The Lottia alveus extinction is quite interesting for a variety of
reasons - one of which is the way its former niche has been taken up by
a similar species at some localities.   In at least two large stands of
Zostera, i have found Lottia testudinalis specialized to inhabit the
eel-grass blades: they are considerably more elliptical - long and
narrow -  than the normal form, and seem to be filling essentially the
same niche that alveus Conrad occupied before the catastrophe that wiped
it out.  I have no idea if these are a genetically distinct population,
or whether some juveniles grow up on the blades, and their shells adapt
to the situation where they live - or whether only the individuals in a
local population whose shells are the required shape (and weight - they
are ligher than their cousins on the ocean floor beside the eel-grass
blades) to remain on the eel-grass, actually manage to do so.  Does
anyone know of a study of this matter, or of similar instances where a
species' niche has begun to be taken over after an extinction or local expatriation?

From the Early Spring pleasantness of the Great Still-White North.