Runcinid species differ from Cephalaspideans because the mantle (or notum) is not transversely divided into two regions, so there is no distinct cephalic region, the mantle cavity is missing and the foot is not split, with no lateral extensions or parapodia. They have no cephalic tentacles and oral tentacles are only present in Ildica. The mantle is well separated from the foot by a groove. Aspect of these animals is similar to a fat worm, somewhat dorsoventrally depressed and their size usually does not exceed 7-8 mm in length.

Runcina coronata by Luis Ángel Díaz Álvarez

Anus position is terminal, lying behind the rear mantle and often slightly right of the midline. The gills are small, between 1 and 3 in numbers, are located between the mantle and the foot to the right of the anus, or they do not exist at all. There could be an outer shell (as in genus Ildica), it could be an internal and rudimentary shell, or it does not exist at all, as in genus Runcina, Runcinella, Runcinida and Ilbia. When present, the internal shell is haliotiform , small, very fragile and in terminal position. They have oral armor, the radula has a wide central rachideal tooth and one or two lateral teeth on each side (radular formulas 2.1.2, 1.1.1). The worn teeth are discarded and not retained in any special sac. Most genus, except Ilbia and Pseudoilbia have four hardened plates or gesial plates in the stomach working as a shredder.

They are very small animals, few specimens exceed 5 mm, they have a smooth back and a usually cryptic coloration. Specialized herbivores, they can often be found among the seaweeds and seagrass (especially in the systems formed by the rhizomes). They can be locally and seasonally very abundant with large population peaks of short duration. Being so small opisthobranchs and being so well camouflaged in the substrate where they live, in order to study them it is necessary to collect masses of algae and rhizomes of Posidonia oceanica, place them in seawater basins, and wait until they surface, to collect them.

The Runcinidae are known in Japanese waters, from Australia to Fiji, in the Galapagos, New Zealand, Azores, Cape Verde, Angola and Canary Islands. In the Mediterranean Sea some species have been cited both in the eastern and western basins. In this family, only the genus Runcina is present in the Mediterranean, home to the largest number of described species.

Within the opisthobranchs the runcinids were formerly considered like a different order and later like a suborder pertaining to the order of the Cephalaspidea, as thus appeared in the catalog of the Iberian opisthobranchs by (Cervera et al., 2004). A later phylogenetic analysis of the order Cephalaspidea (Malaquias et al., 2009) finds important molecular differences between the runcinids and the other groups of Cephalaspidea and proposes to reinstate again the order Runcinacea, with the same category and separated from the Architectibranchia and the true Cephalaspidea. Runcinids were later recovered as a sister group of the Anaspidea (now Aplysiida) and the Pteropoda by Jörger et al (2010) and Göbbeler and Klussmann-Kolb (2011), confirming this position the work by Bouchet et al. (2017). This is the taxonomic position that has been used on this website and that is also accepted in the WoRMS.

The current taxonomy of the Runcinida is:

  • Order Runcinida
    • Superfamily Runcinoidea  H. Adams & A. Adams, 1854
      • Family Runcinidae  H. Adams & A. Adams, 1854
        • Genus Runcina Forbes, 1851
        • Genus Ildica  Bergh, 1889
        • Genus Metaruncina  Baba, 1967
        • Genus Pseudoilbia  Miller & Rudman, 1968
        • Genus Runcinella  Burn, 1963
        • Genus Runcinida  Burn, 1963
        • Genus Edmundsina Ortea, 2013
        • Genus Karukerina Ortea, 2013
        • Genus Lapinura Er. Marcus & Ev. Marcus, 1970
      • Family Ilbiidae Burn, 1963
        • Genus Ilbia  Burn, 1963
        • Genus Fofinha Moro & Ortea, 2015

Runcinida species recorded in the Mediterranean or around the Iberian Peninsula:

Runcina adriatica by Enric Madrenas

Runcina adriatica

Runcina africana (Costa Brava) by Manuel Ballesteros

Runcina africana

Runcina avellana by Enric Madrenas

Runcina avellana

Runcina bahiensis by Enric Madrenas

Runcina bahiensis

Runcina brenkoae by Enric Madrenas

Runcina brenkoae

Runcina capreensis @ Dwejra, Gozo (Malta) 30-06-1993 by Carmel Sammut

Runcina capreensis

Runcina coronata

Runcina coronata

Runcina ferruginea by Enric Madrenas

Runcina ferruginea

Runcina hansbechi

Runcina hansbechi

Runcina ornata 2-3mm @ Qalet Marku, Malta 1m depth 22-03-1993 by Carmel Sammut

Runcina ornata

Runcina zavodniki @ Croatia by Pero Ugarković

Runcina zavodniki


    Bouchet, P., & Rocroi, J. P. (2005). Classification and nomenclator of gastropod families. Malacologia. 47(1-2): 1-397 ISBN 3-925919-72-4.
    Bouchet, P., Rocroi, J. P., Hausdorf, B., Kaim, A., Kano, Y., Nützel, A., Parkhaev, P., Schrödl, M., & Strong, E. (2017). Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61(1-2): 1-526.
    Göbbeler, K., & Klussmann-Kolb, A. (2011). Molecular phylogeny of the Euthyneura (Mollusca, Gastropoda) with special focus on Opisthobranchia as a framework for reconstruction of evolution of diet. Thalassas, 27(2), 121–154. ://WOS:000291629300009
    Jörger, K. M., Stöger, I., Kano, Y., Fukuda, H., Knebelsberger, T., & Schrödl, M. (2010). On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia. BMC Evolutionary Biology, 10(DOI: 10.1186/1471-2148-10-323).
    Malaquias, M. A. E., Mackenzie-Dodds, J., Bouchet, P., Gosliner, T. M., & Reid, D. G. (2009). A molecular phylogeny of the Cephalaspidea sensu lato (Gastropoda: Euthyneura): Architectibranchia redefined and Runcinacea reinstated. Zoologica Scripta 38(1): 23-41.

    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.