Species complexes are a common occurrence in freshwater fish assemblages and may often pose a challenge to the understanding of their ecology. A typical case is the `cryptic' carp gudgeon group Hypseleotris spp. in southeastern Australia, which is thought to comprise four closely related species occurring sympatrically within the Murray-Darling Basin (MDB), where flow delivery scenarios have been proposed for the management and conservation of the native fish faunas. The objective of this study was to provide a deeper insight into the occurrence of piscivory and protracted spawning seasons in Hypseleotris spp. based on six years of sampling, and to cast the findings within the broader context of interrelationships between species complexes and their ecology. It is argued that what has been so far referred to as conspecific predation (cannibalism) in Hypseleotris spp. could be explained alternatively as `congeneric' predation, or even as a combination of both. Furthermore, the presence of larvae all year round should be accounted for when assessing the effectiveness of taxon-specific water allocation programs. Better matching of taxonomy with ecology is, therefore, essential for the successful management and conservation of species complexes, and further insights may be provided through the assessment of altricial/precocial forms.