Larger context for LACK in Corpus USbrown_UKbncw/UK_bncw.txt
The shell-shape patterns reviewed here (and more fully by Crothers, 1983b) coupled with the chromosome variation, can be "explained" by envisaging the N. lapillus populations of the British Isles as comprising two main groups (Fig. 33).
A southwestern faction, bearing the genes for short squat " exposed shore" shells and n = 18 chromosome number, confronting and gradually displacing a northeastern faction which lacks those particular genes and is monomorphic n = 13.
The pattern of "teeth" variation, originally interpreted as purely phenotypic variation dependent upon water temperature, is not far removed (Fig. 20).
Variation in colour, however, does not correlate with the other patterns.
There has been no chromosome work in Shetland(nor investigations of the "teeth"), but the shell-shape data strongly suggest a meeting of two factions - in some places the enclaves show the full "normal" range of variation: in others, only a few miles away, they do not.
It appears that there are two discrete breeding groups on Gore Point (Porlock Weir, Somerset) separate enclaves on the lower and middle shores, with mean lengths of 47.6 mm and 24.3 mm; mean shell-shape ratios of 1.78 and 1.45 respectively.
They feed on different food.
The larger ones lay larger capsules from which larger embryos emerge and grow faster.