Glycogen content and rates of depletion in two limpets with different foraging regime

The storage and utilization of glycogen reserves in the foot tissues of two Mediterranean limpets, Patella rustica L. and Patella caerulea L., were assessed in relation to the differences in their foraging regimes and zonation. No intraspecific difference was found in glycogen content in comparisons between late spring and winter, which roughly correspond to minimum and maximum gonadic development. No evident correlation with body size was detected within each species in both seasons. When comparing the glycogen content of fed specimens of the two species, a significant difference emerged, as concentrations in the foot of P. rustica were, on average, higher than those of P. caerulea in both seasons. When limpets were exposed to increasing periods of starvation (10-30 days) under controlled laboratory conditions, a marked difference between the two species emerged: P. rustica did not show any consistent variation in glycogen level, even after 30 days of starvation, whereas in P. caerulea, glycogen content declined exponentially as starvation proceeded, These findings agree with previous observations on the ecology and behaviour of the two species in the same area: the high shore P. rustica, which is naturally exposed to prolonged periods of starvation on account of the dependence of its foraging activity upon unpredictable and often infrequent favourable conditions (usually during rough sea), is able to store glycogen at greater concentrations, and has an efficient mechanism for saving energy during starvation. The low shore P. caerulea, which usually has a more regular foraging regime (being active at high tide on most days), stores glycogen at lower concentrations and shows a lower capacity to reduce energy losses when starved.