UTILIZATION OF GEOHISTORICAL DATA TO IDENTIFY AN INVASIVE PREDATORY GASTROPOD IN THE COLORADO RIVER DELTA, MEXICO

Throughout the Colorado River basin numerous dams have captured and diverted the entirety of the basin’s fifteen million acre-feet (~1.85x10¹⁰ m³) of water. This complete allocation of the Colorado’s water has led to the devastation of the downstream delta ecosystem. In the absence of pre-dam studies, geohistorical data can play a critical role in elucidating ecological aspects of the past ecosystem, such as species composition and species interactions, that would be otherwise impossible to incorporate into assessments of restoration efforts. Here, we present a case study from the Colorado River delta (CRD) demonstrating the application of geohistorical data in identifying an invasive predatory gastropod.

Due to disagreement in taxon identification in the CRD literature, it is unclear if the two naticid gastropods currently living in the CRD, Polinices recluzianus and Natica chemnitzii, are native or invasive. We expected to observe body fossils of only one species in the pre-dam cheniers (shell-rich beach accumulations) and potentially a second species in tidal flat samples if an invasion had occurred. Given the variation in species-specific behavior in prey handling by naticids, the addition of a second naticid should also have altered the stereotypy of drill holes on bivalve prey and reduced the correlation between predator and prey size. At two localities we target sampled (n=55), bulk sampled (n=30), and collected drilled bivalve prey from cheniers – three of which have previously been dated to pre-dam age – and the tidal flat.

No Natica were found in cheniers of pre-dam age while Polinices were common. Natica and Polinices were found in the actively forming chenier (<70 y.o.) and on the nearby tidal flat at one of our localities. As expected in the case of an invasive naticid species, drill hole stereotypy was significantly different in tidal flat samples compared to those from the pre-dam. The predominant location of drill holes shifted from near the umbo to a split between the umbo and ventral region of prey shells. Similarly, the correlation between predator and prey size was significantly lower in tidal flat samples (r = 0.77 vs r = 0.37). Two lines of evidence, body fossils and trace fossils, suggest that Natica has invaded the CRD in the years since the damming of the Colorado River while Polinices is native to the delta.