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Back to the sea? First ontogenetic data of limnic slugs (Acochlidia, Heterobranchia)

BioSys EU 2013

Authors/Editors: Schrödl M
Brenzinger B
Neusser T P
Publication Date: 2013
Type of Publication: Congress Contributions and Posters
Among slugs, Acochlidia uniquely have conquered the limnic habitat. Based on our current phylogenetic hypothesis on the evolution of Acochlidia there is strong support that the limnic lineages even arose twice independently – in the Western Atlantic and the Indo-Pacific – from marine, meiofaunal ancestors. Dispersal and biogeographic patterns among Indo-Pacific islands in these taxa are however still largely unknown. We collected egg masses of Acochlidium sutteri (Acochlidiidae) from the underside of stones together with adult specimens in freshwater streams on Flores Island (Indonesia). Conspecifity of the egg masses was confirmed by barcoding. Egg masses are gelatinous and amorphous, having a varying amount of up to several hundred egg capsules loosely embedded. Egg capsules are oval; no extracapsular yolk could be detected. Serial semithin sections (0.5 μm) were prepared to characterise early veliger stages. Hatching veliger larvae were actively swimming for up to two days, all larvae kept in freshwater died after a maximum of 48 hours. Scanning electron micrographs of the free veliger stage are provided. Even though comparably small (approx. 95 μm), lecithotrophy is likely based on histology and light-microscopy. Experimentally transferred to sea water, larvae stopped swimming, closed the larval shell and attached to the substratum. Completely inactive they survived for several weeks glued to the bottom of the petri-dish or a sand grain. When removed they reglued themselves. Based on the incapability of survival in freshwater but under marine conditions, amphidromy in limnic Acochlidiidae is likely, as reported for other co-occurring gastropods (e.g. Neritidae). The observed long-living ‘adhesive larva’ might serve as protective stage not to be swamped out in the open ocean or present a dispersal stage by attaching to larger mobile organisms as vectors of dispersal.

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