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The gastropod infraclass Euthyneura comprises at least 30,000 species of snails and slugs, including nudibranch sea slugs, sea hares and garden snails, that flourish in various environments on earth. A unique morphological feature of Euthyneura is the presence of two pairs of sensory head tentacles with different shapes and functions: the anterior labial tentacles and the posterior rhinophores or eyestalks. Here we combine molecular phylogenetic and microanatomical evidence that suggests the two pairs of head tentacles have originated by splitting of the original single tentacle pair (with two parallel nerve cords in each tentacle) as seen in many other gastropods. Minute deep-sea snails of Tjaernoeia and Parvaplustrum, which in our phylogeny belonged to the euthyneurans’ sister group (new infraclass Mesoneura), have tentacles that are split along much of their lengths but associated nerves and epidermal sense organs are not as specialized as in Euthyneura. We suggest that further elaboration of cephalic sense organs in Euthyneura closely coincided with their ecological radiation and drastic modification of body plans. The monotypic family Parvaplustridae nov., superfamily Tjaernoeioidea nov. (Tjaernoeiidae + Parvaplustridae), and new major clade Tetratentaculata nov. (Mesoneura nov. + Euthyneura) are also proposed based on their phylogenetic relationships and shared morphological traits.

Gastropods (slugs and snails) are prominent and species-rich faunal elements in marine and terrestrial habitats of the tropics. While several clades of snails inhabit freshwater systems, slugs are extremely rare in freshwater: only the centimeter-sized Acochlidiidae, with currently three genera, contain more than one species and live in the lower reaches of island streams in an area comprising Eastern Indonesia, Fiji, and Palau. Where known, the species of this unique group are specialized predators of other amphidromous snails’ egg capsules (Neritidae) and their reproductive biology and adaptations to life in freshwater are complex. Acochlidiidae are thus of special interest for evolutionary biology and ecology. We here describe a new genus of unusually bluish-green acochlidiid to date known only from a single locality on the island of Ambon, Indonesia. Previous molecular data found this charismatic species to link slender Strubellia slugs with broad and flattened Acochlidium and Palliohedyle. We establish Wallacellia siputbiru n. gen. n. sp., the “blue slug” in Bahasa Indonesia, by using scanning electron microscopy of cuticular elements, light microscopy of serial semithin histological sections of the soft body, and 3D reconstruction of all organ systems based on these sections. Special structures of this seemingly rare endemic species include the enlarged kidney and the huge copulatory organ. Our data now clarify that, in the invasion of freshwater habitats in Acochlidiidae, sexual selection (the anterior three-part copulatory organ) preceded ecological selection (posterior flattened habitus with branched or multiplied internal organs).

Rhodopemorphs are small, interstitial or psammobiotic heterobranch slugs, which have been troubling to place phylogenetically. Their small size and habit of living in or among sediment have led to a correlated reduction and simplification of morphology, and consequently to contradictory phylogenetic signal from anatomical features. When morphological data have previously been used to generate phylogenetic hypotheses, these were vulnerable to the effects of homoplasy. We collected multiple species of Rhodope, along with another rhodopemorph genus Helminthope, to produce DNA sequence data for the first time to test their monophyly. We sequenced mitochondrial and nuclear genes, and analysed the data under maximum likelihood and Bayesian inference. By analysing these data in a broader heterobranch dataset, we also examined the placement of Rhodopemorpha within Heterobranchia. Despite rhodopemorphs showing aspects of a euthyneurous (and pentaganglionate) condition, their placement in a molecular phylogeny occurs outside the taxon defined by this term (i.e. Euthyneura = Pentaganglionata). Instead, model-based inferences placed rhodopemorphs among basal heterobranch taxa, usually as a clade sister to a shelled group (Murchisonellidae) that was recently removed from the Pyramidelloidea. The Rhodopemorpha + Murchisonellidae clade is herein termed Allomorpha. Three-dimensional reconstruction methods have elucidated potential morphological homologies for Rhodopemorpha and comparisons with Murchisonellidae have also uncovered morphological support for this placement. Thus, we consider the phylogenetic placement of Rhodopemorpha solved, although relationships among lower Heterobranchia lineages remain challenging.

The Murchisonellidae are a small taxon of minute snails with a high-spired shell that occur in shallow marine habitats. Molecular phylogenetics recently revealed that they are not members of the externally similar yet phylogenetically derived Pyramidellidae, but instead potentially one of the oldest clades among the heterobranch Gastropoda. Furthermore, current data surprisingly indicate a sister-group relationship with Rhodopemorpha, highly aberrant marine slugs with previously unclear affinities. Murchisonellidae are characterized by a specialized pincer-like radula, but very little further data exist on soft-body anatomy for most species, and there are only a few observations of living animals. Investigation of the anatomy of Murchisonellidae may thus yield new data providing insights into early heterobranch evolution and that of enigmatic Rhodopemorpha. We collected live specimens of the murchisonellid Koloonella cf. minutissima (Laseron, 1951), a member of a genus known mainly from eastern Australia. We provide detailed live photographs and interactive 3D data on all major organ systems, based on serial histological sections. The mantle cavity is shown to contain several distinct glands, a pair of which is conspicuously similar to glands found in Rhodopemorpha. The anterior digestive system contains a unique four-toothed radula, a feeble pharynx and a special, vacuolated oesophageal bulb. The reproductive system is complex and diaulic, and contains unusual structures. These results highlight structural diversity among minute lower Heterobranchia. Soft-body characters do not contradict, and may even support, the counterintuitive sister-group relationship with shell-less, wormshaped Rhodopemorpha. The classification of Murchisonellidae is discussed and a revised scheme is proposed.

Yasunori Kano1, Hiroaki Fukumori1,2, Bastian Brenzinger3 and Anders Waren4 Department of Marine Ecosystems Dynamics, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan; Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan; Bavarian State Collection of Zoology, Munchhausenstr. 21, D-81247 Munich, Germany; and Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden

The monophyly of the panpulmonate, usually marine benthic Sacoglossa—and its basal division into shelled Oxynoacea and shell-less Plakobranchacea—is undisputed, but family relationships are in doubt. Of particular interest is the potentially basal plakobranchacean family Platyhedylidae, comprising morphologically aberrant members lacking head tentacles or body appendages. Herein we re-describe the type species of the genus Gascoignella, G. aprica Jensen, 1985, from Hong Kong. Morphological data was generated by three-dimensional reconstruction from serial semi-thin sections using Amira software. Our microanatomical results largely confirm the original description. The anterior digestive system is sacoglossan-like but modified, e.g. the ascus is not demarcated externally and pharyngeal pouches are lacking. The digestive gland is bipartite, with two rami separated by a longitudinal, muscular, median septum, but fused in the rear end. The postpharyngeally situated nerve ring contains fused cerebropleural ganglia; the short visceral loop has three ganglia. Two major cerebral nerves were identified as rhinophoral and labiotentacular nerves, innervating sensory areas on the head velum. Gascoignella aprica is a hermaphrodite with a truly androdiaulic genital system of which some originally ambiguous characters are clarified. Bursa and prostate insert into a fertilization chamber proximal to a sac-like albumen gland and a tubular mucus gland. The cephalic copulatory apparatus contains a penis armed with a short and straight stylet and an accessory gland of unclear function; the presumed mode of sperm transfer is discussed. A well-developed heart and a large H-shaped kidney are present; the nephroduct opens into the intestine. Epidermal glands and body tissues are described for the first time. The presence of a unique longitudinal, median septum is considered diagnostic for Platyhedylidae, multiple further apomorphies are given. Morphological evidence supports the molecular phylogenetic hypothesis that the Platyhedylidae could be a basal non-shelled sacoglossan lineage.

The turbellarian-like, radula-lacking Rhodope has been a mystery to taxonomists for over 160 years and was considered a specialized off-shoot of either opisthobranch or pulmonate Euthyneura. Occasionally reported from intertidal waters and sand habitats from all continents, most species of these minute slugs are poorly known and characterized mainly by differences in pigmentation. To understand the evolution of heterobranch microslugs, we established a morphological dataset for Rhodope by describing a new species found in the temperate waters of southern Australia. To set a standard for rhodopids, all major organ systems of R. rousei n. sp. are reconstructed three-dimensionally from series of semithin sections using the software Amira. Microanatomy confirms the loss of many general gastropod features such as foot, cephalic tentacles, shell, radula, mantle cavity, gill and heart. Excretory and digestive systems are heavily modified, with free rhogocytes in the presumed position of the heart, and a secondary buccal bulb replacing the function of the vestigial pharynx. Structural details of the monaulic but hermaphroditic genital system suggest cutaneous fertilization via spermatophores formed in specialized glands. The highly concentrated central nervous system is compared to those of other species of the genus and targets of all detectable nerves are summarized. These characters are compared with adaptations shown by other interstitial gastropods.

Nudibranch molluscs of the genus Corambe differ from most other Doridoidea by having ventral rather than dorsal anus and gills. Because of these and other features, such as separate cerebral and pleural ganglia, corambids have been considered as an archaic or enigmatic group. The first tropical eastern Pacific Corambe species is described in morphological and some histological detail. Selected organs such as circulatory and central nervous features are reconstructed from serial semithin histological slides and visualized in three dimensions using Amira software. Anatomical findings include two separate ganglia on the visceral loop and an additional ganglion on the right side of the body that is connected to the pedal ganglion. Corambe mancorensis n. sp. is dorsoventrally depressed, has an oval, fleshy notum covered with a cuticle, and has a wide posterior medial notch that can be closed completely by unique lobules. Gills are arranged in an unusual horseshoe-like manner including both phanerobranch anal (=medial) gills and corambid lateroventral gill rows, and are connected to the atrium by a complex vessel system. The three medial gills arise from a posterodorsal gill cavity within the notal notch, similar to the case in Corambe evelinae Marcus, 1958. By scanning electron microscopy a vestigial gill cavity is also detectable in C. pacifica MacFarland & O'Donoghue, 1929, but here it is situated ventrally. Our new information on adult corambids is compared with new and published ontogenetic data on phanerobranch and cryptobranch dorids, to contribute to a novel interpretation of the ontogeny of dorid mantle and gill complexes. The progenetic evolution of corambids 'recapitulates' early juvenile dorid stages - turning Haeckel's Law upside down.