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As a result, hybridogenous individuals in the complex are difficult to distinguish from the bisexual parent species on external characters.

An unbalanced sex ratio in a population with more females than males is usually evidence that hybridogenous lineages are present (Kottelat and Freyhof, 2007).

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Hybrid lineages are known, produced by gynogenesis and are nearly all-female.

Males of bisexual lineages are sperm donors but the sperm only induces egg development and contributes no genetic material.

The all-female lineages are therefore sperm parasites and have to occur in sympatry with one of the parental species in a hybrid complex.

An early Oligocene route also existed between the Anatolian landmass and Central Asia (Tang et al., 2008) and cobitids may have invaded the Euro-Mediterranean zoogeographic subregion at least five times independently based on cytochrome b data.

Some members of this family can live in oxygen-poor waters.

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