Coexistence of two morphologically indistinct sturgeon species

Background/Question/Methods

Similar species can coexist via differences in diet or habitat use. These differences can be subtle or dramatic. The coexistence of two morphologically indistinct sturgeon species in the Missouri and Mississippi Rivers of the central United States presents a unique context in which to study processes that lead to species coexistence. As juveniles, and to a lesser extent as adults, distinguishing the endangered pallid sturgeon (Scaphirhynchus albus) from its congener, the shovelnose sturgeon (S. platorynchus) is not feasible without the use of genetic techniques. It is likely the two species forage on the same resources as juveniles and to date, no studies have found significant differences in habitat use. In this study, we examined the internal anatomy and physiology to provide insights into the coexistence of pallid and shovelnose sturgeon. We measured 25 structures (corrected for fish size) that are directly related to food acquisition and processing in the two species. We also conducted intermittent respirometry to detect differences in metabolic rates between the two species.

Results/Conclusions

All external measurements were similar between the two species with the exception of the inner barbel lengths. We found differences in the overall alimentary canal length between the two species (P = 0.007). Total stomach length was greater in pallid sturgeon (86 mm) than in shovelnose sturgeon (68 mm). The distal stomach and gizzard were consistently larger in pallid sturgeon than in shovelnose sturgeon (P < 0.01). The pyloric cecum was also larger in pallid sturgeon (27 mm) compared to shovelnose sturgeon (21 mm), but the overall length of the intestine and spiral valves were similar between the two species. Basal metabolic rates of the two species were similar. Results from this study suggest the two species are similar overall based on their internal morphology as juveniles, with the exception of the stomach. In the wild the shovelnose sturgeon is far more prevalent than the endangered pallid sturgeon. A recent estimate suggests the ratio of pallid to shovelnose sturgeon in the Mississippi River is 1:581. One possibility for this disparity is that shovelnose sturgeon may be outcompeting pallid sturgeon. That is, there is not enough differentiation between the two species internally, externally, or spatially for the two species to coexist. If anthropogenic disturbances are responsible for the low abundance of pallid sturgeon, it remains unclear why the shovelnose sturgeon has not been affected in a similar fashion.