The opalines are a small group of peculiar

heterokont Heterokonts are a group of protists (formally referred to as Heterokonta, Heterokontae or Heterokontophyta). The group is a major line of eukaryotes. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which a ...

s, 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 as endocommensals in the large intestine and cloaca of anurans (

frogs A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" '' Triadobatrachus'' is ...

and toads), though they are sometimes found in

fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is est ...

s and

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...

s. The unusual features of the opalines, first observed by

Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the ...

in 1683, has led to much debate regarding their

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...

position among the

protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...

Taxonomy and phylogeny

The relationship between opalines and other

s 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 The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proje ...

, and they placed the opalines in

Ciliophora The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...

. In the early 20th century other aspects of opaline biology clearly differentiated them from the ciliates and they were placed in Sarcomastigophora, with the

amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...

e and

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and thei ...

s. In the 1980s, detailed ultrastructural studies of ''Opalina ranarum'' revealed that they share many features with the

s 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 In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...

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". Due to the differences in body shape among the different life cycle stages within a species, the use of overall body shape - whether flat or cylindrical - to differentiate the genera has been de-emphasized.

Life cycle

Like many parasites, the life cycle of opalines is rather comple

The most comprehensive study published so far concluded that the life cycles of 10 ''Opalina'' species, 1 ''Zelleriella'' species and 1 ''Protoopalina'' species are all "remarkably similar" (p. 321). A more recent study found that ''Cepedea couillardi'' fits the standard opaline life cycle model described below, while that of ''Opalina proteus'' is completed entirely in the tadpole stage of the host. Very little is known about the life cycles of opalines in fish, reptile or arthropod hosts. Asexual phase in adult anuran host. The basic opaline life cycle begins with the large,

multinucleate Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordina ...

trophonts in the adult anuran cloaca. Through much of the year, the trophonts grow and divide continually to yield more trophonts. Nuclear divisions maintain the appropriate number of nuclei during this phase. As the host's breeding season approaches, the trophonts enter a phase known as palintomy — a series of cell divisions with little or no overall growth or nuclear divisions. The resulting opalines, which become gradually smaller with fewer nuclei per individual, are called tomonts. At some point the small tomonts undergo encystment, and the cysts are released into the environment (i.e. the breeding pool of the anuran host) along with the feces. Sexual and asexual phases in larval anuran host. Once cysts are eaten by foraging tadpoles, they excyst (hatch) to yield gamonts. The gamonts divide further, including a meiotic division, to yield

haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respective ...

gametes. Each gamete has only one nucleus and may be either a

microgamete {{Short pages monitor