Cardiac responses to salinity variations in two differently zoned mediterranean limpets

Cardiac activity of two Mediterranean limpets was tested at different salinities. Patella caerulea inhabits the lower midlittoral where it is exposed to variations in salinity, while P. aspera experiences more stable salinity conditions in the infralittoral fringe. When exposed to moderate hypo- and hypersalinity (23 g l(-1) and 43 g l(-1)) for 24 min, P. caerulea showed no significant variation in heart rate with respect to the control salinity (33 g l(-1)), while P. aspera exhibited a significant increase in heart rate in both conditions. This suggests a rise in metabolic rate due to activation of behavioural responses or physiological regulation. When exposed to extremely low salinity (3 g l(-1)) for 24 min, heart contractions ceased in most specimens of P. caerulea. A smaller number of specimens also displayed cessation of heart beat when exposed to extremely high salinity (63 g l(-1)). The heart beat resumed quickly in all specimens when they were returned to control salinity conditions. In contrast, cardiac activity was not interrupted in any of the P. aspera specimens at the 3 g l(-1) and 63 g l(-1) salinity levels, but strong bradycardia was evident. Contractile activity of the heart ceased in all specimens of P. caerulea and P. aspera when they were exposed to prolonged hypo-osmotic stress (3 g l(-1) for 24 h). This acardia was largely reversible in P. caerulea, but most specimens of P. aspera did not recover from the treatment.