Anatomy and affinities of Micropilina minuta Waren, 1989 (Monoplacophora: Micropilinidae)

The anatomy of the tiny (maximum shell length 1.5 mm) monoplacophoran limpet Micropilina minuta Waren, 1989 was studied by means of semithin serial sections with subsequent 3D computer analysis and visualization (interactive 3D model in the online version). As in other monoplacophorans there are eight pairs of shell muscles ('sectors' A-H). The species has four pairs (unique for monoplacophorans, sectors D-G) of small gills, four pairs of kidneys (sectors A, D-F) which also have gamete-releasing function (sectors D, E), and one (or two) pair(s) of gonadal sacs. Eggs are yolk-rich, but there are no signs of retention of eggs in the mantle cavity for brooding. Only some of these characters, particularly those connected with miniaturization, are shared with Micropilina arntzi. Two characters (absence of a heart and absence of sector G kidneys) are interpreted as synapomorphies of these two species. Based on these new data the supraspecific systematics of Micropilina and Micropilinidae are reevaluated. We also discuss the implications for the evolution of the serial arrangement of organs in the Neopilinoidea.