Acochlidiimorpha

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
http://www.wikipedia.org

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) 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	Hedylopsis spiculifera
Microhedyle glandulifera	Pontohedyle milaschewitchii

Bibliography

1246567 Acochlidiimorpha items 1 entomological-society-of-america author asc 1 https://opistobranquis.info/wp-content/plugins/zotpress/
Bouchet, P., and J. P. Rocroi. 2005. Classification and nomenclator of gastropod families. Malacologia. 47: 1–397.
Bouchet, P., J. P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl, and E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia. 61: 1–526.
Brenzinger, B., T. P. Neusser, K. M. Jörger, and M. Schrödl. 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, J. L., G. Calado, C. Gavaia, M. A. E. Malaquías, J. Templado, M. Ballesteros, J. C. García-Gómez, and C. Megina. 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, F. J., and H. Bertsch. 2009. Diversity and distribution of the Gastropoda Opisthobranchia from the Atlantic Ocean: A global biogeographic approach. Scientia Marina. 73: 153–160.
Hadl, G., H. Kothbauer, R. Peter, and E. Wawra. 1970. Substratwahlversuche mit Microhedyle milaschewitchii Kowalevsky (Gastropoda, Opisthobranchia: Acochlidiacea). Oecologia (Berl) 4(1): 74-82; 2 text figs. Oecologia. 4: 74-.
Jörger, K. M., M. Hess, T. P. Neusser, and M. Schrödl. 2009. Sex in the beach: spermatophores, dermal insemination and 3D sperm ultrastructure of the aphallic mesopsammic Pontohedyle milaschewitchii (Acochlidia, Opisthobranchia, Gastropoda). Marine Biology. 156: 1159–1170.
Jörger, K. M., A. Kristof, A. Klussmann-Kolb, and M. Schrödl. 2008. Keeping Nerves: Central Nervous System of the Interstitial Acochlidiid Parhedyle cryptophthalma (Gastropoda, Opisthobranchia). Journal of Morphology. 269: 1484–1484.
Jörger, K. M., I. Stöger, Y. Kano, H. Fukuda, T. Knebelsberger, and M. Schrödl. 2010. On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia. BMC Evolutionary Biology. 10.
McDonald, G. 2009. Bibliographia Nudibranchia. 2nd Online Edition, Annotated. 1072 pp  Institute of Marine Sciences, University of California, Santa Cruz. (http://escholarship.org/uc/item/8115h0wz).
Neusser, M., T. P. ;. Jörger, K. M. ;. 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, T. P., and M. Schrödl. 2009. Between Vanuatu tides: 3D anatomical reconstruction of a new brackish water acochlidian gastropod from Espiritu Santo. Zoosystema 31(3): 453-469.
Neusser, T. P., A. V. Martynov, and M. Schrödl. 2009. Heartless and primitive? 3D reconstruction of the polar acochlidian gastropod Asperspina murmanica. Acta Zoologica, 90(3): 228-245.
Neusser, T. P., H. Fukuda, K. M. Jörger, Y. Kano, and M. Schrödl. 2011. Sacoglossa or Acochlidia? 3D Reconstruction, molecular phylogeny and evolution of Aitengidae (Gastropoda: Heterobranchia). Journal of Molluscan Studies. 77: 332–350.
Schrödl, M., and T. P. Neusser. 2010. Towards a phylogeny and evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia). Zoological Journal of the Linnean Society. 158: 124–154.
Schrödl, M., K. M. Jörger, and T. P. Neusser. 2008. Phylogeny and Evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia): From Morphology to Sequences. Journal of Morphology. 269: 1464–1465.
Schrödl, M., K. Jörger, A. Klussmann-Kolb, and N. Wilson. 2011. Bye Bye “Opisthobranchia”! a review on the contribution of mesopsammic sea slugs to euthyneuran systematics. Thalassas. 27: 101–112.
Sommerfeldt, N., and M. Schrödl. 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.