INTEGRATING 3D MICROANATOMY AND MOLECULES: NATURAL HISTORY OF THE PACIFIC FRESHWATER SLUG STRUBELLIA ODHNER, 1937 (HETEROBRANCHIA: ACOCHLIDIA), WITH DESCRIPTION OF A NEW SPECIES

Forming a small group of mainly marine meiofaunal slugs, the Acochlidia have recently been separ-ated from the traditional opisthobranch gastropods and placed within a mixed clade of pulmonates, Sacoglossa and Pyramidelloidea on the basis of molecular data. In the light of this new phylogenetic framework, we examined several populations of a comparatively giant Strubellia (Acochlidiidae s. l.) found in rivers of the Solomon Islands and Vanuatu, combining microanatomical and molecular methods (interactive three-dimensional models are given in the online version). Novel features include an extended set of nerves, a ‘cephalic gland ’ of unknown function and an osphradium, all detected here for the first time in Acochlidia. The protandric genital system is characterized by three receptacles in the male phase, a possibly secondary open seminal groove and a complete reduction of the elaborate cephalic copulatory apparatus during ontogeny. Combined evidence from copulatory features and DNA sequences indicate a specific separation between the type species S. paradoxa (Strubell, 1892) from Ambon and the eastern Melanesian Strubellia wawrai n. sp. Live observations show the species to feed on the highly mineralized egg capsules of limnic Neritidae using a special piercing radula. Limnic Pacific acochlidians are suggested to be amphidromic, as are their prey organisms. A unique type of adhesive larva, observed in an Acochlidium species, indicates a possible dispersive stage in Acochlidiidae. Molecular phylogeny confirms the morphology-based placement of Strubellia as sister taxon to other Acochlidiidae