There are many animal groups living in the lake, although not all have been studied. Let's first look at the most important microscopic-sized organisms.
Among the nematodes in the lake, two species, "Crocodolrylaimus thermalis" and "Neoactinolaimus tepidus," were found to be new to science. Among the rotifers, there are some faunistic rarities (Epiphanes brachionus var. Spinosus, Lecane inermis), which are rare organisms found only in thermal waters. The lower crustaceans (Cladocera, Ostracoda, Copepoda) were studied in great detail by Jenő Ponyi. A unique subspecies of Harpacticoidea (Copepoda), "Schizopera clandestina heviziensis," was described from the lake. Outside the lake, on the palisade, lives a unique Hydracarina mite. In and around the lake, 66 species and 8 variants of snails are known, which is a rich fauna compared to the 190 known species of snails in Hungary. Apart from the tiny creatures and snails, the higher animal life in the lake consists of only a few fish and amphibians. The higher animal life of the lake, except for fish, is hardly researched.
Among amphibians, frogs, the fire-bellied toad (Bombina bombina), and the laughing frog (Rana ridibunda) live along the plant-bordered edge of the outlet, while the green frog (Rana esculenta) and the spotted newt (Triturus vulgaris) live in the water. Among reptiles, the grass snake (Natrix natrix) and the dice snake (Natrix tesselata) live here in small numbers. Occasionally, the marsh turtle (Emys orbicularis) appears in the outlet and reed beds.
Various ducks appear in the outlet, especially in autumn and spring. When Lake Balaton freezes, mute swans (Cygnus olor), black-headed gulls (Larus ridibundus), and sometimes even coots (Fulica atra) flee here. Every year, swallows gather in the forests around the lake, and many non-migratory birds find shelter here for the winter.
Among mammals, the water shrew (Neomys fodiens) feeds on small fish, while the forest shrew (Sorex araneus) consumes insects. The mole (Talpa europaea) digs up the grassy areas.
The native species of the lake is the common carp (Carassius carassius), which likes the marshy reed beds, as well as the small-scaled tench (Tinca tinca), the common bream (Abramis brama), and the surface-feeding ide (Leuciscus idus). The now-extinct mirror carp (Cyprinus carpio) also once lived in the lake. Up to the dam of the lake, many other species, such as the loach (Misgurnus fossilis), also known from Lake Balaton, ascend, but it is not confirmed that it has been found in the Hévíz Lake. Additionally, a dwarf carp (Cyprinus carpio) was found in the lake, which according to researchers (Dr. Tamás Müller, Szent István University) is a unique population both in terms of its genetics and ecological properties (heat tolerance).
It has very small growth and never exceeds 25 cm in size, with most individuals being palm-sized. It has unique genetic characteristics and environmental tolerance. It has adapted to extreme temperature and chemical conditions. Like the domesticated carp breeds, it has fat reserves, despite almost exclusively feeding on mud. Its fat composition is different from that of other carp, containing more saturated fatty acids than its counterparts. They become sexually mature even at small sizes. In recent years, there have been many observations suggesting that due to the unique habitat, the Hévíz wild carp is capable of reproducing year-round.
Researchers from the Hungarian University of Agriculture and Life Sciences have been studying this carp variety for more than 15 years.
Three introduced fish species still live in the lake. The small live-bearing mosquito fish (Gambusia affinis), which feeds on mosquito larvae and small plankton, is very characteristic. These tiny Gambusias were introduced in the autumn of 1937 from southern North American waters with the specific aim of eliminating malaria mosquito larvae. In addition to Gambusia, the introduced goldfish (Carassius auratus) and the egg-eating sunfish (Lepomis gibbosus) live in the lake and the outlet.
