Trait-based mechanisms of invasion: Divergence in morpho-functional traits between the native and invasive Carassius species

Functional trait divergence underpins many biological invasions, yet its role in native- invasive fish interactions remains poorly understood. Invasive species significantly threaten the native biodiversity by disrupting the functional structure of aquatic communities. The widespread expansion of the invasive gibel carp (Carassius gibelio) throughout the European freshwater systems has potentially led to notable decline of the native crucian carp (Carassius carassius), yet the specific functional trait mechanisms driving this shift remains unclear. To address this unexplored aspect, we aimed to study the detailed morphometric measurements with a standardized functional trait approach to in order to understand whether divergence in the key morpho- functional ecological traits gives the invasive gibel carp a competitive edge. Samples for both native crucian carp and invasive gibel carp were collected across five sympatric pond systems in Czech Republic, wherein we quantified fourteen morpho-functional traits related to feeding and locomotion in both the species. Our analysis revealed that gibel carp consistently exhibited higher gut length ratios and larger oral gape surfaces than crucian carp, reflecting broader digestive capabilities and an increased ability to exploit diverse food resources, particularly plant and detrital material. Gibel carp also displayed significantly hightened values for the fin morphological traits, including pectoral fin aspect ratio and fin surface to body size ratio, suggesting enhanced maneuverability and habitat access. On the other hand, certain traits such as eye position and body shape showed substantial overlap between the two Carassius species, indicating convergence in certain ecological functions. The observed divergence in feeding and locomotory traits under sympatric conditions indicates that the invasion success of gibel carp is closely associated with increased trophic flexibility and resource acquisition. These functional trait advantages likely underpin rapid population growth and heightened competitive pressure on the native crucian carp population, which continues to decline in the invaded habitats. Our findings therefore, underscore the importance of trait-based analysis for elucidating the ecological processes underlying invasion and highlight the urgent need for targeted conservation actions to protect the critically endangered native crucian carp population.