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PHYLUM RAPHIDIOPHYTA

INTRODUCTION TO THE RAPHIDIOPHYTA

 Raphidiophyta (ra-fi-de-O-fa-ta) is derived from two Greek roots that mean sew (ravo -ράβω); and plant (phyto -φυτό).  The reference may be to the needle-like trichocysts that are abundant just beneath the cell membrane.

The raphidiophytes, also called the chloromonads, make up a small group of photosynthetic motile unicellular organisms.  They are relatively large and sluggish members of protists associated with rotting vegetation in fresh water (Figures A&B) or they can become locally abundant in the marine plankton and often cause fish kills in near shore and brackish water environments (Figure C).  They became taxonomic nomands for some time because of the occurrence of a set of characters that seemed to intersect with the dinoflagellates (trichocysts) and the chrysophytes (a rhizoplast and heterokont motile cell).  

SYNOPTIC DESCRIPTION OF THE RAPHIDIOPHYTA

SYSTEMATICS OF THE RAPHIDIOPHYTA

This phylum is considered to be a class of the Chrysophyta by Margulis and Schwartz (1988 Pr-4 and 1998 Pr-13) and Bold and Wynne (1985). Lee (1980) considers the chloromonads to occupy a position of uncertain status (as did Smith 1950). Protozoologists like Kudo (1966), Grell (1976) and Lee et al (1985) treat the chloromonads as an order within the phytoflagellates. Because they have a unique set of cellular characters (e.g. trichocysts, mucocysts, rhizoplast, kinetochores, many more than 2 chloroplasts per cell) Taylor (1976) places the chloromonads in an isolated position but near the chrysophytes. Similarly, Heywood (1989) cautiously suggests that the chloromonads might occupy a unique position within the Chromophyta (chlorophyll a and c containing algae). Dodge (1973) shows a similar kind of association.  Heywood (1990) argues that the term chloromonad is inappropriate as a root for taxa in the phylum because Chloromonas does not belong to this phylum; thus was the basis for the change in designation from Chloromonada to Raphidiophyta. 

The following system follows the phylogeny of Taylor (1976) and the analysis of Dodge (1973) in which the group is given isolated or phylum status.  Sogin and Patterson (Tree of Life Project) also shows them as a natural group.  Cavalier-Smith and Chao (1996) in a cladistic analysis suggest that the raphidiophytes are sisters to the eustig-chrysophyte clade.  Earlier, Cavalier-Smith (1986) united most of the photosynthetic heterokonts into a single phylum, the Ochrophyta, a taxon adopted by Graham and Wilcox (2000) to include the diatoms, raphidiophytes, chrysophytes, eustigs, silicoflagellates, xanthophytes, and phaeophytes.  Although these taxa do seem to be part of the same clade, I interpret them as related within the same subkingdom because of the enormous variation between these groups.

Heywood (1990) contends that much more work is required before a valid taxonomy can be created for this group.  Since that time, much more confirmation for the association of the raphidiophytes with the other heterokonts has been made (e.g. Cavalier-Smith and Chao 1996).  However, they still seem to be isolated enough to warrant phylum-level status.  Thus I offer the following with that cautionary note.

HIERARCHICAL CLASSIFICATION OF THE RAPHIDIOPHYTA

This page is maintained by Jack R. Holt.  Last revised: 03/17/2008.