Evidence for intense biotic interactions in the eastern Gulf of Mexico after a two million year hiatus: inferences from muricid edge-drilling behaviour

Edge drilling by the muricid gastropods Chicoreus dilectus and Phyllonotus pomum has been observed in the laboratory under conditions simulating an enemy-rich environment, and evidence for it is found in Florida’s Pliocene fossil record. However, this behaviour has never been observed in the wild and was presumed to be ecologically extinct for the last two million years. This study documents for the first time expression of enemy-induced edge-drilling behaviour in the wild by C. dilectus and P. pomum. These observations are based on a multi-year survey between 2002 and 2015 in St Joseph Bay, Florida, a coastal lagoon in the northeastern Gulf of Mexico. Edge drilling was not recorded in 2002 in St Joseph Bay, but expression of edge-drilling behaviour was confirmed by direct observation for both species and two stations in the bay between 2007 and 2011. Edge-drilling predation traces produced by C. dilectus and P. pomum accounted for more than half of all their drilling attacks in bulk samples collected in 2007, with the frequency of occurrence in bulk samples declining at both stations over time. Intensive sampling along the west Florida shelf over the same time interval failed to find any other unambiguous evidence for expression of edge drilling by these species. St Joseph Bay has the highest densities of predators and lowest prey survival rates in the region and also highest secondary productivity in any seagrass habitat globally, confirming the use of edge drilling by C. dilectus and P. pomum as an indicator of Florida’s ‘enemy hotspots’ and their fates in modern times and in the fossil record. The re-emergence of enemy-induced edge drilling by C. dilectus and P. pomum in Florida after two million years, but only in St Joseph Bay, suggests the present-day ecology of the bay is unique on geological time scales.