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This study presents a detailed comparative analysis of external morphology of four of the most invasive goby species in Europe (round goby Neogobius melanostomus, bighead goby Ponticola kessleri, monkey goby Neogobius fluviatilis and racer goby Ponticola gymnotrachelus) and interprets some ecological requirements of these species based on their morphological attributes. The results are evaluated within an ontogenetic context, and the morphological differences between the species are discussed in terms of the question: can special external shape adaptations help to assess the invasive potential of each species? The morphometric analyses demonstrate important differences between the four invasive gobies. Neogobius melanostomus appears to have the least specialized external morphology that may favour its invasive success: little specialization to habitat or diet means reduced restraints on overall ecological requirements. The other three species were found to possess some morphological specializations (P. kessleri to large prey, N. fluviatilis to sandy habitats and P. gymnotrachelus to macrophytes), but none of these gobies have managed to colonize such large areas or to reach such overall abundances as N. melanostomus.

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Key-words: biological invasions, diet composition, feeding strategy, diet competition, DanubeBighead goby (Neogobius kessleri) and round goby (Neogobius melanostomus) have been invading new non-native areas about two decades successfully. In this study, diet spectrum, seasonal variation, feeding strategy and diet overlap between these two invasive species were assessed. Materials were collected from the Danube at Bratislava by fishing rods and/or electrofishing. The diet spectrum of both species was diverse: a total of 46 food types in bighead goby and 51food types in round goby were observed. Dikerogammarus sp., chironomid larvae and Corophium sp. were the most predominant food types in bighead goby, whereas in round goby, chironomid larvae, Corophium sp., bryozoans and Cladocera predominated. The diet varied over seasons. In the Slovak part of the Danube, bighead goby and round goby have adapted to local food resources, consuming diverse food from small to large items, both with soft and/or hard body. This enhances the capability of these invasive species to spread successfully. It appears that even if both exploit similar food resources, their proportional content differs. Further differences between these gobies were also found in their food behaviour and feeding strategy. Both species tend to be specialists where possible, but round goby demonstrates higher flexibility towards general feeding strategy. RÉSUMÉLes gobies invasifs de Kessler Neogobius kessleri et le gobie à tâches noires N. melanostomus (Teleostei, Gobiidae) sont-ils en compétition pour la nourriture ? le gobie de Kessler et le gobie à tâches noires se sont adaptés à des ressources alimentaires locales, consommant une nourriture variée allant de petites à de grandes proies, à corps mou et/ou dur. Cela augmente la capacité de ces espèces envahissantes à se propager avec succès. Il semble que même si les deux espèces exploitent des ressources alimentaires similaires, leurs proportions diffèrent. D'autres différences entre ces gobies ont également été trouvées dans leur comportement alimentaire et leur stratégie d'alimentation. Les deux espèces ont tendance à être des spécialistes lorsque cela est possible, mais le gobie à tâches noires démontre une plus grande flexibilité allant vers une stratégie alimentaire plus généraliste. Mots