Structure and development of nephridia in Annelida and related taxa

Abstract

Two different kinds of filtration nephridia, protonephridia and metanephridia, are described in Polychaeta. During ontogenesis protonephridia generally precede metanephridia. While the latter are segmentally arranged, protonephridia are characteristic for the larva and are the first nephridial structure formed during ontogenesis. There is strong evidence that both organs depend on the same information and that their specific structure depends on the way in which the coelom is formed and which final expansion it gains. While metanephridia are regarded to be homologous throughout the polychaetes, protonephridia seem to have evolved in several lineages. Some of the protonephridia closely resemble less differentiated stages of metanephridial development, so that protonephridial evolution can be explained by truncation of the metanephridial development. Nevertheless, structural details are large enough to allow us to expect information on the polychaete evolution if the database on polychaete nephridia increases. A comparison of the polychaete metanephridia with those of the Clitellata and Sipuncula reveals some surprising details. In Clitellata the structure of the funnel is quite uniform in microdrilid oligochaetous Clitellata and resembles that of the aeolosomatids. Like the nephridia in the polychaete taxa Sabellida and Terebellida, those of the Sipunucla possess podocytes covering the coelomic side of the duct.

Three consecutive generations of nephridia occur during development of Platynereis dumerilii (Annelida, Polychaeta)

“…The origin and development of nephridia in larval and juvenile Platynereis dumerilii has been investigated up to now mainly on a morphological and ultrastructural level (Bartolomaeus and Quast, 2005, and references therein). In order to learn more about the molecular properties of those nephridia, we investigated if two markers, alkaline phosphatase and acetylated b-tubulin, are appropriate as a reference system to analyse the stages of nephridial development in more detail.…”

“…ae, adult eye; g, gut; le, larval eye; sd, stomodaeum. Quast, 2005). Despite morphological similarity to a metanephridium, the larval nephridium is clearly distinguishable from the definitive nephridia: (1) the larval nephridium forms in the deutomeric, synchronously developing larval segments, when the coelomic cavity and a functional circulatory system are not yet fully established, (2) it is a transitory structure, which disappears in worms with about 20 segments (and therewith correlates to the onset of gametogenesis, Rebscher et al, 2007), and (3) it forms meanders over time (Figs.…”

“…3). Rupture of the terminal cell in protonephridia has been reported during ontogeny of several species, such as the polychaete Ophelia (for an overview, see Bartolomaeus, 1999;Bartolomaeus and Ax, 1991;Bartolomaeus and Quast, 2005;Quast and Bartolomaeus, 2005). Protonephridia may acquire metanephridial characteristics by a passive (partial) opening of the flame bulb.…”

“…The filtering structure in the terminal cell consists of slit-like slashes in the cell, which are covered by the extracellular matrix. Single or multiple cilia in the lumen of the terminal cell generate an inward flow, which allows filtration of the body fluid across the extracellular matrix (Bartolomaeus and Quast, 2005).…”

“…The primary urine is transported into the nephridium by a ciliated funnel, where modification by resorption and secretion occurs, like in protonephridia, in the canal (Smith and Rupert, 1986). Annelid nephridia have been very well investigated on a morphological level (Bartolomaeus and Quast, 2005). They develop as segmental organs and typically span segment boundaries with the ciliated funnel opening in the anterior segment, and the canal as well as nephridiopore, located in the posterior one.…”