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
Opalescence refers to the optical phenomena displayed by the mineraloid gemstone opalopalescent. 2019. In Noah Webster's 1828 American Dictionary of the English Language. Retrieved January 7, 2019, from https://1828.mshaffer.com/d/word/opale ...
appearance of these microscopic organisms when illuminated with full sunlight.
Most opalines live as
endocommensals in the large intestine and cloaca of
anurans (
frogs and
toads), though they are sometimes found in
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
,
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 rhynchocephalians ( ...
s,
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 esti ...
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 pairs ...
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 o ...
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 exc ...
s.
Taxonomy and phylogeny
The relationship between opalines and other
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 exc ...
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 projecti ...
, 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
The phylum Sarcomastigophora belongs to the Protista or protoctista kingdom and it includes many unicellular or colonial, autotrophic, or heterotrophic organisms. It is characterized by flagellae, pseudopodia, or both.
Taxonomy
It is divided in ...
, with the
amoebae 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 their ...
s. In the 1980s, detailed
ultrastructural
Ultrastructure (or ultra-structure) is the architecture of cells and biomaterials that is visible at higher magnifications than found on a standard optical light microscope. This traditionally meant the resolution and magnification range of a co ...
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
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 gro ...
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 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, respectively ...
gametes. Each gamete has only one nucleus and may be either a