Well, Tom,
Identifying sponges by email is always perilous, but since sponges are
common and sponge experts are rare, I'll chance it. Various species of
Cliona often encrust shells and also bore into them. The borings start out
as a radiating system of small, round cavities connected to one another
(and to the surface of the shell) by short, narrow galleries. Later, the
system develops into a network that may be more or less dense depending on
the species of sponge. Cliona may grow on living oysters, but attacks only
the dead shell and not the inner surface, at least until the mollusk has
died. It is therefore common to see shells that have a clean inner surface
and a riddled outer surface.

The name Cliona is similar to Clione and Ciona, which refer to other marine
animals. The names are sometimes confused in the literature.

Andrew K. Rindsberg
Geological Survey of Alabama
"Diagnosis at a Distance"