Phylogeny and evolution of acochlidian opisthobranchs

February 6, 2006
From: M. Schrödl & T. P. Neusser

Acochlidian opisthobranchs are amongst the most fascinating gastropods combining an almost incredible number of aberrant morphological and biological features. Most of the 27 species known worldwide are marine, with tiny specimens inhabiting interstitial spaces of intertidal or (shallow) subtidal sands. As in other mesopsammic opisthobranchs, such as Rhodopemorpha, Philinoglossidae (Cephalaspidea), Platyhedyle (Sacoglossa), and Pseudovermidae (Nudibranchia), extreme ecological conditions are thought to have caused drastic morphological adaptations: miniaturization, development of an elongate, worm-like body shape, and reductions of shell, mantle cavity organs, pigments, and eyes. In addition, many interstitial acochlidian species show special reproductive features, e.g. the copulatory organs are modified to allow hypodermal fertilization, or are completely reduced. In the latter case, sperm transfer occurs via spermatophores. Several species forming spermatophores have separate sexes, a condition that is unique amongst the otherwise hermaphroditic opisthobranchs.

 Some acochlidian species invaded brackish or even freshwater systems; again unique amongst opisthobranchs. With exception of the small limnic Tantulum elegans that was so far only reported from a mountain spring swamp on the Caribbean St. Vincent Island, there is an array of large (up to 3 cm) acochlidian species occurring in freshwater systems of several Indo-Pacific islands. Besides showing a remarkable (secondary?) gigantism, most of the latter species have giant penial papillae armed with cuticular spines and thorns, and a complex system of penial glands with so far unknown homology and functions.

Morphological knowledge on acochlidians still is largely restricted to external features and gross anatomy presented in old original descriptions. The current structural and systematic knowledge on acochlidians was reviewed recently by Sommerfeldt & Schrödl (2005). Within our current project on the "Phylogeny and Evolution of Acochlidia", we are preparing a detailed species by species account using modern histological techniques and 3D-visualization (see Neusser et al., 2005). Every help regarding biological observations, distributional data, or sending specimens for histological, ultrastructural or molecular analyses would be highly appreciated.

  • Neusser, T.P., Haszprunar, G., Hess, M., Schrödl, M. 2005. Computerbased 3-dimensional reconstruction of the anatomy of Microhedyle remanei (Marcus, 1953), an interstitial acochlidian gastropod from Bermuda. Journal of Morphology 267: 231-247.
  • 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: 153-165.

Michael Schrödl & Timea P. Neusser

Schrödl, M. & Neusser, T.P., 2006 (Feb 6) Phylogeny and evolution of acochlidian opisthobranchs. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Michael & Timea,
I look forward to many more messages on these interstitial animals. They certainly deserve more 'press' than they normally get.
Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Feb 6). Comment on Phylogeny and evolution of acochlidian opisthobranchs by M. Schrödl & T. P. Neusser. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from



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