First, the correction: as our alert friend Emilio pointed out, all
Muricidae are not carnivorous, and N. lappilus is on the "not" side.  I
always wondered why they have habitat preferences more suitable to a
vegetarian!
 
Second,  i was wondering  if anyone would remember  poor old "Lottia"
alveus (Conrad), a critter who made the mistake of becoming  too picky
in neighbourhood preferrances, and apparently paid the ultimate price
when the only place it liked to live (Zostera, ie.) suffered a massive
die-back nearly simultaneousy throughout its range. (thank you, Dr.
Lee!)   The Zostera  came back, but alveus probably didn't:  she's never
been found again alive.  One might say she lost big,  in the Lottiary of
existance!
 
Third, i have checked extremely carefully, and our blade-dwelling
friend is definitely T. testudinarius, which also lives on the bottom at
the same locality (Crescent Beach, Lunenburg  Co., N.S.).  However, the
specimens living on the Zostera blades are signifigantly narrower than
their relatives on the seafloor below.  I have heard other anecdotal
reports of the same thing  occuring at other  locations where a
long-lived stand of Zostera remained, but do not know of any literature
references.  What would be fascinating  to do, would be to try to breed
the elongate form in a place where there is no Zostera, and see if the
progeny maintain a statistical propensity towards ellipticality: if so,
then we could posit the emergance of a genetically-distinct variety of
testudinaris, specialized for living  on the green-and-narrow, so to
speak.  If not, we could at least say that it is an adaptable species
which is taking advantage of a niche left vacant by the departure of
miss alveus, which might in time develop a true variety, and perhaps
eventually, a new species, if it proved adventageous for the
Zostera-inhabitants to breed at a different time from the
bottom-dwellers, or if an introduced predator were to eat-up most of the
nominate form over a large area, but could not reach their higher-minded
cousins, leaving  them to form a small gene-pool which might become
rather different from the species in general, so that in time,
interbreeding  became impossible.
 
Of course the above, fascinating as it may be to some, is pure
speculation, and several other hypotheses could be ventured which
involve no evolutionary musings.
 
Finally,  all other  considerations aside,  this is an example of  a
complex community (8 spp of mollusc, plus algae and one or two other
attatched organisms, plus transients such as fish, and interactions with
the benthic beasties) which exists only because of structural complexity
in a local environment- similar to a forest or reef, where biodiversity
is  at least partially dependant upon the number and nature of layers
present.  Does anyone out there have time to study this ecosystem before
it gets buried under a parking  lot??