Now almost a decade after the discovery of leptin in teleostean fishes, all components of the mammalian leptin system have been identified in fish: multiple leptin genes, a single leptin receptor (with different splice variants and accompanying different signalling capacities) and a leptin receptor-overlapping transcript (LEPROT) (Kurokawa et al., 2008). Also, mutant models are generated and used to study the effects of leptin or its receptor in fish. With this toolkit, comparative leptin research can elucidate