DESCRIPTION OF THE PHYLUM ORTHONECTIDA (GIARD 1877)

EUKARYA> UNIKONTA> OPISTHOKONTA> ANIMALIA> METAZOA> BILATERIA> PROTOSTOMATA> LOPHOTROCHOZOA> EUTROCHOZOA> ORTHONECTOZOA

Orthonectozoa (OR-tho-nek-to-ZO-a) is made of two Greek roots meaning “straight swimming” [straight -ortho (ορθό); swimming -nekto (νηκτικο)]; -ida is a standard new Latin ending for a higher taxon. The name was created by Giard (1877).

INTRODUCTION TO THE ORTHONECTOZOA The orthonectids, like the rhombozoans, are among the simplest of the bilaterians in form. As mature organisms, they are little more than a jacket of ciliated cells that enclose a gonad; however, their structural simplicity likely is a consequence of their parasitic lifestyle. In their asexual stages, the dominant forms, all are tissue parasites of an eclectic mix of invertebrates (echinoderms, nemertenians, turbellarian flatworms, polychaetes, and clams; Margulis and Schwartz 1998). The sexual stages are free-swimming (Figures 1 and 2). Aside from the gametes and zygotes, they form three distinct structural forms in their lifecycle, one of which is a syncitial plasmodium (text with tooltip) A plasmodium is a multinucleate cell formed by nuclear divisions without cytokinesis. , which can become large enough in some taxa to castrate or do other great harm to their hosts. The other two forms are the free-swimming sexual adults: large female and small male (Figures 1 and 2). These are very problematic taxa, not only because they are poorly known, but also because they are so simple in form. Once they were grouped together with the Rhombozoa as the Mesozoa, which, as the name implied, were considered transitional organisms at the base of the metazoans (Hyman 1959; Storer and Usinger 1965). Now, especially in morphological studies, both the rhombozoans and orthonectids are viewed as problematic taxa with no clear affinities (Margulis and Schwartz 1998; Brusca and Brusca 2003; and Tudge 2000). Brusca and Brusca (2003) suggest that the rhombozoans and orthonectids may only appear similar through convergence. That is, highly simplified parasitic animals at the lower end of structural complexity would likely converge on similar body plans; so, their taxonomic association is necessarily suspect. The suggestion by Hyman (1940), Stunkard (1954), Margulis and Schwartz (1998) that they are related to other acoelomates (particularly the flatworms) is equally suspect. The simplicity caused Giard (1877) to suggest that they were very primitive animals. Cavalier-Smith (1993) considered the mesozoans to be multicelluar protozoans. That the mesozoan (rhombozoans and orthonectids) were only superficially associated seemed to be supported by molecular work (Hanelt et al. 1996), but more recent work by Petrov et al. (2010) showed that the rhombozoans and orthonectids were associated, and perhaps should be reunited as the Mesozoa. Thus, they appear to be highly reduced triploblastic animals (Pawlowski et al 1996; Petrov et al. 2010) related to the Annelida + Mollusca (Suzuki et al. 2010) or as a sister to the Annelida (Petrov et al. 2010). We have elected to keep them in the Spiralia as a sister to the Rhombozoa, the other mesozoan group as suggested by Petrov et al. (2010). The mesozoan clade is a sister to Mollusca + (Annelida + Nemertea) as shown in Figure 3.

FIGURE 1. A diagram of an adult male (left) and female (right) of Rhopalura. Image from http://biodidac.bio.uottawa.ca/

A. Male plasmodium E-G. Development of the sexual male in the plasmodium B. External view of female C. Longitudinal section of B D. External view of male H. Fertilization (male is the smaller animal) Sexual animals come from respective plasmodia (each plasmodium is one sex). The free-swimming, ciliated sexual animals leave the host and the male directly fertilizes the female. Fusion occurs within the female where ciliated larvae develop and emerge to seek out an appropriate host. In the host, the larva invades a cell or tissues and then forms a feeding asexual plasmodium.

FIGURE 2. Stages in the lifecycle of an orthonectid, Rhopalura. Figures and text from Hyman (1940)

FIGURE 3. A cladogram that shows the relationships of Orthonectida within the Protostomata.

LITERATURE CITED Barnes, R. D. 1980. Invertebrate Zoology. Saunders College/Holt, Rinehart and Wilson, Philadelphia. Barnes. R. S. K. 1984a. Kingdom Animalia. IN: R. S. K. Barnes, ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc., Sunderland, MA. pp. 129-257. Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass. Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago. Cavalier-Smith, T. 1993. Kingdom protozoa and its 18 Phyla. Microbiological Reviews. 57: 953-994. Giard, A. M. 1877. Sur les Orthonectida, classe nouvelle d’animaux parasites des Échinodermes et des Turbellariés” (in French). Comptes Rendus. 85: 812–814. Giribet, G., C. W. Dunn, G. D. Edgecombe, and G. W. Rouse. 2007. A modern look at the Animal Tree of Life. Zootaxa. 1668: 61-79. Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis. Hyman, L. H. 1940. The Invertebrates. Volume 1, Protozoa through Ctenophora. McGraw-Hill Book Company, Inc. New York and London. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. Meglitsch, P. A. and F. R. Schramm. 1991. Invertebrate Zoology. Oxford University Press, New York, Oxford. Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. 2nd Edition. Oxford University Press. Oxford. Pawlowski, J., J. I. Montoya Burgos, J. F. Fahrni, J. Wuest, and L. Zaninetti. 1996. Origin of the Mesozoa inferred from 18S rRNA gene sequences. Molecular Biology and Evolution 13:1128-1132. Petrov, N. B., V. V. Aleshin, A. N. Pegova, M. V. Ofitserov, and G. S. Slyusarev. 2010. New insight into the phylogeny of Mesozoa: evidence from the 18S and 28S rRNA genes. Moscow University Biological Sciences Bulletin. 65(4): 167-169 [in English]. Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. 6th edition. Saunders. Ft Worth, TX. Ruppert, E. E., R. S. Fox, and R. D. Barnes. 2004. Invertebrate Zoology: A Functional Evolutionary Approach. Seventh Edition. Thomson, Brooks/Cole. New York. pp. 1-963. Storer, T. I. and R. L. Usinger. 1965. General Zoology. 4th Edition. McGraw-Hill Book Company. New York. Stunkard, H. W. 1954. The life history and systematic relations of the Mesozoa. Quart. Rev. Biol. 29: 230-244. Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York. Walker, J. C. and D. T. Anderson. 2001. The Platyhelminthes, Nemertea, Entoprocta, and Gnathostomulida. In: Anderson, D.T., ed. Invertebrate Zoology. Oxford University Press. Oxford, UK. pp. 59-85. [L] Valentine, J. W. 2004. The Origin of Phyla. University of Chicago Press. Chicago. 614 pp.

By Jack R. Holt. Last revised: 04/10/2013