This material is published by Ecology and Evolution (2016, 6: 514-531).
The influence of simulated exploitation on Patella vulgata populations: protandric sex change is size-dependent
C. Debora. G. Borges, Stephen J. Hawkins, Tasman P. Crowe and C. Patrick Doncaster
1. Grazing molluscs are used as a food resource worldwide and limpets are harvested commercially for both local consumption and export in several countries.
2. This paper describes a field experiment to assess the effects of simulated human exploitation of limpets Patella vulgata on their population ecology in terms of protandry (age-related sex change from male to female), growth, recruitment, migration and density regulation.
3. Limpet populations at two locations in south-west England were artificially exploited by systematic removal of the largest individuals for 18 months in plots assigned to three treatments at each site: no (control), low and high exploitation.
4. The shell size at sex change (L50: the size at which there is a 50:50 sex ratio) decreased in response to the exploitation treatments, as did the mean shell size of sexual stages. Size-dependent sex change was indicated by L50 occurring at smaller sizes in treatments than controls, suggesting an earlier switch to females. Mean shell size of P. vulgata neuters changed little under different levels of exploitation, while males and females both decreased markedly in size with exploitation.
5. No differences were detected in the relative abundances of sexual stages, indicating some compensation for the removal of the bigger individuals via recruitment and sex change since no migratory patterns were detected between treatments. At the end of the experiment 0-15 mm recruits were more abundant at one of the locations but no differences were detected between treatments.
6. We conclude that sex change in P. vulgata can be induced at smaller sizes by reductions in density of the largest individuals reducing inter-age class competition. Knowledge of sex-change adaptation in exploited limpet populations should underpin strategies to counteract population decline and improve rocky shore conservation and resource management.
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