DESCRIPTION OF THE PHYLUM ACTINOPHRYIDA (CLAUS 1874)

EUKARYA> CHROMALVEOLATA> HETEROKONTAE> ACTINOPHRYIDA

Actinophryida (ak-tin-o-FRY-e-da) is derived from two Greek roots that mean ray (aktina -ακτίνα); and eyebrow (ophrys -οφρύς). The reference is to the rayed or bushy appearance of the cell, particularly in reference to the axopods. The name of the phylum is derived from Actinophrys, a common genus.

INTRODUCTION TO THE ACTINOPHRYIDA These are the classic heliozoans, or, as Antony van Leeuwenhoek called them, the sun animacules. These cells resemble a model of the sun with a spherical body and stiff, radiating axopods (text with tooltip) An axopod is a thin pseudopodium that is permanent and stiff due to the presence of an internal microtubular structure. This is characteristic of the Actinopodotista. . Although relatively few recognized species, the Actinophryids are the most common heliozoans (literally, sun animals) in fresh water (Figures 1 and 2). Typically, they occur as filter feeders among vegetation in still ponds and lakes. Images of these cells show axopods in regular arrays and a distinct difference between the outer and inner cytoplasm (called ectoplasm and endoplasm, respectively; see Figure 2). The ectoplasm is vacuolated, which may serve as a means of controlling the density of the cell. The structure of the cell and of the axopod array is a consequence of a very geometric cytoskeleton. Each axopod is supported by microtubules which originate at a nucleus and extend outward in a double polygonal spiral. They operate like conveyor belts with cytoplasm moving up the center and down toward the cell on the outside. Particles, bacteria, small unicellular eukaryotes, etc. that adhere to the cell membrane are carried to the base of the axopod and incorporated into a food vacuole. Actinophrys tends to have a single nucleus while Actinosphaerium is multinucleate. Little else is known about their life histories. In times of unfavorable conditions, they encyst. Perhaps, gametic fusion occurs in the cyst. Exactly where (or if) meiosis occurs is not known. Since their discovery by Anton van Leeuwenhoek, the “sun animacules” have been taxonomic nomads. For some time they seemed to have found a home in the “sarcodina” because they produced axopods (e.g. Kudo 1966; and Grell 1973). The presence of axopods also seemed to ally them with numerous other heliozoan-like organisms (e.g. Radiolaria, Centrohelida, etc.) and led to the union of all axopod-bearing taxa into the phylum Actinopoda (Febvre-Chevalier 1990; and Margulis and Schwartz 1998). Smith and Patterson (1986) reviewed ultrastructural and life history details and suggested that the groups of heliozoans were only superficially alike and in need of systematic revision. Patterson (1999) separated the heliozoans into several sisterless groups, some of which were later incorporated into the Cercozoae. The Actinophryids, however, showed a set of characters that seemed to ally them with the silicoflagellates and the pedinellids in the kingdom Heterokontae (Mikrjukov and Patterson 2001). Unfortunately, even the two authors (Mikrjukov and Patterson 2001) could not agree on a single taxonomic scheme that would unite them. Nikolaev et al. (2004) in their supertree analysis of a spectrum of eukaryotes also show the Actinophryids emerging within the stramenopiles (=Heterokontae) but not particularly associated with the silicoflagellates. Thus, we elect to keep this group separated as a phylum until more solid confirmations can be made.

FIGURE 1. Actinophrys showing the vacuolate cytoplasm and large central nucleus. Image from http://biology.kenyon.edu/Microbial_Biorealm/eukaryotes/actinophrydae/actinophrydae.htm	FIGURE 2. Actinosphaerium taken with a DIC microscope. The axopods arise at the border between the ectoplasm and endoplasm. Image from The Systematics Biodiversity Image Archive

FIGURE 3. A cladogram showing the relationship between Actinophryida (in shaded box) and other members of the Ocrophyte clade (taxa in bold). This supports a relationship with the silicoflagellates and the secondary loss of chloroplasts. The overall topology of the cladogram is a summary of the analyses of Andersen (2004), Brown and Sorhannus (2010), and Yang et al. (2012).

LITERATURE CITED Andersen, R. A. 2004a. Biology and systematics of heterokont and haptophyte algae. American Journal of Botany. 91(10): 1508-1508. Brown, J. W. and U. Sorhannus. 2010. A molecular genetic timescale for the diversification of autotrophic stramenopiles (Ochrophyta): substantive underestimation of putative fossil ages. PLoS ONE 5(9): e12759. doi:10.1371/journal.pone.0012759. Claus, C. 1874. Grandzuge der Zoologie. 3 Aufl. 1 Lief. Febvre-Chevalier, C. 1990. Heliozoa. 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. 347-362. Grell, K. G. 1973. Protozoology. Springer-Verlag. New York. Kudo, R.R. 1966. Protozoology. 5th ed. Charles C. Thomas Publisher. Springfield. Mikrjukov, K.A. and D.J. Patterson. 2001. Taxonomy and phylogeny of Heliozoa. III. Actinophryids. Acta Protozoologica. 40: 3-25. Nikolaev, S. I., C. Berney, J. Fahrni, I. Bolivar, S. Polet, A. P. Mylnikov, V. V. Aleshin, N. B. Petrov, and J. Pawlowski. 2004. The twilight of Heliozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes. Proceedings of the National Academy of Sciences. USA. 101(21): 8066-8071. Patterson, D. J. 1999. The diversity of eukaryotes. American Naturalist. 154 (Suppl.): S96–S124. Smith, R. M. and D. J. Patterson. 1986. Analyses of heliozoan interrelationships: an example of the potentials and limitations of ultrastructural approaches to the study of protistan phylogeny. Proceedings of the Royal Society of London. B. 227:325-266. Yang, E. C., G. H. Boo, H. J. Kim, S. M. Cho, S. M. Boo, R. A. Andersen, and H. S. Yoon. 2012. Supermatrix data highlight the phylogenetic relationships of photosynthetic stramenopiles. Protist. 163: 217-231.

By Jack R. Holt. Last revised: 02/19/2013