Where a limpet starts its life is largely a matter of chance.  The
gametes (reproductive cells) are freely released into the water,
fertilization takes place there, and the veligers (early embryos) swim
around until they start to develop a shell. The weight of the shell
drags them to the bottom.  If they land on sand or mud, they die.  If
they happen to land on a rock, or a larger shell, or some other hard
surface, they attach and begin their new life. Sometimes tremendous
numbers of gametes are released in a small area, resulting in great
numbers of spat (young, newly-shelled individuals) settling down on the
available growing space all at once.  In such a case, a surface may be
virtually covered with tiny limpets.  But this situation can't last
long. Limpets grow. They feed largely on microscopic algae which grow on
the hard surface where they live.  A limpet does not remain in one
place. It roams about over a small area, feeding, and returns to the
same resting place before the tide goes out. There is just so much food
to go around, and soon the population of growing limpets must start
thinning out. Some individuals will die from lack of food (and perhaps
some from predation), leaving more room for algae to grow. That allows
the remaining individuals to browse and grow somewhat; but pretty soon
the same situation develops again.  The limpets, now larger, need more
food, and they are grazing it faster than it can regrow.  So, some of
them die, leaving more space per limpet.  By the time they are fully
mature and ready to release gametes, the population consists of
individuals which are much larger in size, but much smaller in number
(probably far less than 1%) of the original population, and necesssarily
separated by much greater distance (you can't grow food for a cow in the
same space as food for a hamster).

Take a look at the following picture, which shows young slipper limpets
(Crepidula fornicata) on the inner surface of a dead softshell clam (Mya
arenaria) shell.  These are not "true limpets" of course, such as you
are studying, but the principle is the same.  These animals are not
going to grow much until somebody vacates the premises.  In the end,
there may be only one or two slipper limpets calling this shell home.

http://members.home.net/paulcyp/CREPIDUL.JPG

Regards,
Paul M.