3)  The form of the species is largely determined by environmental
factors, as noted by a previous respondant: the sturdier shells, with
the thickest  and narrowest lips, occur where the crabs are thickest.
Interestingly enough, when the "green crab", a more southerly species,
invaded the Bay of Fundy in the late 1970s, the thickness of N. lapp.'s
lips, as well as the occurance of "teeth" in the aperature (which
effectively narrow the aperature, keeping hungry baby crabs out!)
increased substantially.
 
        The lammelose form "imbricata", is somewhat of a mystery to most
observers (ok- it is to me, at least!!).  It occurs in small frequencies
in most populations in the Minas Basin, the interior portion of the Bay
of Fundy, quite randomly.  Largely absent from the Atlantic coast, it
can be found in a few very protected  back-bays, such as at Cresent
Beach, where it forms the predatory componant (this time i'm certain!!)
of the Zostera communities i wrote about a while ago. (however, the
imbricate form is found on the bottom, as well as on the eel-grass) .
THEORIES WELCOME  HERE!!I hope this has not been "too much too late",
but this species would seem to demostrate many of the principles of
variability  rather nicely, so is hopefully worthy of a good discussion.
 
                                                                                                -Ross M.