Phylogenetic analysis of the highly enigmatic and aberrant acochlidian opisthobranch gastropods is mainly hindered by the lack of reliable anatomical data. Due to their small size, marine interstitial acochlidian species are not suitable for anatomical investigations by dissecting. The tinyness and complexity of, e.g., opisthobranch central nervous and reproductive systems make data derived from older paraffin-based histology and interpretation by hand-based graphical reconstruction questionable. In a former study, a hermaphroditic member of the basal acochlidian genus Hedylopsis ("Hedylopsacea") has been examined in detail. The present study uses the gonochoric Microhedyle remanei (Microhedylidae) as a model organism of the other traditional major acochlidian subgroup, the "Microhedylacea." More than 20 specimens with up to 2 mm body length were extracted from coarse subtidal sand near Castle Roads, Bermuda Islands. Their central nervous, digestive, circulatory, excretory, and genital systems were reconstructed three-dimensionally from serial semithin histological sections using AMIRA software. The radula was analyzed by SEM. Our specimens closely resemble the original description of M. remanei (Marcus, 1953; as Unela) from Brazil; apparent differences regarding the number of visceral loop ganglia or details of male genitalia are assumed to be due to oversights by the former author or because of different ontogenetic stages. Microhedyle remanei differs from all congeners due to the lack (vs. presence) of eyes; further distinguishing features are discussed. In contrast to members of the hedylopsacean Hedylopsidae, Acochlidiidae, and Tantulidae, M. remanei shows a nervous system with numerous precerebral accessory "ganglia" that are not differentiated into cortex and medulla. While all Microhedylidae previously were thought to lack a heart, M. remanei shows a two-chambered heart, as is usual for opisthobranchs. The oocytes of M. remanei are yolky and large in relation to body size and suggest intracapsular larval development. A comparative microanatomical discussion and the distribution of characters within acochlidian taxa indicate that the current classification of the Acochlidia does not reflect phylogenetic relationships. Detailed structural investigations on further poorly known species are required; the computer-based 3D reconstruction of semithin serial sections with AMIRA is shown to be an ideal tool for efficient analysis and presentation of the microanatomy of small specimens.

The Acochlidia are unique among opisthobranch gastropods in combining extremely high morphological and ecological diversity with modest species diversity. The phylogeny of acochlidians has never been addressed by cladistic means, as their evolution has remained unknown. This study gives a first overview on more than 150 biological and morphological characters that are potentially useful for phylogenetic analysis. Based on 107 characters, a parsimony analysis (PAUP) was performed for all 27 valid acochlidian species together with 11 (plus two) outgroup taxa. The resulting strict consensus tree shows a moderate overall resolution, with at least some bootstrap support for most resolved nodes. The Acochlidia are clearly monophyletic, and originate from an unresolved basal opisthobranch level. The Acochlidia split into the Hedylopsacea (Tantulum (Hedylopsis (Pseudunela (Strubellia ('Acochlidium', 'Palliohedyle'))))) and Microhedylacea (Asperspina (Pontohedyle, 'Parhedyle', 'Microhedyle', (Ganitus, Paraganitus))). The formerly enigmatic Ganitidae, resembling sacoglossan opisthobranchs by having dagger-like rachidian radular teeth, are likely to be highly derived microhedylids. The paraphyly of some of the traditionally recognized family level taxa induced a preliminary reclassification. From the phylogenetic hypothesis obtained, we conclude that the acochlidian ancestor was marine mesopsammic. The colonization of limnic systems occurred twice, independently: first in the Caribbean (with the development of the small interstitial Tantulum elegans), and second in the Indo-Pacific, with a radiation of large-sized benthic acochlidian species. The evolution of extraordinary reproductive features, such as hypodermic impregnation by a complex copulative aparatus in hedylopsaceans, cutaneous insemination via spermatophores in microhedylaceans, and gonochorism in Microhedylidae s.l. (including Ganitidae), is discussed.

Abstract. Acochlidian gastropods combine several aberrant biological and morphological features. The poorly known Caribbean Tantulum elegans is one of the few opisthobranch species inhabiting a freshwater system, and the only one found in muddy interstices of a Caribbean mountain spring swamp. Morphological details of this tiny species were either unknown or not fully reliable, especially with regard to the complex central nervous and reproductive systems. We critically re‐examined original paratype section series and prepared semi‐thin serial sections of two additional paratypes. All organ systems were three‐dimensionally reconstructed using AMIRA software. Our results show several discrepancies from the original description: the pharynx is a complex system of different muscles, but similar to that of other acochlidian species; the circulatory system shows a two‐chambered heart; in the nervous system there are separate optic and rhinophoral ganglia, the latter innervating a pair of small sensory pits we assume to be Hancock's organs, and large aggregations of precerebral accessory ganglia were found. Nephropore, anus, and female gonopore open dextroventrally. To our surprise, adults of T. elegans are sequential hermaphrodites with an unusual androdiaulic reproductive system and a well‐developed cephalic penial complex. In T. elegans, there is a mix of character conditions found in different genera, e.g., Pseudunela and Asperspina. The phylogenetic position of T. elegans still remains unclear.

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