Economic importance of Opalinida
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Opalinids
Opalinids are a small group of peculiar cosmopolitan organisms that belong to the kingdom Protista. Recent classification places the opalinids as heterotrophic stramenopiles (heterokonts) within the phylum Placidozoa, class Opalinea, and order Slopalinida. There are over 200 species, and, although opalinids are typically endocommensals (that is, living within the host without affecting it) in the large intestine and cloaca of anurans (frogs and toads), they have also been reported from fish, salamanders, reptiles, and some invertebrates (mollusks and insects). They are of no medical or economic importance, but they are interesting because their reproductive cycles are apparently controlled by the host’s hormones. In addition, opalinids are routinely encountered in dissections of frogs in college biology laboratories, and most students are surprised to know that they even exist. Several surveys have reported opalinids from Arkansas anurans, including from a toad in Lonoke County and chorus frogs in Union County. The state supports twenty-nine species and subspecies of anurans, so it is likely that a number of species of opalinids occur in frogs and toads in the state.
Initially, biologists thought that the hair-like structures that covered opalinids’ surface were cilia, and they placed them within the phylum Ciliophora (ciliates). However, in the early twentieth century, other aspects of their biology clearly helped differentiate them from the ciliates, and they were placed in the phylum Sarcomastigophora, with the amoebae and flagellates. In the 1980s, detailed ultrastructural studies of Opalina ranarum using electron microscopy revealed that they share morphologies with the heterokonts of the family Proteromonadidae. Then, in 1985, a new order, the Slopalinida was proposed to include the members of the families Proteromonadidae and Opalinidae. In 2004, the first reliable opaline molecular sequence data supported the monophyletic origin of the order Slopalinida. Therefore, researchers now consider the opalines to be a family (Opalinidae) within the order Slopalinida.
The cell surface of opalinids possesses numerous rows of flagella (kineties) in oblique and delicate parallel rows that allow interference of reflected light and opalescence; superficially, they look like ciliates. For traditional (typical) opalinids, the anterior end of the organism is defined as the direction of travel. The anterior margin is usually hyaline, against which the anterior ends of the kineties abut, which is termed the falx. They have a left-handed spiral path of movement, no mouth, no contractile vacuole, but large nuclei in binucleate forms and small in multinucleate forms; there is no macro- or micronucleus.
Lacking a mouth, opalinids get nutrition via pinocytosis (ingestion of liquid into a cell by using small vesicles from the membrane) and reproduce by means of syngamy (sexual reproduction) with union or fusion of two gametes, which differ in size and/or form (anisogamy). While some experts consider the opalines to be parasites, evidence suggests they are actually endocommensals that do not injure their hosts. Since the host absorbs nutrients from what it ingests in the small intestine, the opalines are likely not robbing them of essential nutrients. It is therefore assumed that the opalines are simply living off the leftover nutrients in the feces, possibly supplemented by the biochemical contributions of the lush bacterial flora that also reside there. Anurans often contain many hundreds and sometimes thousands of opalines in their lower bowel and feces, and they appear to be completely healthy, with no evident pathologies in their intestinal or cloacal epithelia.
Very little is known about the life cycles of opalines in fish, reptilian, or arthropod hosts. However, the life cycle of a standard opalinid (like Opalina ranarum) includes an asexual stage of multinucleate trophozoites (trophonts) in the cloacal area of adult frogs. They divide by binary fission most of year and yield more trophonts, and from late winter to early spring, production of cysts and the sexual reproductive cycle occurs. About two weeks prior to frogs arriving at breeding sites, hastening of binary fission occurs. Numerous smaller forms (tomonts), about thirty to ninety micrometers long and with fewer nuclei, are generated (referred to as palintony). These encyst, and cysts thirty to seventy micrometers in diameter and
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