This time we are going to speak about one of the most dangerous fish parasites for discus fish (Symphysodon discus). It can be detrimental for young discus fish and can be a serious problem for adult fish. Why only discus fish, you ask? The monogenetic flukes, belonging to the family of Dactylogyridae and genus Sciadicleithrum called Sciadicleithrum variabilum only attack discus fish and parasitize on the gills. The behaviour of these gill flukes is described as being host-specific, which implies that species of this genus parasitizes only one species of fish: discus. Conversely, scientists have so far found no other species of gill flukes that parasitizes discus fish, although the total number of parasite species is several thousand. Such a high species specificity with respect to parasite and host is characteristic of many Monogenea of the Neotropics region (Middle and South America).
Sciadicleithrum variabilum are small, translucent worms that can only be seen when using a microscope. They are hermaphrodites and belong to the group of oviparous worms. The life cycle of this species is direct, so there is no need for intermediate hosts. There are no exact data on the prepatent period nor of the viability of the eggs, but it is possible to refer to the general data known for the family of parasites to which this species belongs. In closely related species, at a water temperature of 27 °C, the complete cycle from egg to mature individuals can take 11-13 days, and the maximum recorded period of viability of the eggs is 28 days. These matters should be considered when treating aquariums against flukes.
Like many other monogenic species, Sciadicleithrum variabilum attaches itself to the gills of fish by an attachment disk which for this species consist of 2 pairs of median (large) hooks and 5 pairs of small hooks. When holding on to the gills, S. variabilum perforates the (single-layer) cell epithelium of the respiratory lamella of the gills, causing microhemorrhages and disruption of gas exchange in the damaged area. In severe cases, when the number of worms on the gills of one fish can reach several tens or even hundreds, this causes asphyxia and death.
If you see the following symptoms in your fish, you need to make an accurate diagnosis, preferably by a microscopic examination, and if necessary take measures to control gill flukes’ infection in your fish:
- Fish stay near the surface of the water, hovering and gasping for air;
- Fish breathe heavily and fast;
- Gill covers are protruding and the fish are spreading the gills wide open;
- Only one side of the gill covers moves / one sided breathing;
- The fish are under stress; they scurry about the aquarium when a somebody approaches.
Is there a cure? Yes, luckily there is: eSHa gdex!