Ultrastructural observations of the protonephridia of Polymerurus nodicaudus (Gastrotricha: Paucitubulatina)

Abstract

Kieneke, A. and Hochberg, R. 2012. Ultrastructural observations of the protonephridia of Polymerurus nodicaudus (Gastrotricha: Paucitubulatina). -Acta Zoologica (Stockholm) 93: 115-124.We studied different regions of the protonephridia of the limnic gastrotrich Polymerurus nodicaudus by means of light and electron microscopy to determine how freshwater species might differ from their marine relatives. Microscopic and ultrastructural characters are in accordance with another limnic species of Paucitubulatina, Chaetonotus maximus, whose protonephridial system has been previously reconstructed. Shared protonephridial characters of both species include the presence of highly elongate terminal organ cilia, microvilli, and the canal cell lumen as well as the presence of a conspicuous anterior loop of the protonephridial lumen. These features are not present in representatives of earlier, marine, paucitubulatan lineages (i.e., Xenotrichulidae) and so are assessed as evolutionary novelties that were likely important for the successful colonization of the freshwater environment.

1. Gastrotricha

“…1.19 A). This "intermicrovillar" ECM resembles the one that can be observed between the eight circumciliary microvilli of the terminal cells of protonephridia of species like Turbanella cornuta (Teuchert 1973), Chaetonotus maximus (Kieneke et al 2008b), or Polymerurus nodicaudus (Kieneke & Hochberg 2012). The number of 10 circumciliary microvilli in mechanoreceptive sensory cells ( Fig.…”

“…The aim of these studies was to get a broader database of more species to infer the evolution of these organs as well as the character pattern of the stem species of Gastrotricha. Such a basic protonephridial anatomy is also much likely in the freshwater paucitubulatinan species Polymerurus nodicaudus (Kieneke & Hochberg 2012). Data on Xenotrichula carolinensis and Chaetonotus maximus proved that the protonephridial system of the stem species of Paucitubulatina, whose monophyly is supported in almost all phylogenetic analyses (see chapter Phylogeny), consists of a single pair of protonephridia, each composed of a bicellular terminal organ with a so-called composite filter, a voluminous and aciliar canal cell with a convoluted lumen, and a nephridiopore cell (Kieneke et al 2008c).…”

“…ECM surrounds the muscle cells, the Y-cells, the protonephridial and gonadal tissue (Ruppert 1991). Comparable widenings of the intercellular space filled with ECM are known from the region of the protonephridial terminal cells in different gastrotrich species (e.g., , 2008c, Kieneke & Hochberg 2012. Ruppert (1991) indicates tiny lacunar spaces in some places, but it is not clear whether such spaces constitute true lacunes of a primary body cavity or regions within the ECM.…”

“…According to Teuchert (1973), there is no continuous lumen in the protonephridia of Turbanella cornuta (i.e., a connection from the terminal cell lumina via the canal cell lumen to the nephridiopore cell lumen). The cleft between those podocyte-like processes is spanned by a filter diaphragm (Kieneke et al 2008c, Kieneke & Hochberg 2012. The number of protonephridia was reduced to a single pair (see Figs.…”

“…Within the stem lineage of Paucitubulatina, different evolutionary transformations have happened. A remarkable evolutionary novelty is the strong elongation of the canal cell lumen: in all species studied so far, Xenotrichula carolinensis, Chaetonotus maximus, and Polymerurus nodicaudus, the narrow lumen runs strongly convoluted through the canal cell (Kieneke et al 2008c, Kieneke & Hochberg 2012Figs. 1.26 E and 1.30 A).…”