This is a small group of Heterobranchs comprising about 30 species worldwide and living in the interstitial marine or freshwater medium. They have been very poorly studied, they are very small and have a somewhat aberrant body morphology for being molluscs. They have adapted to live and move between the sediment particles.

Acochlidium fijiiensis Haynes & Kenchington, 1991

They have an elongated body, with no shell when adults, and a mantle that usually has numerous spicules. They have no gills nor mantle cavity and the head has oral tentacles and rhinophores. Generally the sexes are separated. They possibly feed on microorganisms that live in the interstitial medium.

Traditionally considered a peculiar group of opisthobranchs, recent works by Jorger et al. (2010) and Schrödl et al. (2011) tend to classify them closer to pulmonate gastropods within the clade Panpulmonata.

The taxonomy of Acochlidiacea according to recent work by Schrödl & Neusser (2010) is:

  • Order Acochlidiacea (= Acochlidia) Odhner, 1937
    • Clade Hedylopsacea
      • Family Acochlidiidae Küthe, 1935
        • Genus Acochlidium Strubell, 1892
        • Genus Palliohedyle Rankin, 1979
        • Genus Strubellia Odhner, 1937
      • Family Hedylopsidae Odhner, 1952
        • Genus Hedylopsis Thiele, 1931
      • Family Pseudunelidae Rankin, 1979
        • Genus Pseudunela Salvini-Plawen, 1973
      • Family Tantulidae Rankin, 1979
        • Genus Tantulum Rankin, 1979
    • Clade Microhedylacea
      • Family Aitengidae Swennen & Buatip, 2009
        • Genus Aiteng Swennen & Buatip, 2009
      • Family Asperspinidae Rankin, 1979
        • Genus Asperspina Rankin, 1979
      • Family Parhedylidae Thiele, 1931
        • Genus Microhedyle Hertling, 1930
        • Genus Parhedyle Thiele, 1931
        • Genus Pontohedyle Golikov & Starobogatov, 1972
Pontohedyle milaschewitchii (Kowalevsky, 1901) A) Esquema de un ejemplar entero en vista dorsal (dg: glándula digestiva; ey: ojos; lt: tentáculo labial; sp1 y sp2: espículas; vs: saco visceral) B, C) Fotografias ejemplar vivo mostrando la variación de su forma externa en movimiento. (Jorger et al, 2008)

Pontohedyle milaschewitchii (Kowalevsky, 1901)
A) Schema of a typical specimen in dorsal view (dg: digestive gland; ey: eyes; lt: labial tentacle; sp1 & sp2: spicules; vs: visceral sac)
B, C) Pictures showing living specimen. (Jorger et al, 2008)

In the Mediterranean and around the Iberian Peninsula the following families have been reported:

Asperspina loricata @ Mesa do Con, Ría de Arousa, 05.05.1981 by V. Urgorri


Hedylopsis spiculifera @ Cariño, Ría de Ferrol, 10.02.2004 by V. Urgorri


Microhedyle glandulifera @ Cariño, Ría de Ferrol, 10.02.2004 by V. Urgorri


So far no Acochlidiacea has been cited in the Catalan coast, because of the lack of studies of the interstitial medium directed towards this group.


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) "Acochlidiacea" in OPK-Opistobranquis, Published: 04/02/2013, Accessed: 24/04/2017 at (

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