DINNER ON THE DRILLED SHELL: OCTOPODE BORE HOLE PATTERNS IN THE ANCIENT AND RECENT

Feeding behavior including drill hole boring by octopodes has been previously documented (e.g. Arnold and Arnold, 1969), but documentation in the fossil record is scarce (Klompmaker et al., 2013, 2014). Octopods feeding on gastropods often drill a hole in the shell, predominantly in the spire, weaken the prey with venom, then removing the prey from its shell, and then feeding. The drill holes vary in size and shape and often resemble those made by muricids. Drilled gastropod shells from midden in the Bahamas and fossil collections from universities were examined so drill holes could be attributed, with certainty, to octopodes. The modern shells, with the exceptions of cowries, were principally drilled in sutures between whorls of the spire. The placement of the drill holes appears to be most closely linked with the position of the columnar muscle. The same pattern held for the fossil material. The columnar muscle holds the body of the gastropod within its shell, and if damaged, would enable a predator to remove the entire prey item with ease. In cowries (Family Cypraeidae, Subfamily Cypraeinae), the spire is generally concealed in mature individuals by the most recent shell growth. There is indeed a preferred region of drilling. Cowrie shells were consistently drilled through the inner lip below the spire where the columnar muscle would be attached. It appears that the placement of the drill hole is a key characteristic with potential for identifying octopodes drilling in the fossil record. However, not all gastropods appear to be drilled by octopodes. Some species recovered from the recent midden (e.g. Toona) were not drilled. In the case of cowries, octopodes strongly predate Lucria and only a single specimen out of 51 Erosaria shells was drilled. It is therefore significant to document that octopi do not drill all gastropod species and rates of predations could be underestimated from recent death assemblages.