3D microanatomy of a gastropod ‘worm’, Rhodope rousei sp. nov. from southern Australia
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 sp. nov. 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 other species of the genus, and targets of all detectable nerves are summarized. These adaptations are discussed in relation to other interstitial gastropods.