Runcinacea

Runcinacean 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.

Biology
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.

Distribution
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.

Taxonomy
Within the Opisthobranchia, the Runcinacea were first considered as a different order and later as a suborder to the order of Cephalaspidea, as stated in the updated catalog of the Iberian opisthobranchs (Cervera et al., 2004). A recent phylogenetic analysis of the Cephalaspidea order (Malaquias et al., 2008) found significant molecular differences between Runcinacea and other groups Cephalaspidea and proposes reinstating again the order Runcinacea, with the same category and separated from the Architectibranchia and the true Cephalaspidea. This is the taxonomic position that has been adopted on this website and which is also accepted in WoRMS. In the phylogenetic analysis by Jörger et al., 2010, Runcinacea appear as the sister group of the Anaspidea (sea hares) and the Pteropoda (sea butterflies) within the clade called Euopisthobranchia.

The current taxonomy for Runcinacea is:

  • Order Runcinacea Burn, 1963
    • 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  Odhner, 1924
        • Genus Runcinida  Burn, 1963
      • Family Ilbiidae  Burn, 1963
        • Genus Ilbia  Burn, 1963

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

Runcina adriatica by Manuel Ballesteros

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

Runcina brenkoae

Runcina capreensis

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

Bibliography

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.

Cite this article as:

Ballesteros, Manuel, Enric Madrenas, Miquel Pontes et al. (2012-2017) "Runcinacea" in OPK-Opistobranquis, Published: 16/04/2013, Accessed: 28/06/2017 at (http://opistobranquis.info/en/5euCU)

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