This group comprises about 300 opisthobranch species characterized, among other things, by an strictly herbivorous diet 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 order contains two suborders: Oxynoacea, which includes the shelled sacoglossans, that feed almost exclusively on caulerpal group seaweed and do not retain the chloroplasts of the algae, and Plakobranchacea that integrates the “naked sacoglossan” species, which have a more varied diet, while most species retain the algae plasts.

The taxonomy of Sacoglossa according to WoRMS is:

  • Order SACOGLOSSA von Ihering, 1876
    • Suborder Oxynoacea H. Adams & A. Adams, 1854
      • Family Cylindrobullidae Thiele, 1931
        • Genus Cylindrobulla P. Fischer, 1857
      • Family Juliidae E. A. Smith, 1885
        • Genus Berthelinia Crosse, 1875
        • Genus Julia Gould, 1862
      • Family Volvatellidae Pilsbry, 1895
        • Genus Ascobulla Ev. Marcus, 1972
        • Genus Volvatella Pease, 1860
      • Family Oxynoidae H. Adams & A. Adams, 1854
        • Genus Oxynöe Rafinesque, 1819
        • Genus Lobiger Krohn, 1847
        • Genus Lophopleurella Zilch, 1956
        • Genus Roburnella Ev. Marcus, 1982
    • Suborder Plakobranchacea Rang, 1829
      • Family Plakobranchidae Gray, 1840
        • Genus Elysia Risso, 1818
        • Genus Thuridilla Bergh, 1871
        • Genus Elysiobranchus Pruvot-Fol, 1930
        • Genus Pattyclaya Ev. Marcus, 1982
        • Genus Plakobranchus van Hasselt, 1824
      • Family Boseliidae Marcus, 1982
        • Genus Bosellia Trinchese, 1890-91
      • Family Platyhedylidae Salvini-Plawen, 1973
        • Genus Gascoignella Jensen, 1985
        • Genus Platyhedyle Salvini-Plawen, 1973
      • Family Caliphyllidae Tiberi, 1881
        • Genus Caliphylla A. Costa, 1867
        • Genus Cyerce Bergh, 1870
        • Genus Mourgona Er. Marcus & Ev. Marcus, 1970
        • Genus Polybranchia Pease, 1860
        • Genus Sohgenia Hamatani, 1991
      • Family Hermaeidae H. Adams y A. Adams, 1854
        • Genus Hermaea Lovén, 1844
        • Genus Aplysiopsis Deshayes, 1853
      • Family Limapontiidae Gray, 1847
        • Genus Placida Trinchese, 1876
        • Genus Alderella Odhner in Franc, 1968
        • Genus Alderia Allman, 1845
        • Genus Alderiopsis Baba, 1968
        • Genus Calliopaea d’Orbigny, 1837
        • Genus Costasiella Pruvot-Fol, 1951
        • Genus Ercolania Trinchese, 1872
        • Genus Limapontia Johnston, 1836
        • Genus Olea Agersborg, 1923
        • Genus Stiliger Ehrenberg, 1828

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

Aplysiopsis-elegans @ Qajjenza, Malta on beached red algae 15-09-1993 by Carmel Sammut

Aplysiopsis elegans

Aplysiopsis formosa 15mm @ Blairgowrie Pier, Australia 3.5m 14-04-2016 by PT Hirschfield

Aplysiopsis formosa

Ascobulla fragilis @ Malta by Constantino Mifsud

Ascobulla fragilis

Bosellia mimetica

Bosellia mimetica

Caliphylla mediterranea 8-9mm @ Qajjenza, Malta 1m depth 06-02-1995 by Carmel Sammut

Caliphylla mediterranea

Calliopaea bellula

Calliopaea bellula

Cyerce cristallina by Gilles Cavignaux

Cyerce cristallina

Cyerce graeca @ Punta del Romaní, L'Escala, Spain 6-02-2016 by Enric Madrenas

Cyerce graeca

Elysia fezi according to the Vilella´s description

Elysia fezi

Elysia flava by Enric Madrenas

Elysia flava

Elysia gordanae by Fabio Vitale

Elysia gordanae

Elysia hetta

Elysia hetta

Elysia manriquei @ Taliarte, Gran Canaria 27-05-2014 in 6m by Pablo Samper Méndez

Elysia manriquei

Elysia ornata by Sonja Ooms

Elysia ornata

Elysia papillosa @ Key Largo, Florida (USA) by Manuel Malaquías

Elysia papillosa

Elysia subornata @ Key Largo, Florida (USA) by Manuel Malaquías

Elysia subornata

Elysia timida

Elysia timida

Elysia tomentosa by Baki Yokes @ 20080925 Fethiye, Turquia 24m, on Caulerpa racemosa var. lamourouxii, size 38mm

Elysia tomentosa

Elysia translucens by Enric Madrenas

Elysia translucens

Elysia viridis @ Laguna Fusaro, Bacoli, Napoli by Guido Villani

Elysia viridis

Ercolania caerulea @ Porto Cesareo (Lecce), Northern Ionian Sea by Fabio Vitale

Ercolania coerulea

Ercolania viridis by Fabio Vitale

Ercolania viridis

Hermaea bifida @ Es Caials 20/02/2010 by Miquel Pontes

Hermaea bifida

Hermaea cantabra @ Isla de la Torre, Santander, Cantabria, Spain 22-01-2004 by Manuel Caballer

Hermaea cantabra

Hermaea paucicirra

Hermaea paucicirra

Hermaea variopicta @ Banyuls (France) 01-07-1992 by Géry Parent

Hermaea variopicta

Jenseneria borgninii 12mm @ Qalet Marku, Malta 1m depth 31-03-1994 by Carmel Sammut

Jenseneria borgninii

Limapontia capitata 3mm @ Menai Strait (Wales) April 2011 by Ian Smith

Limapontia capitata

Limapontia senestra 3,7mm by Ian Smith

Limapontia senestra

Lobiger serradifalci by Enrico Ricchitelli

Lobiger serradifalci

Oxynoe olivacea

Oxynoe olivacea

Oxynoe viridis by Murat Draman - @ 20090900 Kas, Antalya, Turquia, 25m on Caulerpa racemosa var. lamourouxii

Oxynoe viridis

Placida brevicornis by José Carlos García Gómez

Placida brevicornis

Placida cremoniana @ Punta del Plom, Begur (Spain) 20080614 by Miquel Pontes

Placida cremoniana

Placida dendritica by Enric Madrenas

Placida dendritica

Placida tardyi, original artwork by Salvatore Trinchese (1879: Lam. XV, Fig. 1)

Placida tardyi

Placida verticillata by Enric Madrenas

Placida verticilata

Placida viridis 6mm @ Qajjenza, Malta 1m depth 6-02-1995 by Carmel Sammut

Placida viridis

Polybranchia viridis @ Malta by Enric Madrenas

Polybranchia viridis

Thuridilla hopei

Thuridilla hopei