| Abstract: | Heart rates were monitored in situ in the shore crab, Carcinus maenas, in relation to variations in depth, salinity, oxygen tension, temperature, light intensity and pH. Experiments were performed in the Looe Estuary, Cornwall, England and in Batson Creek in the Salcombe-Kingsbridge Estuary, Devon, England. Experiments in the Looe Estuary were conducted in the vicinity of a storm water storage discharge whereas the experiments in Batson Creek were performed on a clean site. Tidal rhythms in heart rates were commonly detected but diel rhythms in heart rate were also observed frequently. Both types of rhythm were more evident in animals from Batson Creek than from Looe. In Batson Creek, 12 out of 15 crabs expressed tidal rhythms in heart rate, whereas 6 out of 15 crabs expressed diel rhythms. In the two studies in the Looe Estuary, 6 out of 15 crabs and 3 out of 15 crabs expressed tidal and diel rhythm in heart rate, respectively. At both experimental sites, heart rates were positively correlated with increasing changes in depth and salinity, whereas heart rates were negatively correlated with light intensity. In addition, heart rates appeared to be positively correlated with increasing oxygen tension in the experiments performed in the Looe Estuary. The study suggests that depth and oxygen availability are more important to in situ heart rates in shore crabs within tidal estuaries than are salinity, light intensity and pH. Also, sewage discharge appears to cause an acute increase in heart rate, which may affect expression of biological rhythms in shore crabs. |
