VOLTZOW, J.*; MARTIN, C.-L.; Univ. of Scranton, Pennsylvania, USA; Univ. of New England, National Marine Science Centre, New South Wales, Australia: Flow in Fissurellids: A Story Full of Holes

Most fissurellid gastropods, or keyhole limpets, are characterized by a cap-shaped shell with an apical opening. Their mantle cavities contain paired organs, including a large pair of gills, which are believed to represent the primitive condition for gastropods. The apical opening has been proposed to function to prevent incoming clean seawater from mixing with waste-laden excurrent water. This is supported by the pattern of flow in these animals; water enters at either side of the head, passes through the gills, and exits through the apical opening. Some species of keyhole limpets, however, lack apical openings. We compared the patterns of flow and morphology of fissurellids that have apical openings with those that do not. The Australian fissurellid Amblychilepas negrita and the North American Diodora aspera have apical openings and the expected flow-through pattern of circulation. The Australian fissurellids Scutus antipodes, Clypidina rugosa, and Tugali parmophoidea, which lack apical openings, formed siphons at the central anterior regions of their mantles through which the water exited their mantle cavities. In addition, they brought their gills together in a point above their heads, thus using the gills to manipulate the flow. Anatomical differences in the gills and roof of the mantle cavity also appeared to contribute to these contrasting flow patterns.