Morphological Plasticity of Invasive Silver Carp in Divergent Midwestern Rivers

In the past decade, silver carp (SC; Hypopthalmicthys molitrix) have incited a great quantity of scientific research because their establishment and success in the Midwestern U.S. has led to concerns that they could invade the Great Lakes Basin. These previous studies have identified phenotypic plasticity in SC behavior and spawning as they invade novel environments. Although divergent habitats have been shown to elicit morphological plasticity in multiple fish species, similar research has not been conducted for SC despite their observed plasticity in other traits. I examined SC collected from two hydrogeomorphically divergent rivers, the Illinois River (IR) and middle Wabash River (MWR), as examples of rivers that support SC populations at different invasion stages. I compared differences in SC body shapes through geometric morphometric analysis and life history characteristics through population demographics between rivers. MWRSC were in better condition, lived longer, attained greater total lengths, and had higher GSIs than IRSC. Morphometric analysis revealed a significant divergence in body morphology between MWRSC and IRSC, whereby MWRSC had deeper body sections and narrow, tapered heads, while IRSC had shallower bodies and deeper heads. Principle component analysis indicated that contrasts in morphology were most strongly associated with river, as opposed to other factors like sex, hybrid status, and time sampled. While I do not assert causal evidence for the difference in shape based on specific environmental characteristics of the two rivers, I speculate that IRSC and MWRSC are undergoing different selection pressures due to population demographics and environmental characteristics of the two rivers. Regardless of the specific causal factors, it is clear that body shapes were different between the two rivers, suggesting that location-based selective agents are driving phenotypic outcomes in invasive SC