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 Acochlidiimorpha according to recent work by Bouchet et al. (2017) is:

  • Superorder Acochlidiimorpha
    • Superfamily Acochlidioidea Küthe, 1935
      • Family Acochlidiidae Küthe, 1935
        • Genus Acochlidium Strubell, 1892
        • Genus Palliohedyla Rankin, 1979
        • Genus Palliohedyle Rankin, 1979
        • Genus Strubellia Odhner, 1937
      • Family Aitengidae Swennen & Buatip, 2009
        • Genus Aiteng Swennen & Buatip, 2009
      • Family Bathyhedylidae Neusser, Jörger, Lodde-Bensch, Strong & Schrödl, 2016
        • Genus Bathyhedyle Neusser, Jörger, Lodde-Bensch, Strong & Schrödl, 2016
      • 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
    • Superfamily Parhedyloidea Thiele, 1931
      • Family Asperspinidae Rankin, 1979
        • Genus Asperspina Rankin, 1979
        • Genus Minicheviella Starobogatov, 1983
      • 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 species have been reported:

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

Asperspina loricata

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

Hedylopsis spiculifera

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

Microhedyle glandulifera

Pontohedyle milaschewitchii @ Brasil by Jörger et al, 2008

Pontohedyle milaschewitchii

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.


    Bouchet P, Rocroi JP. 2005. Classification and nomenclator of gastropod families. Malacologia. 47(1–2):1–397.
    Bouchet P, Rocroi JP, Hausdorf B, et al. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia. 61(1–2):1–526.
    Brenzinger B, Neusser TP, Jörger KM, et al. 2011. Integrating 3D microanatomy and molecules: natural history of the Pacific freshwater slug Strubellia Odhner, 1937 (Heterobranchia: Acochlidia), with description of a new species. Journal of Molluscan Studies 77(4): 351-374.
    Cervera JL, Calado G, Gavaia C, et al. 2004. An annotated and updated checklist of the opisthobranchs (Mollusca: Gastropoda) from Spain and Portugal (including islands and archipelagos). Boletín Instituto Español de Oceanografía, 20 (1-4): 1-111. L.
    Garcia FJ, Bertsch H. 2009. Diversity and distribution of the Gastropoda Opisthobranchia from the Atlantic Ocean: A global biogeographic approach. Scientia Marina. 73(1):153–160.
    Hadl G, Kothbauer H, Peter R, et al. 1970. Substratwahlversuche mit Microhedyle milaschewitchii Kowalevsky (Gastropoda, Opisthobranchia: Acochlidiacea). Oecologia (Berl) 4(1): 74-82; 2 text figs. Oecologia. 4(1):74-.
    Jörger KM, Hess M, Neusser TP, et al. 2009. Sex in the beach: spermatophores, dermal insemination and 3D sperm ultrastructure of the aphallic mesopsammic Pontohedyle milaschewitchii (Acochlidia, Opisthobranchia, Gastropoda). Marine Biology. 156(6):1159–1170.
    Jörger KM, Kristof A, Klussmann-Kolb A, et al. 2008. Keeping Nerves: Central Nervous System of the Interstitial Acochlidiid Parhedyle cryptophthalma (Gastropoda, Opisthobranchia). Journal of Morphology. 269(12):1484–1484. Available from ://WOS:000261623800149.
    Jörger KM, Stöger I, Kano Y, et al. 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).
    McDonald G. 2009. Bibliographia Nudibranchia. 2nd Online Edition, Annotated. 1072 pp  Institute of Marine Sciences, University of California, Santa Cruz. Available from
    Neusser M TP; Jörger, KM; Schrödl. 2011. Cryptic species in tropic sands - Interactive 3D anatomy, molecular phylogeny and evolution of meiofaunal Pseudunelidae (Gastropoda, Acochlidia). PLoS ONE 6(8): e23313. [Nomenclaturally available from separate print-only edition].
    Neusser TP, Schrödl M. 2009. Between Vanuatu tides: 3D anatomical reconstruction of a new brackish water acochlidian gastropod from Espiritu Santo. Zoosystema 31(3): 453-469.
    Neusser TP, Martynov AV, Schrödl M. 2009. Heartless and primitive? 3D reconstruction of the polar acochlidian gastropod Asperspina murmanica. Acta Zoologica, 90(3): 228-245.
    Neusser TP, Fukuda H, Jörger KM, et al. 2011. Sacoglossa or Acochlidia? 3D Reconstruction, molecular phylogeny and evolution of Aitengidae (Gastropoda: Heterobranchia). Journal of Molluscan Studies. 77:332–350.
    Schrödl M, Neusser TP. 2010. Towards a phylogeny and evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia). Zoological Journal of the Linnean Society. 158(1):124–154.
    Schrödl M, Jörger KM, Neusser TP. 2008. Phylogeny and Evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia): From Morphology to Sequences. Journal of Morphology. 269(12):1464–1465. Available from ://WOS:000261623800048.
    Schrödl M, Jörger K, Klussmann-Kolb A, et al. 2011. Bye Bye “Opisthobranchia”! a review on the contribution of mesopsammic sea slugs to euthyneuran systematics. Thalassas. 27(2):101–112.
    Sommerfeldt N, Schrödl M. 2005. Microanatomy of Hedylopsis ballantinei, a new interstitial acochlidian gastropod from the Red Sea, and its significance for phylogeny. Journal of Molluscan Studies, 71(2): 153-165.
    Wawra E. 1978. Zur Identität von Microhedyle glandulifera (Kowalewski, 1901) und Microhedyle lactea (Hertling, 1930) (Gastropoda, Opisthobranchia, Acochlidiacea). Annalen des Naturhistorischen Museums in Wien 81: 607-617.
    Wawra E. 1989. Zur Kenntnis der interstitiellen Opisthobranchierart Hedylopsis spiculifera (Kowalevsky) (Mollusca, Gastropoda). Zoologica Scripta, 18(3): 397–403.

    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.