Variation within and between populations of Littorina nigrolineata Gray on Holy Island, Anglesey

Populations of the rough winkle, Littorina nigrolineata, from a range of sites mostly on Holy Island, Anglesey, have been examined with respect to shell colour, shell shape, shell aperture width, penial gland number and shell size and thickness. In addition, reproductive investment has been investigated in four of these populations.
      There is a remarkably disjunct distribution of the lined and unlined forms on Holy Island and on nearby Anglesey itself. At the one site at which both forms occur, the distinction between them is much less marked. There is a strong association between the broadness and squatness of shells and the exposure of the site at which they are found, and also between aperture width and exposure. Shell size and thickness both tend to be greatest at those sites with the greatest risk of shell injury by crushing. There is a surprising amount of variation in the number of penial glands. At a single site, Abraham’s Bosom, two distinct populations (one occurring in crevices, the other amongst boulders) differ in their shell shape, shell size, shell thickness, aperture width, their size at maturity and their relative reproductive investment.
      In all cases, these results are discussed in the context of the environmental selective forces acting on the winkles.