Actinodine

The actinodines are a small group of protists, comprising a few genera which range from colored flagellates to colorless heliozoa, found in freshwater and marine habitats. They typically have one anterior flagellum, extended into a wing-like shape, surrounded by tentacles or axopods supported by triads of microtubules, which may also arise over the rest of the body. They are included among the axodines, a group of heterokont algae, together with the silicoflagellates.

Colored actinodines may be autotrophic or mixotrophic, feeding on small prey which are captured by the tentacles. These include the genera Pedinella, Apedinella, Pseudopedinella, and Mesopedinella. Colorless forms feed exclusively by phagocytosis, and include Parapedinella, Actinomonas, Pteridomonas, and Ciliophrys. The last alternates between a mobile stage, similar in form to the other genera, and a feeding stage, where the body is contracted with extended axopods over the entire surface, and the flagellum is curled up into a figure eight.

Mikrjukov and Patterson have argued that the actinophryids also belong here. These are spherical heliozoa which lack flagella altogether, and are covered in axopods supported by microtubules in a unique double coil pattern. There are two genera, the uninucleate Actinophrys and the multinucleate Actinosphaerium, comprising the most frequently found freshwater heliozoa. Some other heliozoans have also been suggested as possible members, but these typically show considerably different cell structure.

The name actinodines specifically includes the actinophryids, and prior to their inclusion the group was called the pedinellids. The colored forms were originally treated as a family of Ochromonadales, promoted to the order Pedinellales by Zimmermann et al. in 1984. The colorless forms were originally treated among the Heliozoa as the orders Ciliophryida and Actinophryida.

References

Mikrjukov K.A., Patterson D.J. (2001) Taxonomy and Phylogeny of Heliozoa. III. Actinophryids. Acta Protozoologica 40: 3-25