Social control of sex change in the shelf limpet, Crepidula norrisiarum: Size-specific responses to local group composition

Crepidula norrisiarum (Williamson) is found on the shells of the gastropod Norrisia norrisia (MacGinitie and MacGinitie) in groups consisting of 1-13 individuals of varying sizes. A combination of field collections and laboratory experiments revealed that sex change in this protandrous hermaphrodite is strongly affected by the size of the individual and the composition of the local group. Field data indicate that solitary shelf limpets change from male to female when they are about 16 mm in length, and we used this as a guide to investigate the social circumstances under which individuals accelerate or delay sex change. Females at sizes smaller than 16 mm (indicative of accelerated sex change) were present when they were the largest member of their local group; conversely, males larger than 16 mm (indicative of delayed sex change) were present when only larger females are present in the group. When relatively larger and smaller group members are present, sex expression appeared to depend on the individual's place in the size hierarchy. These results suggested hypotheses about social control of sex change that were tested in a series of laboratory experiments using artificially assembled groups. In accord with predictions, males changed sex when they became the largest member of a group (13 of 14 males tested), regardless of their initial size. Individuals changed sex faster in the presence of smaller males than when alone, and smaller males changed sex faster than did larger males. In contrast, no male changed sex in groups where larger females were present (n=15). Sex changing individuals had significantly higher growth rates, and comparisons among the groups suggest that the growth differences are conditional responses and not simply due to food limitation. This work suggests that in gastropods the presence of relatively permanent small local mating populations is the critical factor associated with social control of sex change, rather than mode of larval development and dispersal.