Studies on the Structure and Function of the Larval Kidney Complex of Prosobranch Gastropods

The larval kidneys of prosobranch gastropods have long been assumed to be involved in handling wastes, but with little supporting evidence. In this study, the larval kidneys of Searlesia dira and Nucella canaliculata were studied with light, electron, and fluorescence microscopy. They consist of three cell types: (1) a large external absorptive cell swollen with heterophagosomes and possessing an endocytotically active external surface; (2) an internal crystal cell with numerous vacuole-bound crystals of a calcium salt and with morphologically complex canaliculi; (3) an internal pore cell characterized by slit-pores that lead to subsurface cisternae, a tubular network, and one or two ciliated ducts that open into the hemocoel. Empirical evidence indicates that the absorptive cell rapidly takes up and stores albumen proteins from the capsular fluid. Absorptive cells were found in 17 of 19 species tested, representing three prosobranch orders, but were not found in 2 opisthobranch or 1 pulmonate species. We hypothesize that the absorptive cells have become specialized for the uptake of capsular albumen prior to the functional differentiation of the gut. However, the nutritional importance of the absorbed albumen proteins and the functions of the crystal and pore cells are presently unknown. No evidence for an excretory function was found for the larval kidney complex; it may be a vestigial protonephridium, the components of which have become disorganized and functionally altered.