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Keywords:

  • Mollusca;
  • Opisthobranchia;
  • three-dimensional reconstruction;
  • phylogeny

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.