Ultrastructure of protonephridia in Xenotrichula carolinensis syltensis and Chaetonotus maximus (Gastrotricha: Chaetonotida): comparative evaluation of the gastrotrich excretory organs

Abstract

In an attempt to obtain detailed information on the entire protonephridial system in Gastrotricha, we have studied the protonephridial ultrastructure of two paucitubulatan species, Xenotrichula carolinensis syltensis and Chaetonotus maximus by means of complete sets of ultrathin sections. In spite of some diVerences in detail, the morphology of protonephridia in both examined species shows a common pattern: Both species have one pair of protonephridia that consist of a bicellular terminal organ, a voluminous, aciliar canal cell and an adjacent, aciliar nephridiopore cell. The terminal organ consists of two monociliar terminal cells each with a distal cytoplasmic lobe. These lobes interdigitate and surround cilia and microvilli of the terminal cells. Where both lobes interdigitate, a meandering cleft is formed that is covered by the Wltration barrier. We here term the entire structure composite Wlter. The elongated, in some regions convoluted protonephridial lumen opens distally to the outside via a permanent nephridiopore. A comparison with the protonephridia of other species of the Gastrotricha allows hypothesising the following autapomorphies of the Paucitubulata: The bicellular terminal organ with a composite Wlter, the convoluted distal canal cell lumen and the absence of cilia, ciliary basal structures and microvilli within the canal cell. Moreover, this comparative survey could conWrm important characteristics of the protonephridial system assumed for the ground pattern of Gastrotricha like, for example, the single terminal cell with one cilium surrounded by eight microvilli.

Comparative morphology and evolution of the nephridia in Nemertea

“…If two or more terminal cells form the suspending structure of the filtration barrier ( Figure 1C), it is called "compound filter" (Neuhaus 1988). It has been shown for several bilaterian taxa that supporting structures may display a large variation of forms (Kieneke et al 2008). This variation might be informative for phylogenetic inferences.…”

“…This hypothesis seemed to find support in Oudemans' (1885), Nawitzki's (1931) and Friedrich's (1935a, b) observations of a direct connection filtration barrier: (C) compound filter is formed by several terminal cells; (D) filter formed by a single terminal cell; (E) transcellular duct. Notes: ECM, extracellular matrix; DC, duct cell; NPC, nephridiopore cell; PC, podocyte; TC, terminal cell; (A, B) modified from Bartolomaeus and Ax (1991); (C-E) modified from Kieneke et al (2008). ; (B) metanephridial system; arrows indicate the direction of filtration as well as secretory and resorption processes in the duct; (C, D) supporting structure for the between the blood fluid and nephridial lumen in certain palaeonemertean species.…”

A phylogenetic approach to species delimitation in freshwater Gastrotricha from Sweden

“…A recent study of the phylogeny of Thaumastodermatidae showed that, within this family, not only molecular data and morphology are in general accordance, but also that there is homoplasy in the evolution of some cuticular structures. Indeed, additional data, such as the morphology of different organ systems (see e.g., Hochberg & Litvaitis, 2003;Leasi et al, 2006;Kieneke et al, 2008a;Leasi & Todaro, 2008) as well as gene sequence data should also be utilized for classification.…”

Nephridial development and body cavity formation in Artemia salina (Crustacea: Branchiopoda): no evidence for any transitory coelom

“…The duct cells bear several microvilli extending into the duct. The duct is either percellular, i.e., its lumen is enfolded by a cell (sensu Kieneke et al 2008, Wg. 1D), or it is formed by two cells so that the lumen is intercellular (Fig.…”