sacoglossans are also called “solar-powered sea slugs” (Rudman, W.B., 1998 Solar-powered sea slugs. [In] Sea Slug Forum. Australian Museum, Sydney). The defensive mechanisms of the sacoglossans range from homochromy on the algal substrate, crypsis on it due to their body shapes, the detachment of body parts (or autotomy), the presence of a shell, or the chemical defense by the use of secondary metabolites. In this last case there are sacoglossans that incorporate these metabolites from the algae they feed on and accumulate them unmodified in the animal tissues, releasing them when disturbed, while other species also obtain the metabolites from the alga but modify them slightly before releasing. Finally there are sacoglossans which may biosynthesize their own defense molecules without incorporating them from the diet. The sacoglossans are hermaphrodites, have internal fertilization through reciprocal copulation and the spawn is made as a coiled cord, ribbon, or other forms. The hatched veliger larvae may, depending on the species, feed on plankton (planktotrophic larvae) or from the egg own reserves (lecytotrophic larvae or yolk sac larvae).

Distribution
Most sacoglossan species have a tropical or subtropical distribution. They are found in the Indo-Pacific, the Caribbean or the Atlantic, and always have a close relationship with the distribution of the algal species they feed on. In European waters there have been cataloged about 50 sacoglossan species, most of them also present in the Iberian Peninsula coast. In the Catalan coast there have been cited a dozen species.

Taxonomy
The Sacoglossa is now a Superorder (formerly an Order) within the Subterclass Tectipleura. It contains three Superfamilies: Oxynooidea, which includes the shelled sacoglossans that feed almost exclusively on caulerpal group seaweed and do not retain the chloroplasts of the algae, the Plakobranchoidea that integrates the “naked sacoglossan” species, which have a more varied diet, while most species retain the algae plasts, and the Platyhedyloidea.

The taxonomy of Sacoglossa according to Bouchet et al. (2017) and used at WoRMS is:

  • Superorder Sacoglossa 
    • Superfamily Oxynooidea Stoliczka, 1868 (1847)
      • Family Cylindrobullidae Thiele, 1931
        • Genus Cylindrobulla P. Fischer, 1857
      • Family Juliidae E. A. Smith, 1885
        • Subfamily Bertheliniinae Keen & A. G. Smith, 1961
          • Genus Berthelinia Crosse, 1875
        • Subfamily Juliinae E. A. Smith, 1885
          • Genus Julia Gould, 1862
      • Family Oxynoidae Stoliczka, 1868 (1847)
        • Genus Lobiger Krohn, 1847
        • Genus Lophopleurella Zilch, 1956
        • Genus Oxynoe Rafinesque, 1814
        • Genus Roburnella Ev. Marcus, 1982
      • Family Volvatellidae Pilsbry, 1895
        • Genus Ascobulla Ev. Marcus, 1972
        • Genus Volvatella Pease, 1860
    • Superfamily Plakobranchoidea Gray, 1840
      • Family Costasiellidae K. B. Clark, 1984
        • Genus Costasiella Pruvot-Fol, 1951
        • Genus Panderevela Moro & Ortea, 2015
      • Family Hermaeidae H. Adams & A. Adams, 1854
        • Genus Aplysiopsis Deshayes, 1853
        • Genus Caliphylla A. Costa, 1867
        • Genus Cyerce Bergh, 1870
        • Genus Hermaea Lovén, 1844
        • Genus Mourgona Er. Marcus & Ev. Marcus, 1970
        • Genus Polybranchia Pease, 1860
        • Genus Sohgenia Hamatani, 1991
      • Family Limapontiidae Gray, 1847
        • Genus Alderella Odhner in Franc, 1968
        • Genus Alderia Allman, 1845
        • Genus Alderiopsis Baba, 1968
        • Genus Calliopaea d’Orbigny, 1837
        • Genus Ercolania Trinchese, 1872
        • Genus Kerryclarkella K. Jensen, 2015
        • Genus Limapontia Johnston, 1836
        • Genus Olea Agersborg, 1923
        • Genus Placida Trinchese, 1876
        • Genus Sacoproteus Krug, Wong, M. R. Medina, Gosliner & Valdés, 2018
        • Genus Stiliger Ehrenberg, 1828
      • Family Plakobranchidae Gray, 1840
        • Genus Bosellia Trinchese, 1891
        • Genus Elysia Risso, 1818
        • Genus Plakobranchus van Hasselt, 1824
        • Genus Thuridilla Bergh, 1872
    • Superfamily Platyhedyloidea Salvini-Plawen, 1973
      • Family Platyhedylidae Salvini-Plawen, 1973
        • Genus Gascoignella Jensen, 1985
        • Genus Platyhedyle Salvini-Plawen, 1973

Sacoglossan superfamilies cited in the Mediterranean Sea or around the Iberian Peninsula:

Oxynoe olivacea by Miquel Pontes

Oxynooidea

Elysia flava by Enric Madrenas

Plakobranchoidea

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