>The borings are predatory.  If a drilling predator can get at a
>shell without boring, it does not drill.  For example, moon snails
>attack razor clams through the gape between the shells and do not
>drill into them.
>
>There are two possible reasons for the predator to attack a thick
>part of the shell.  First, it may be the easiest spot to attack,
>either because of the life habits of the victim or the prey handling
>techniques of the predator.  Secondly, the thickest part of the
>shell is generally protecting the meatiest part of the animal, so
>the extra effort of drilling there may be rewarded with a better
>meal.
>
>At least some muricids and naticids prefer different parts of the
>shell to attack, given the same kind of bivalve as victim.

It's amazing just where drillers will have a go. The best borings I
have seen are in Oligopcene & Miocene fossils roundabout here. Big
Latiarca & large thick Limopsis (the former  up to 100mm long and 5mm
thick, the latter up to 50mm long, 3mm thick) drilled through the
ventral commissure (where valves meet)... successfully... that's
about 5-6mm the muricid had to bore. Patience pays off! (and the only
large muricids in most of these localities are Typhis, so I guess
these must be the culprits).

A muricid need not drill where the bivalve flesh is... the proboscis
(NOT the siphon, please note!) is evertable and can stretch a long
way once through the prey's armor. Naticids on the other hand
probably don't have long probosces, and so have to be more careful
where they excavate (I'd welcome comments on the reach of naticid
probosces).

Certainly many boring gastropods have preferences as to how to locate
the prey shell, and bore accordingly. Often a hole is incomplete
(failure), and sometimes there are more than one completed boring in
a shell (indicating that a second and STOOPID snail has bored a dead
shell!).

Turritellids are often targeted by naticids, and are generally
drilled into a midspire whorl (not the bodywhorl, as these snails
retract a LONG way up into the spire... a tactic for avoiding
crab-peeling predation)... often bored into the suture, puncturing 2
whorls at once (smart move... if one whorl isn't filled with tasty
flesh, the other probably IS).

Bivalves are commonly bored on the early part of the disc, though
borings can occur anywhere, including right through the commissure at
the hinge (have 2 Latiarca bored through the teeth, and therefore
also through the ligament).
--
Andrew Grebneff
165 Evans St, Dunedin, New Zealand
64 (3) 473-8863
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Fossil preparator
Seashell, Macintosh & VW/Toyota van nut