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Animal EvolutionInterrelationships of the Living Phyla$

Claus Nielsen

Print publication date: 2011

Print ISBN-13: 9780199606023

Published to Oxford Scholarship Online: December 2013

DOI: 10.1093/acprof:oso/9780199606023.001.0001

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(p.381) 66 Problematica
Animal Evolution

Claus Nielsen

Oxford University Press

Abstract and Keywords

Two of the problematic groups in the animal kingdom in terms of phylogenetic positions are Buddenbrockia (and the Myxozoa as a whole) and Symbion. More recent molecular studies have placed the former in the Cnidaria and the latter in its own phylum, closely related to Entoprocta and Ectoprocta. However, Salvinella and the ‘Mesozoa’ – that is, the parasites Orthonectida and Dicyemida (Rhombozoa) – remain enigmatic, necessitating the addition of a new taxon: Diurodrilus. The Dicyemida and Orthonectida are sometimes placed close to the metazoan stem, and both are even regarded as multicellular organisms or as specialised parasitic flatworms. The two groups may not be sister groups and provide no insights about animal evolution. The dicyemids inhabit the kidneys of cephalopods, while the orthonectids parasitise a variety of marine invertebrates.

Keywords:   evolution, Buddenbrockia, Symbion, Salvinella, Orthonectida, Dicyemida, Diurodrilus, dicyemids, orthonectids, phylogenetic positions

After the publication of the second edition of this book, new information, especially from molecular studies, has made it possible to assign well-founded phylogenetic positions to two of the problematic groups, viz. Buddenbrockia (and the Myxozoa as a whole), which have been included in the Cnidaria (Chapter 13), and Symbion, which has been placed in its own phylum (Chapter 37), closely related to Entoprocta and Ectoprocta. However, Salvinella and the ‘Mesozoa’, i.e. the Orthonectida and Dicyemida (Rhombozoa), remain enigmatic, and a new taxon must be added, viz. Diurodrilus.

The completely perplexing Salinella salve, which was obtained by Frenzel (1892) from a saline culture of material from Córdoba, Argentina, has never been found again. The description shows a tube of cells with cilia both on the inner and the outer side and with special cilia around both openings. Various developmental stages and an encystation after ‘conjugation’ were also described. The whole description could well be a complete misunderstanding, and it seems futile to discuss it further.

Dicyemida and Orthonectida are well-established ‘Problematica’, which are discussed in all major textbooks and encyclopedia. Their position is sometimes regarded as close to the metazoan stem (as indicated by the name Mesozoa) or even as multicellular organisms evolved separately from the metazoans, whereas other authors regard them as specialized parasitic flatworms. The latter view is supported by some molecular studies. The two groups may not be sister groups. They do not contribute to our understanding of animal evolution.

Dicyemids are parasites in the kidneys of cephalopods. A review of their ultrastructure and life cycle is given by Horvath (1997). The polarized epithelia with cell junctions show that they are indeed metazoans, but they lack nerves, and their structure appears highly modified, probably in connection with the parasitic life style. Molecular data support the view that they are bilaterians and perhaps more specifically spiralians, but not related to the platyhelminths (Kobayashi et al. 1999; Aruga et al. 2007; Kobayashi et al. 2009; Suzuki et al. 2010).

Orthonectids are parasites of various marine invertebrates (Kozloff 1990). The free-swimming females consist of an outer layer of ciliated and unciliated cells in species-specific patterns, an inner mass of cells, and longitudinal muscle cells (Slyusarev 2003); nervous cells have not been observed. The much smaller males have a similar structure. Small ciliated larvae enter the host, but this part of the life cycle needs more study. A study of 18S rDNA (Hanelt et al. 1996) indicates that the orthonectids are bilaterians, not related to platyhelminths, but without further details.

Diurodrilus, with six described species, is a genus of small interstitial worms found in tidal and subti (p.382) dal sands around the world (Worsaae and Kristensen 2005). They were originally placed in the ‘Archiannelida’, but new investigations (Worsaae and Rouse 2008) demonstrate that they have a unique morphology, lacking several key annelid characteristics and showing a few similarities with micrognathozoans. Molecular studies were restricted by the material available, but showed no support for an annelid relationship.

The Rhinogradentia (see the chapter vignette) were obviously vertebrates (Stümpke 1961), but the dramatic annihilation of the whole group makes further studies impossible.


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Frenzel, J. 1892. Untersuchungen über die mikroskopische fauna Argentiniens. Salvinella salve nov. gen. nov. spec. Arch. Naturgesch. 58: 66–96.

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