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KINGDOM ANIMALIA
PHYLUM CHOANOFLAGELLATA

INTRODUCTION 

Choanoflagellata (ko-a-no fla-ge-LA-ta) is derived from a combination of Greek and Latin roots meaning funnel flagellates [funnel -choni (χωνί); and whip -flagellum).  The reference is to the funnel-like collar that surrounds the flagellum.

The choanoflagellates are free-living aquatic organisms (freshwater to marine) that range from unicellular to colonial species and resemble choanocytes, the flagellated collar cells of sponges (see A-D above).  The collar usually is made of microvilli that surround the single flagellum and serves as a filter for catching particles caught in the vortex created by the flagellum.  Some taxa secrete a single sheath, and sometimes the microvilli are silicaceous (Figure A), a feature that seems to connect them with the heterokont algae.  That some taxa also have cellulosic loricas, also suggested a link to the algae.  

The persistent microscopist can find choanoflagellates attached to a substrate by a stalk (B) or free-floating (C).  I have encountered them as members of the periphyton or plankton in small, eutrophic ponds.

A. SEM micrograph of the silicaceous lorica and "filter" that surrounds the flagellum of Diplotheca costata.  Cell body and collar together is about 12 µm long.

B. Drawing of a colonial choanoflagellate, Sphaeroeca lackeyi. Each cell body is about 10 µm long.

C. Colonial choanoflagellate, Proterospongia haeckeli, long thought to be a "missing link" between sponges and "protozoa". Colony is 40–50 µm across.

D. Fluorescence photomicrographs of chonaoflagellates from the Georges Banks.  Note the flagella and collar.

Images A-C from: http://www.niwa.co.nz/pubs/wa/09-2/evolution.htm ;
Image D from: http://www.bigelow.org/cytometry/Image_gallery/CHOANO.html

SYNOPTIC DESCRIPTION OF THE CHOANOFLAGELLATA

The following information came from Margulis and Schwartz (1998),  Barnes (1984), Brusca and Brusca (2003), Lee et al. (1985), Cavalier-Smith (1997), Tudge (2000), and Adl et al. (2005).  Because these clearly are not Metazoans, I have used the "protozoan" description format for this phylum.

I. SYNONYMS: The formal taxon Choanomonada was defined by Kent in 1880.  Other synonyms include: choanoflagellates, choanomastigotes, choanocytes, collar cells.

II. NUMBER: >140 species known.

III. PHYLUM CHARACTERISTICS:

A. Structure and Physiology

Cell Form: Unicellular to colonial.

Flagella: One whiplash flagellum; cells swim with flagellum trailing.

Basal Bodies: Microtubular flagellar rootlets radiate from all around the active basal body and terminate beneath the cell membrane without making a connection with the nucleus or the barren orthogonal basal body.  The basal body has a central filament in the transition zone.

Cell Covering: Base of flagellum surrounded by a ring (collar) of tentacular microvilli (costae) used in filter feeding; some secrete a membranous sheath, a cellulosic lorica (theca), or basket-like lorica of silicaceous bars.

Chloroplasts: Sometimes with green plastids.

Food Reserves: Not reported.

Mitochondria: Flat cristae.

 Golgi:   Present.

Nucleus: Uninucleate.  Endosome?

Centrioles: Not reported.

Inclusions and Ejectile Organelles:  Food vacuoles and contractile vacuoles.

B. Reproduction:

Mitosis: Not reported.

Meiosis: Not reported.

Sexual Reproduction and Life History: Not reported; however, a life history with multiple life stages has been reported in Proterospongia.

C. Ecology: Freshwater to marine; free living, phagotrophic.

SYSTEMATICS OF THE CHOANOFLAGELLATA

Until recently the choanoflagellates were considered to be part of the zooflagellates (Margulis and Schwartz 1988, 1998; and Buck 1990).   However, the choanoflagellates do not fit within that group very well, which prompted Sleigh et al. (1984) to raise the choanoflagellates to phylum-level status.  Taylor (1976), Lee (1980) and Sze (1986) tried to solve the same problem by associating the choanoflagellates with the chrysophytes.  However, details of the choanoflagellate cells are remarkably similar to those of the sponges, a view first proposed by Metschnikoff (1886) following the discovery of Proterospongia (Figure C).  Molecular data support that contention (Tudge 2000; Nielsen 1995).  Brusca and Brusca (2003) indicate that because the choanoflagellates seem to lack affinities elsewhere, they may be highly reduced sponges.  Molecular evidence also confirms their animal association (Wainright et al. 1993; Cavalier-Smith et al. 1996a)Adl et al. (2005) presented the choanoflagellates, a group that they called Choanomonada, as a rank equal to all of the Metazoa, Fungi, and nuclearid amoebae. Here, I consider them to occupy the animal subkingdom Choanozoa.  Please consult The Major Clades of the Animal Kingdom for some views on the relationships of the choanoflagellates with the other phyla of the animal kingdom.

The group has been divided according to the nature of the lorica that surrounds the cells.  Lee et al. (1985) separated the taxa into two families according to the types of loricas: cells naked or with a cellulosic theca and another group with a lorica constructed of silicaceous strips.  Cavalier-Smith and Chao (1997) divided them into two groups according to the presence or absence of a lorica (theca).  The system presented here divides them into three groups and follows the divisions, but not the ranks, given by Adl et al. (2005).

HIERARCHICAL CLASSIFICATION OF THE CHOANOFLAGELLATA

This system is a modification of Lee et al. (1985) who also consider the choanoflagellates to be a natural group within the Zoomastigophora.  I have elevated their Class Choanomastigotes to phylum-level status (Phylum Choanoflagellata) with a single class (Class Choanoflagellida).  Within that system, I have used the divisions of Adl et al. (2005). 

 

ORDER MONOSIGIDA.  Unicellular or colonial; all cells naked, without visible lorica or periplast.

Codosiga, Monosiga, Sphaeroeca.

ORDER SALPINGOECIDA.  Unicellular to colonial with a clearly-visible cellulosic lorica (theca) or periplast.

Desmarella, Proterospongia, Salpingoeca, Choanoeca, Pachysoeca.

ORDER ACANTHOECIDA.  Unicellular or colonial; planktonic or attached; each cell with a basket-like lorica of silicaceous microvillae (=costae), each of which is made of rod like costal strips

Acanthoeca, Bicosta, Acanthoecopsis, Diplotheca, Pleurosiga, Stephanoeca, Saepicula.


LITERATURE CITED

Adl, S. M., A. G. B. Simpson, M. A. Farmer, R. A. Andersen, O. R. Anderson, J. R. Barta, S. S. Bowser, G. Brugerolle, R. A. Fensome, S. Fredericq, T. Y. James, S. Karpov, P. Kugrens, J. Krug, C. E. Lane, L. A. Lewis, J. Lodge, D. H. Lynn, D. G. Mann, R. M. McCourt, L. Mendoza, O. Moestrup, S. E. Mozley-Standridge, T. A. Nerad, C. A. Shearer, A. V. Smirnov, F. W. Spiegel, and M. F. J. R. Taylor. 2005. The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology. 52(5):399-451. [3,L]

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. 2nd Edition. Sinauer Associates. Sunderland, MA. [C,L]

Buck, K. R. 1990. Choanomastigotes (choanoflagellates).  In: Margulis, L., J.O. Corliss, M. Melkonian, and D.J. Chapman, eds. 1990. Handbook of the Protoctista; the Structure, Cultivation, Habits and Life Histories of the Eukaryotic Microorganisms and Their Descendants Exclusive of Animals, Plants and Fungi. Jones and  Bartlett Publishers. Boston. pp. 194-199. [L]

Cavalier-Smith, T. and E. E. Chao. 1997. Sarcomonad ribosomal RNA sequences, rhizopod phylogeny, and the origin of euglyphid amoebae. Archiv fur Protistenkunde. 147: 227-236.

Cavalier-Smith, T., M. T. E. P. Allsopp, E. E. Chao, N. Boury-Esnault, and J. Vacelet. 1996a. Sponge phylogeny, animal monophyly, and the origin of the nervous system: 18S rRNA evidence. Canadian Journal of Zoology 74:2031-2045. [L]

Lee, J. J., S. H. Hunter, and E. C. Bovee, eds. 1985. An Illustrated Guide to the Protozoa. Society of Protozoologists. Lawrence, Kansas.

Lee, R. E. 1980. Phycology. Cambridge University Press. Cambridge. [C,L]

Margulis, L. and K. Schwartz. 1988. Five kingdoms, an illustrated guide to the phyla of life on earth. 2nd Edition. W.H. Freeman and Co.  New York. [C]

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. [C]

Metschnikoff, E. 1886. Embryologische Studien an Medusen : Ein Beitrag zur Genealogie der Primitive Organe. Atlas Figures 1-12. Alfred Hölder. Vienna. pp. 45-71.

Nielsen, C. 1995. Animal Evolution: Interrelationships of the Living Phyla. 1st Edition. Oxford University Press. Oxford.

Sze, P. 1986. A Biology of the Algae. Wm. C. Brown Publishers. Dubuque, Iowa . [C,L]

Taylor, F. J. R. 1976. Flagellate Phylogeny: A Study in Conflicts. Journal of Protozoology. 23(1):28-40. [C,L]

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.

Wainright, P. O., G. Hinkle, M. L. Sogin, and S. K. Stickel. 1993. Monophyletic origins of the Metazoa: an evolutionary link with Fungi. Science. 260: 340-342. 

This page is maintained by Jack R. Holt.  Last modified: 02/11/09