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.

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).

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.

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


Elysia flava by Enric Madrenas



    Baba, K. 1961. The shells and radulae in Berthelinia, a bivalved sacoglossan genus. Venus 21(4): 389-401, pl 21.
    Baba, K. 1952. Record of a rare sacoglossan mollusk, Lobiger (Lobiger) sagamiensis s sp from Sagami Bay, Japan. Zool Mag., Zool Soc Japan, 61(11): 337-338 (21-22), text figs.1-3.
    Baba, K. 1937. A new noteworthy species of the Sacoglossan Genus Alderia, from Amakusa, Japan. Zoological Magazine, Japan Dobutsugaku Zasshi 49(7):249-251.
    Baba, K. 1959. The family Stiligeridae from Japan (Opisthobranchia-Sacoglossa). Publications of the Seto Marine Biological Laboratory VII(3): 327-334.
    Baba, K. 1968. A revised description of Alderia nigra Baba, 1937, type species of Alderiopsis, n.g., from Japan (Opisthobranchia-Sacoglossa). Bijdragen tot de Dierkunde 38: 5-11, pls. 1-2.
    Baba, K. 1974. Notes on Lobiger souverbii Fischer, 1856, re-identified, of Japan (Opisthobranchia: Sacoglossa: Lobigeridae). Collecting & Breeding 36(4): 74-77.
    Baba, K. 1974. Some Comments On Lobiger souverbii Re Identified Of Japan Opisthobranchia Sacoglossa Lobigeridae. Veliger 16 (3). 1974. 253-257.
    Baba, K. 1966. Gross anatomy of the specimens of the shelled sacoglossan Volvatella (= Arthessa) collected from Okino-Erabu Island, Southern Kyushu, Japan (Nudibranchia). Publ. Seto Mar. Biol. Lab. 14(3): 197-205, pls 7-10.
    Baba, K., and I. Hamatani. 1971. Record Of Phyllobranchillus orientalis Synonym Branchophyllum orientale Var sagamiense From Cape Shiono-Misaki Kii Middle Japan Opisthobranchia Sacoglossa. Publ. Seto Mar. Biol. Lab. 19(4):197-201.
    Baba, K., and I. Hamatani. 1970. The Anatomy Of Ercolania boodleae From Seto Kii Middle Japan Opisthobranchia Sacoglossa. Publ. Seto Mar. Biol. Lab. 18 (4). 1970. 215-222.
    Baba, K., and I. Hamatani. 1970. Occurrences Of Specimens Presumably Identifiable With Stiliger ornatus At Seto Kii Middle Japan Opisthobranchia Sacoglossa. Publ. Seto Mar. Biol. Lab. 18 (3). 1970. 199-206.
    Bailey, K. H., and J. S. Bleakney. 1967. First Canadian report of the sacoglossan Elysia chlorotica Gould. Vel 9(3): 353-354.
    Ballesteros, M. 2007. Lista actualizada de los opistobranquios (Mollusca: Gastropoda: Opisthobranchia) de las costas catalanas. Spira. 2: 163–188.
    Barletta, G. 1980. Guide per il riconoscimento delle specie animali delle acque lagunari e costiere italiene. 3. Gasteropodi nudi (Pleurobranchomorpha, Sacoglossa, Aplysiomorpha e Nudibranchia). ...pp.
    Barletta, G. 1981. Gasteropodi nudi (Pleurobranchomorpha, Sacoglossa, Aplysiomorpha e Nudibranchia) Consiglio Nazionale delle Richerche, Guide per il riconoscimento delle specie animali delle acque lagunari e costiere italiane AQ/1/92, 3. Quaderni della Civica Stazione Idrobiologica di Milano (9):1-124.
    Becerro, M. A., G. Goetz, V. J. Paul, and P. J. Scheuer. 2001. Chemical defenses of the sacoglossan mollusk Elysia rufescens and its host alga Bryopsis sp Journal of Chemical Ecology November, 2001. 27 (11): 2287-2299.
    Bleakney, J. S. 1990. Indirect Evidence of a Morphological Response in the Radula of Placida dendritica (Alder and Hancock, 1843) (Opisthobranchia, Ascoglossa Sacoglossa) to Different Algal Prey. Veliger 33(1):111-115.
    Bleakney, J. S. 1989. Morphological Variation In The Radula Of Placida dendritica Alder And Hancock 1843 Opisthobranchia Ascoglossa-Sacoglossa From Atlantic And Pacific Populations. Veliger 32(2):171-181.
    Bleakney, J. S., and K. H. Bailey. 1967. Rediscovery of the salt-marsh sacoglossan Alderia modesta Lov‚n in eastern Canada. Proc Mal Soc Lond 37: 347-9.
    Bouchet, P., and J. P. Rocroi. 2005. Classification and nomenclator of gastropod families. Malacologia. 47: 1–397.
    Bouchet, P., J. P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl, and E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia. 61: 1–526.
    Brandley, B. K. 1984. Aspects of the ecology and physiology of Elysia cf. furvacauda (Mollusca: Sacoglossa). Bull. Mar. Sci. 34(2): 207-219.
    Burn, R. F. 1979. Lophopleurella Wilsoni New-Record Opisthobranchia Sacoglossa From Southern Australia. J Malacol Soc Aust 4 (3). 1979. 128.
    Burn, R. F. 1998. Order Sacoglossa, In: Mollusca: The Southern Synthesis. Fauna of Australia. vol 5. CSIRO Publishing: Melbourne, Part B; pp 961-974.
    Busacca, M. 1976. Feeding Specificity As Related To Chloroplast Retention In The Sacoglossa. Florida Sci. 39 (Suppl). 1976. 8.
    Caballer, M., and J. Ortea. 2007. Nueva especie del género Hermaea Lovén, 1844 (Mollusca: Sacoglossa), de la costa norte de La Habana, Cuba. Avicennia 19: 127-132.
    Caballer, M., and J. Ortea. 2013. The genus Hermaea Lovén, 1844 (Mollusca: Sacoglossa) in the Caribbean, with the description of a new species from Cuba. Revista de la Academia Canaria de Ciencias, 25: 67-78. [Dated.
    Caballer, M., J. Ortea, and L. Moro. 2006. Una nueva especie del género Hermaea Loven, 1844 (Mollusca: Sacoglossa: Hermaeidae) de Ghana y las islas de Cabo Verde. Revista de la Academia Canaria de Ciencias, 17(4): 143-150.
    Caballer, M., J. Ortea, and L. Moro. 2009. Descripcion de una nueva especie de Stiliger Ehremberg [sic], 1831 (Mollusca: Sacoglossa) de las islas Canarias. Vieraea 37: 85-90.
    Carlson, C. H., and P. J. Hoff. 1977. A Sand Dwelling Elysia From Guam Opisthobranchia Sacoglossa. Veliger 20 (1):14-16.
    Carlson, C. H., and P. J. Hoff. 1978. The Identifiable Elysia From Guam Elysiidae Sacoglossa Opisthobranchia. Micronesica 14 (1):89-114.
    Casalduero, F. G., and C. Muniain. 2008. The role of kleptoplasts in the survival rates of Elysia timida (Risso, 1818): (Sacoglossa : Opisthobranchia) during periods of food shortage. Journal of Experimental Marine Biology and Ecology. 357: 181–187.
    Cattaneo-Vietti, R., and G. Barletta. 1984. Elenco preliminare dei molluschi opistobranchi viventi nel Mediterraneo (Sacoglossa, Pleurobranchomorpha, Acochlidiacea, Aplysiomorpha, Nudibranchia). Bollettino Malacologico (Pubblicazione Mensile Edita dalla Societa Italiana di Malacologia) 20(9-12):195-218.
    Cavas, L., K. Yurdakoc, and M. B. Yokeş. 2005. Antioxidant status of Lobiger serradifalci and Oxynoe olivacea (Opisthobranchia, Mollusca). Journal of Experimental Marine Biology and Ecology. 314: 227–235.
    Cervera, J. L., and P. J. López-González. 1996. New records of two uncommon sacoglossans (Gastropoda: Opisthobranchia) from the coasts of the Iberian peninsula. Veliger. 39: 93–95.
    Cervera, J. L., J. C. García, and J. A. Ortea. 1988. A New Species Of The Genus Hermaea Gastropoda Opisthobranchia Sacoglossa And Redescription Of Two Rare Sacoglossans Of The European Malacofauna. Iberus 8 (2). 1988. 215-224.
    Cesari, P. 1990. La malacofauna della Laguna Veneta - 2. I gasteropodi bullomorfi, aplisiomorfi, sacoglossi, pleurobranchomorfi e nudibranchi. Malacofauna of the Venice Lagoon: 2. Bullomorph, aplysiomorph, sacoglossan, pleurobranch, and nudibranch gastropods. Bollettino del Museo Civico de Storia Naturale di Venezi 39:21-43.
    Chia, F. S. 1971. Oviposition, fecundity, and larval development of three sacoglossan opisthobranchs from the Northumberland coast, England. Veliger 13(4): 319-325.
    Chia, F. S., and M. Skeel. 1973. The effect of food consumption on growth, fecundity, and mortality in a sacoglossan opisthobranch, Olea hansineensis. Veliger 16(2):153-158.
    Christa, G., L. Wescott, T. F. Schaeberle, G. M. Koenig, and H. Waegele. 2013. What remains after 2 months of starvation? Analysis of sequestered algae in a photosynthetic slug, Plakobranchus ocellatus (Sacoglossa, Opisthobranchia), by barcoding. Planta. 237: 559–572.
    Cimino, G., and M. T. Ghiselin. 1998. Chemical defense and evolution in the Sacoglossa (Mollusca: Gastropoda: Opisthobranchia). Chemoecology. 8: 51–60.
    Cimino, G., A. Fontana, and M. Gavagnin. 1999. Marine opisthobranch molluscs: Chemistry and ecology in sacoglossans and dorids. Current Organic Chemistry 3(4):327-372.
    Clark, K. B. 1994. Ascoglossan (=Sacoglossa) Molluscs in the Florida Keys: Rare Marine Invertebrates at Special Risk. Bull. Mar. Sci. 54(3):900- 916, tbls. 1-3, fig 1.
    Clark, K. B. 1992. Plant-like Animals and Animal-like Plants: Symbiotic Coevolution of Ascoglossan (=Sacoglossan) Molluscs, Their Algal Prey, and Algal Plastids. in W. Reisser, Ed.,Algae and Symbioses. Biopress Ltd., Bristol, U.K.; pp 515-530, 3 figs.
    Clark, K. B., and D. R. Franz. 1969. Occurrence of the Sacoglossan Opisthobranch Hermaea dendritica Alder & Hancock in New England. Veliger 12(2):174-175.
    Clark, K. B., and K. R. Jensen. 1981. A comparison of egg size, capsule size, and development patterns in the order Ascoglossa (Sacoglossa)(Mollusca Opisthobranchia). International J. Invertebr. Reprod. 3:57-64.
    Clark, K. B., K. R. Jensen, H. M. Stirts, and C. Fermin. 1981. Chloroplast Symbiosis in a non-Elysiid Mollusc, Costasiella lilianae Marcus (Hermaeida: Ascoglossa (=Sacoglossa): Effects of Temperature, Light Intensity and Starvation on Carbon Fixation Rate. Biol. Bull. 160(1):43-54; figs. 1-10, tbl 1.
    Clark, K. B., K. R. Jensen, and H. M. Stirts. 1990. Survey for Functional Kleptoplasty Among West Atlantic Ascoglossa (=Sacoglossa) (Mollusca: Opisthobranchia). Veliger 33(4):339-345, tbls. 1-2, fig 1.
    Crane, S. V. 1971. The feeding and reproductive behaviour of the sacoglossan gastropod Olea hansineensis Agersborg, 1923. Veliger 14(1):57-59.

    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.