Cytogenetics as a Tool Box in Fish Conservation: The Present Situation in Europe 2.1 CHAPTER Petr Ráb 1* , Jörg Bohlen 1 , Marie Rábová 1 , Martin Flajšhans 2,3 and Lukáš Kalous 1, 4 INTRODUCTION Fishes are by far the most species-rich group of vertebrates. More than 25,000 of the 35,000 vertebrate species comprise fishes. The exploration of this enormous biodiversity is still a challenge for ichthyologists. This is Address for Correspondence: * Petr RÆb, Laboratory of Fish Genetics, Section of Evolutionary Biology and Genetics, Institute of Animal Physiology and Genetics (IAPG) AS CR, 277 21 Libchov, Czech Republic. E-mail: rab@iapg.cas.cz 1 Laboratory of Fish Genetics, Section of Evolutionary Biology and Genetics, Institute of Animal Physiology and Genetics (IAPG), AS CR, 27721 Lib chov, Czech Republic. 2 Department of Genetics and Breeding, Research Institute of Fishery and Hydrobiology (RIFH), University of South Bohemia, Vod`any, Czech Republic. 3 Joint Laboratory of Genetics, Physiology and Reproduction of Fishes of IAPG AS CR Libchov and RIFH USB Vod`any, Czech Republic. 4 Department of Zoology and Fisheries, Faculty of Agrobiology, Food and Natural Resources, Czech University of Agriculture, Praha 6  Suchdol, Czech Republic.