Intertidal vertical zonation is ubiquitous on temperate rocky intertidal shores worldwide and one of the most well studied ecological phenomena. A species’ upper vertical limit on the shore is generally attributed to abiotic stress, particularly thermal and desiccation stress at low tide. However, there is little direct evidence for this hypothesis, and other mechanisms such as food or oxygen limitation may also influence vertical limits. As part of a broader study of the vertical zonation of the temperate intertidal barnacle Balanus glandula, we quantified oxygen consumption of barnacles during a five-hour low tide exposure and for six hours of subsequent immersion. Animals were maintained in the lab under a typical tidal cycle with 10°C water temperatures and 19°C air temperatures. For each trial, 3-9 barnacles were then exposed to one of 8 low tide temperature treatments: 10, 15, 20, 25, 30, 35, 38, or 40°C and then returned to a 10 °C high tide. Rates of oxygen consumption were calculated from oxygen concentrations measured at 10-second intervals in both air and water.
Results/Conclusions
Aerial respiration was detected at all temperatures except 40 °C. However, individual barnacles showed very different patterns, with some barnacles inactive at a particular temperature at which others consumed high amounts of oxygen. The overall pattern of aerial respiration across temperatures followed the general shape of a thermal performance curve. Barnacles showed comparable rates of aerial respiration between 15-35°C, with declining respiration outside those temperatures. Similarly, upon resubmersion, aquatic respiration was similar between 10-30°C with reduced rates above 30°C. At aerial temperatures above 30°C all barnacles showed suppression of aquatic respiration for at least 3 hours after submersion. We also detected temporal shifts in oxygen use between temperatures. Animals exposed to the coldest and warmest temperatures show low initial oxygen consumption over the first few hours, but at moderate temperatures, respiration rates were highest during the first 1-2 hours. Our results suggest that B. glandula is capable of maintaining high levels of oxygen consumption during aerial exposure, even at warm temperatures. Thus, upper vertical limits are unlikely to be set by limits on aerobic metabolism, but may be set by other mechanisms of energy limitation, such as food limitation.