Females of Littorina obesa, L. scabra and L. littorea have been shown by other workers to grow more rapidly than the males so that they predominate in the larger size groups. They are significantly more numerous than the males among the larger specimens of L. neritoides on the Plymouth Breakwater.

More male than female snails are infected by a larval trematode, Metacercaria A, when it occurs alone, and the percentage infection falls when the proportion of males falls. A second larval trematode, Cercaria B, infects more female snails than males. Infections with this species alone remain consistently low, but where it occurs in the same snails as Metacercaria A the percentage infection rises rapidly, that is, in the larger size groups of the snails where the females predominate. Females infected by Cercaria B appear then to be attractive to the metacercariae.

Evidence is given to show that it is extremely unlikely that these large female snails were originally males in which sex reversal has occurred.

This different intensity of infection of the sexes of L. neritoides is considered to be primarily responsible for the shapes of the curves when infection is plotted against size, and it does not appear likely that primary infections with larval trematodes cause acceleration of the growth rate in this species.

It is thought that ecological factors on the breakwater are likely to be the chief causes of the large size attained there by L. neritoides.