Protoopalina sp.

In fish, Protoopalina sp. are most commonly found in the intestines of discus. To our knowledge, in literature only one species is described to be found in discus intestines: Protoopalina symphysodonis. Protoopalina sp. can cause a colossal outbreak in discus of all ages, leading to deterioration of health and even, although rarely, to death. However, in our practice with fish parasites, we visually noted at least four types of protopalines in discus, which differed significantly in appearance, pointing at the possibility that more than one species can be found in discus.

Common (visible) symptoms of a Protoopalina sp.:

  • Darkening (blackening) of the color or paleness;
  • Weight loss and retarded growth of juvenile fish;
  • Stressful behaviour, fish sticking together in a group and staying in a corner of the tank;
  • Stringy, white or translucent, long feces as a general sign of intestinal parasite infestation.

Unfortunately, these symptoms are common to most intestinal infections of fish caused by both protozoa or by helminths. An exact diagnosis can be made by microscopic examination of fish feces.

So far, no specific treatment is known to eradicate Protoopalina completely, however, eSHa HEXAMITA will often do the job or at least reduce the number of parasites, prevents mass mortality and enables fish and their immune system to recover and cope with this parasite. Protoopalina sp. are rather large unicellular microorganisms, clearly visible at a low (40×) magnification under a microscope. They have an elongated, zeppelin-like cell shape and a round cross-section. The front end of the cell is bluntly rounded, and the back end of the cell is tapered and sharply pointed. Their bodies are covered in flagella, which have a striking similarity with cilia. There is also one big flagellum at the tail.


Opalinidae is a small, rather poorly studied group of unicellular flagellated microorganisms, of which many species are found inhabiting the intestinal tract of amphibians and for the most part are endocommensal protozoa. The group Opalinidae consists of only five genera, one of which is Protoopalina. Some representatives of the genus Protoopalina parasitize in the digestive tract of freshwater and marine fish species. The first Opalinidae were discovered by van Leeuwenhoek back in 1683 and for a long time were classified as ciliated microorganisms. Indeed, if you look under a microscope at a Protoopalina cell, you can see that the entire cell is covered with a uniform layer of what seem to be cilia, just like the cell of Ichthyophthirius. The structure of these “cilia” is different and they are actually flagella. This was discovered by modern ultrastructural studies and phylogenetic analysis which showed a clear distinction of Opalinidae from other ciliated microorganisms and showed they belong to Heterokonts, flagellated microorganisms. Protopalinids are able to multiply not only by binary fission, but also by sexual reproduction, forming gametes carrying a flagellum.

Fun fact: the group got its name “Opalinidae” because of the organisms opalescent glow (like precious opal), which emerges when light hits the multitude of flagella they are covered in.


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