In addition to gastrochaenids and lithophagids, there are worms and pholadoidean bivalves that may be found boring into shells for shelter.  Unlike predatory holes, these will avoid breaking through to the far side; if they do, many are capable of secreting minerals to cover up the hole.  They also will generally not be aimed straight into the shell, unless it is very thick.  The pholadoideans are close relatives of the shipworms.

Several other kinds of animals can make borings into shells, though not usually tunnel-like.  For example, sometimes you see little slits into a shell, lens-shaped (like a pair of parentheses with no space between them).  These are made by acrothoracian barnacles.  Dead shells or epifaunal species often have a network of tunnels in the shell due to the work of clionid sponges.

All of these are just taking advantage of the shell as a place to find shelter.  They are not predatory, but a heavy infestation may be detrimental to the host shell by making it easy to break.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa