We looked at 649 middens from 39 O. vulgaris dens in Bonaire. 60% of the gastropod shells were drilled. The holes were tiny and beveled, easy to miss unless you looked very closely. My guess is that drilling takes a lot of time and that other methods will be used first, if possible. For example, a lot of the bivalves were simply pulled apart - something a moon snail would not easily be able to do. Octopuses generally drill just a single hole in a strategic location near the adductor muscle. It has been a while but I think there were a few rare cases when more than one hole had been attempted - Dr. Roland Anderson would know for sure.
Aquarists captive rearing octopuses have a unique opportunity to try some novel experiments involving drilling in octopuses. I'd be happy to help advise. For example, do octopuses that have never encountered gastropods know where to drill or is this learned? Anyone with an octopus can look at: What is the food handeling time needed to drill through diff thicknesses of shells? An uw hydrophone would be a useful tool and should pick up the drilling sound. Am I right that drilling is not a preferred method to get a meal? Or do other factors come into play - such as the fact that drilling can be done back in the security of the octopuses lair. . .
James
By the way, the really interesting discovery in the work below was that while O. vulgaris as a group eat a diversity of prey, but individual octopuses often specialize.
Reference:
Octopus vulgaris in the Caribbean is a specializing generalist
Anderson, Wood, and Mather
ABSTRACT: The diet of Octopus vulgaris was determined from the remains of 649 prey items gathered from the middens of 38 dens in a small area off the Caribbean island of Bonaire. Remains of 35 species of gastropod (19% of the total), 19 bivalves (51%) and 21 crustaceans (30%) were identified and examined for mode of entry into hard-shelled prey. Although 60% of the gastropods were drilled, neither the size/weight ratio nor the presence of an operculum determined whether drilling occurred. There were strong differences in prey preference among individual octopuses, and the Cardona niche breadth index (B’) of the midden items was 0.08, indicative of specialization. Examples include the exclusive preference for Pinna carnea by one den occupant. This study, by focusing on assessment of preference at specific den locations, is the first to show that while the population had a wide choice of prey items, the individual choices were much narrower, indicating that octopuses were specializing generalists.