The majority of known acochlidian sea slug species are marine mesopsammic, while some others are limnic. The structural, functional and evolutionary background of the invasion of freshwater systems was hardly explored. During the expedition SANTO 2006 to Espiritu Santo, Vanuatu, we discovered a unique new acochlidian species in a brackish water habitat. Pseudunela espiritusanta n. sp. inhabits the underside of intertidal rocks deeply embedded into coarse sand, the interstices of which are filled with a mixture of fresh subsoil and seawater. Pseudunela espiritusanta n. sp. is herein described in full external and anatomical detail using computer-based 3-dimensional reconstruction techniques from serial histological sections. This new species possesses a typical acochlidian central nervous and digestive system; it is a simultaneous hermaphrodite with a special androdiaulic reproductive system and complex, stylet-bearing copulatory organs with associated glands. Such penial features may indicate a relationship with marine mesopsammic Pseudunela (Pseudunelidae) species, while e.g., the larger body size, the broad foot, and the presence of a special ventricular cell layer may be potential synapomorphies with limnic, benthic Acochlidiidae (Strubellia and Acochlidiidae s.s.). Pseudunela espiritusanta n. sp. shares its special shape of head tentacles with both Pseudunela and Strubellia, while other characters are potentially synapomorphic with either one or the other taxon. Regardless of its unresolved exact systematic position, Pseudunela espiritusanta n. sp. evidently links marine and limnic taxa by its intermediate ecological and morphological features. Its considerable body size and well-developed heart and kidney can be considered as preadaptations to overcome osmotic challenges when colonising rivers from brackish coastal sands.
Mollusca, Opisthobranchia, marine, freshwater, limnic, morphology, histology, penial stylet, Vanuatu, new species.