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
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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: