DESCRIPTION OF THE PHYLUM RHOMBOZOA (VAN BENEDEN 1876)
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RHOMBOZOA LINKS
| Rhombozoa (rom-bo-ZO-a) is made of two Greek roots that mean a rhomboid animal [rhomboid -rhomboeides (ρομβοειδές); animal -zoo(ζώο)]. |
| INTRODUCTION TO THE RHOMBOZOA The rhombozoans, also called dicyemids, are among the simplest of the bilaterians in form with twenty to thirty somatic cells enclosing a long, cylindrical axial cell with one to several hundred axoblasts (text with tooltip) Axoblasts are rhombozoan reproductive cells that occur within the Rhombozoan axial cell. They each develop into vermiform larvae within the axial cell. (Figure 1). However, because they are parasites of cephalopod kidneys, their structural simplicity likely is a consequence of their parasitic lifestyle. Figure 2 is the infusiform, the infective larval stage. Their lifecycles are poorly known, but seem to be elaborate with at least four different structural (and functional) stages. Figure 3 [Figure 1 of Suzuki et al. (2010) and Furuya and Tsuneki (2003)] illustrates the developmental changes in a typical dicyemid in the renal sac of a cephalopod. The nematogen stage is asexual and produces vermiform embryos, some of which may develop into rhombogens, the sexual forms. They generate zygotes which develop into infusiform embryos, which leave the host. The free-swimming and infective stages that develop from the infusiform embryos are unknown. The early cleavage patterns of the zygote are spiral (Furuya et al. 1992). Once they were grouped together with the Orthonectida as the Mesozoa, which, as the name implied, were considered transitional organisms at the base of the metazoans (Storer and Usinger 1965). Now, 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 Stunkard (1954) and Margulis and Schwartz (1998) that they are related to other acoelomates (particularly the flatworms) is equally suspect. The superficial association seemed to be supported by molecular work (Hanelt et al. 1996), but more recent work by Suzuki et al. (2010) showed that they were not associated with Platyhelminthes. Furthermore, they appeared to be transitional between the diploblastic and triploblastic animals. Other early work (e.g. Katayama et al. 1995) supported the theory that they were highly reduced triploblastic animals. The discovery of spiral cleavage (Furuya et al. 1992) further places the dicyemids within the Spiralia, either as a sister group to Annelida + Mollusca (Suzuki et al. 2010) or as a sister to the Annelida (Petrov et al. 2010). Noto and Endoh (2004) repackage an old theory that the dicyemids really are protist parasites that have acquired metazoan genes through lateral gene transfer. Such a theory is interesting but does not adequately explain the range of cellular, developmental, and molecular homologies with triploblastic animals. We have elected to keep them in the Spiralia as a sister to the Orthonectida, 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 4. |
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| FIGURE 1. Diagram of an adult Dycema. Image from http://biodidac.bio.uottawa.ca/ | FIGURE 2. Photomicrograph of the infusiform, the infective larval stage. Image from http://www.tarleton.edu/~dekeith/mesozoa.jpg |
 | FIGURE 3. A figure used in Suzuki et al. (2010) and Furuya and Tsuneki (2003) to illustrate the life cycle of a typical dicyemid. Nematogens produce asexual vermiform embryos, some of which can develop into sexual rhombogens, which produce infusiform embryos, the infective stage. |
 | FIGURE 4. A cladogram that shows the relationships of Rhombozoa 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. Furuya, H. and K. Tsuneki. 2003. Biology of dicyemid mesozoans. Zoological Science. 20: 519-532. [C] Furuya, H., K. Tsuneki, and Y. Koshida. 1992. Development of the infusoriform embryo of Dicyema japonicum (Mesozoa: Dicyemidae). Biol. Bull. 183: 248-257. 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. Katayama, T., H. Wada, H. Furuya, N. Satoh, and M. Yamamoto. 1995. Phylogenetic position of the dycyemid Mesozoa inferred from 18S rDNA sequences. Biological Bulletin 189:81-90. 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. Noto, T. and H. Endoh. 2003. A “chimera” theory on the origin of dicyemid mesozoans: evolution driven by frequent lateral gene transfer from host to parasite. BioSystems. 73: 73-83. 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. [C] Suzuki, T., K. Ogino, K. Tsuneki, and H. Furuya. 2010. Phylogenetic analysis of dicyemod mesozoans (Phylum Dicyemida) from innexin amino acid sequences: dicyemids are not related to Platyhelminthes. Journal of Parasitology. 96(3): 614-625. 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 |
