SEXUAL SELECTION ON FEMALE SIZE IN A MARINE SNAIL, LITTORINA-LITTOREA (L)

The mating pattern of the common shore-dwelling gastropod Littorina littorea was studied in five boulder shore populations on the Swedish west coast, to examine the occurrence of size assortative mating and sexual selection on size. We looked for correlations between sizes of males in copulating male-female pairs in three of the populations to test the possibility of assortative mating. Furthermore, we compared the distribution of small and large individuals among copulating and non-copulating snails in all five populations to reveal components of sexual selection among males and females. Size assortative mating was found in one of three populations. In three of five populations, sexual selection on size among females were found, large females being favoured as mating partners over small ones. There was a trend towards this pattern in a fourth population. In no population did we find any indications of sexual selection on male size. Deviation from random mating and/or observation of sexual selection at a particular time may be effects of a non-random size distribution of snails, a pre-copulatory choice and/or different copulation durations among pairs of different composition. We performed laboratory experiments to test these possibilities. Although encounter rates may have been the same, large females were mounted at a higher rate and initiated in copulations more often than small females, and large females copulated for longer than did small females. These observations supported our field data in that large females were favoured as mates by most males. The shell length of females of Littorina littorea is positively correlated with their fecundity, and thus, males seem to prefer to mate with the more profitable large females.