The influence of simulated exploitation on Patella vulgata populations: protandric sex change is size‐dependent

Abstract

Grazing mollusks are used as a food resource worldwide, and limpets are harvested commercially for both local consumption and export in several countries. This study 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. Limpet populations at two locations in southwest 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. The shell size at sex change (L 50: 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 L 50 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. 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 as 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. We conclude that sex change in P. vulgata can be induced at smaller sizes by reductions in density of the largest individuals reducing interage 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.

Exploitation promotes earlier sex change in a protandrous patellid limpet, Patella aspera Röding, 1798

“…These results are also very similar to those found by Borges et al. () for P. vulgata in the British Isles.…”

“…As exploitation is mostly targeting the larger individuals (see Figure b), results suggest that males are compensating the removal mainly of the larger females, by undergoing sex change at smaller and presumably earlier sizes. That the size of the smallest male has remained relatively stable throughout the archipelago also suggests that the switch of neuters into males has not been affected (in contrast to Borges et al., in P. vulgata ). These results suggest that the numbers of males in the population must have been reduced in the most exploited islands.…”

Growth and reproduction of the north-eastern Atlantic keystone species Patella aspera (Mollusca: Patellogastropoda)

“…Imbalances in the sex ratio of this species might be related to the possibility of P. aspera being a protandric hermaphrodite. Also, size selective harvest of this species might induce phenotypic pasticity in protandric hermaphrodite limpets as recently reported for Patella vulgata by Borges et al [56]. In heavily exploited populations of protandric hermaphrodite limpets, sex-change has been reported to occur at smaller sizes as a consequence of the size selective nature of limpet harvest, skewing sex-ratios and altering the reproductive output of those populations.…”

Predicting the fate of the most endangered marine invertebrate of the Mediterranean: The power of long‐term monitoring in conservation biology

“…In such source populations, a density‐dependent effect could occur while carrying capacity is being achieved. In fact, Boaventura et al () and Borges, Hawkins, Crowe, and Doncaster () have demonstrated several effects of density on biological traits of limpets such as sex ratio, sex change, weight, and shell growth or survival. Nevertheless, the increasing density of the population in Parque del Mediterraneo is indicative of weak inter‐age class interactions, providing evidence that the carrying capacity has not yet been reached.…”