DESCRIPTION OF THE PHYLUM CHOANOFLAGELLATA (KENT 1880)

EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>CHOANOZOA>CHOANOFLAGELLATA

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 name was coined by Kent (1880), and a common synonym for the phylum is Choanozoa (Cavalier-Smith 1993a).

INTRODUCTION TO THE CHOANOFLAGELLATA 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 Figures 1-4). 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 1), 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 substrates by stalks (Figure 2) or free-floating (Figure 3). We have encountered them as members of the periphyton or plankton in small, eutrophic ponds. 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 3). Molecular data support that contention (Snell et al. 2001, see Figure 5 below; 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, we 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 cell. 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).

FIGURE 1. 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. Image from http://www.niwa.co.nz/pubs/wa/09-2/evolution.htm	FIGURE 2. Drawing of a colonial choanoflagellate, Sphaeroeca lackeyi. Each cell body is about 10 µm long. Image from http://www.niwa.co.nz/pubs/wa/09-2/evolution.htm	FIGURE 3. Colonial choanoflagellate, Proterospongia haeckeli, long thought to be a “missing link” between sponges and “protozoa”. Colony is 40–50 µm across. Image from http://www.niwa.co.nz/pubs/wa/09-2/evolution.htm	FIGURE 4. Fluorescence photomicrographs of chonaoflagellates from the Georges Banks. Note the flagella and collar. Image from http://www.bigelow.org/cytometry/Image_gallery/CHOANO.html

FIGURE 5. A figure from Snell et al. (2001) based on amino acid sequences. Monosiga (see Figure 6), a choanoflagellate emerges in the clade with animals as a sister to the metazoa.	FIGURE 6. Phase-contrast photomicrograph of Monosiga. Note the outline of the collar and the flagellum. Image by Stephen Fairclough, Wikimedia Commons

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By Jack R. Holt and Carlos A. Iudica. Last revised: 04/07/2013