Opalinidae

The opalines are a small group of peculiar heterokonts, currently assigned to the family Opalinidae, in the order Slopalinida. Their name is derived from the opalescent appearance of these microscopic organisms when illuminated with full sunlight. Most opalines live in the large intestine and cloaca of anurans (frogs and toads), though they are sometimes found in fish, reptiles, molluscs and insects; whether they are parasitic is not certain. The unusual features of the opalines, first observed by Antonie van Leeuwenhoek in 1683, has led to much debate regarding their phylogenetic position among the protists. Taxonomy and phylogeny The relationship between opalines and other protists has been a subject of great controversy since the late 19th century, and is not completely resolved at present. Initially, microscopists believed that the thousands of rhythmically beating hair-like structures which cover their surface were cilia, and they placed the opalines in Ciliophora. In the early 20th century other aspects of opaline biology clearly differentiated them from the ciliates and they were placed in Sarcomastigophora, with the amoebae and flagellates. In the 1980s, detailed ultrastructural studies of Opalina ranarum revealed that they share many features with the heterokonts of the family Proteromonadidae. A new order—Slopalinida Patterson 1985—was proposed to include the members of the families Proteromonadidae Grassé 1952 and Opalinidae Claus 1874. In 2004, the first reliable opaline genetic sequence data supported the monophyletic nature of the order Slopalinida. The authors of that study considered the opalines to be a family (Opalinidae) within the order Slopalinida. There are currently about 200 recognized species of opalines in 5 genera: Opalina Purkinje and Valentin 1835, Protoopalina Metcalf 1918, Cepedea Metcalf 1920, Zelleriella Metcalf 1920, and Protozelleriella Delvinquier et al. 1991. Two additional genera, Hegneriella Earl 1971 and Bezzenbergeria Earl 1973, have not been considered as valid by subsequent authors (p. 249) The 5 recognized genera differ in terms of the number of nuclei, the appearance and location of the falx (two short, sickle-shaped rows of flagella), and whether the long rows of flagella (called "kineties") cover the body evenly or if there is a "bald spot".