|TAXA OF LIFE||
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:
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 . 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).|
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This page is maintained by Jack R. Holt. Last modified: 02/11/09