EXPLORATORY SPATIAL ANALYSES AS AN ALTERNATIVE APPROACH FOR UNDERSTANDING THE MECHANISMS THAT DRIVE THE NATICID DRILL HOLE DISTRIBUTION

A number of different procedures for the identification of stereotypy in drilling predation are available. Typically these involve partitioning the prey’s shell into sectors, which, however, results in the loss of information. The spatial nature of the drilling process, by which the predator makes a single hole through the surface of the prey’s shell, allows us to combine morphometrics and spatial analysis to better understand the mechanisms that drive the naticid drill hole location.

In order to quantify the position of the traces on bivalve prey, we use a 2-D morphometric approach. A procrustes superimposition method was used to remove shell size, orientation and position information from the analyses. Because drillhole points can be also plotted within a spatial sampling region (i.e., prey outline), we considered the landmark configuration and that sampling region as a 2-D point pattern. From the point pattern it is possible to produce average density maps of drillholes per unit area.

To test the null hypothesis that drillholes are randomly distributed within the sampling region, we used a number of clustering and interaction tests. We found that intensity varies from place to place on the prey shell in each case. Drillholes tend to cluster in certain areas of a given prey. This is typically around the umbo or center on the shell of a bivalved prey, but can also be at a point on the commissure between the closed valves. The species studied include the extant Iliochione subrugosa, Protothaca asperrima and the fossils Lirophora latilirata, L. glytocyma, and Panchione mactropsis.

To understand if the clusters found are related to particular morphological traits of the prey, such as prey thickness and ornamentation, we combine the drilling intensity plots with the trait plots. This graphic approach makes it possible to recognize if drillhole distributions may be related to any of the morphological traits. These maps are also useful to compare and visualize large data sets. Finally, we used variograms to quantify the variation in drill hole spatial configuration across space and time. A preliminary study of naticid predation on I. subrugosa along the Pacific coast of Central America using our approach suggests that spatial patterns in distribution of drillholes do not vary predictably through space or across environments.