Tolerance to ocean acidification has not been very well studied, and even fewer studies have examined the physiological basis for this tolerance. The velvet swimming crab Necora puber has been shown to be tolerant to acidification on a short-term basis. A recent study examined the physiological implications of a medium-term exposure to acidified conditions (Small et al. 2010). The authors examined a variety of parameters to attempt to get a complete look at how this species tolerates decreases in pH. They found that most physiological processes, including thermal tolerance and immune response, were unaffected by lowered pH levels and that the crabs were able to compensate for these harsh conditions. A decrease in oxygen uptake suggests that decreased energy consumption is associated with internal pH regulation, even though more of the remaining energy consumption is presumably diverted to active excretion of hydrogen ions from the gills. This will likely make the crabs less active predators. —Emily Putnam
Small, D., Calosi, P., White, D., Spicer, J.I., Widdicombe, S., 2010. Impact of medium-term exposure to CO2 enriched seawater on the physiological functions of the velvet swimming crab Necora puber. Aquatic Biology 10, 11–21.
Small and his colleagues at the University of Plymouth studied the effects of acidification on a variety of physiological characteristics of a medium–term exposure length of 30 days. Male N. puber individuals were collected and held in three acidification conditions––the current pH, the projected pH for the year 2100, and a lower value to mimic conditions of extra CO2 inputs in addition to acidification. After 30 days, crabs were placed in a chamber for 50 minutes and the amount of oxygen taken up by the crabs was measured as an indicator of metabolic rate. The upper thermal tolerance was determined by finding the temperature at which the crabs began to spasm and the temperature at which they were unable to right themselves from an upturned position. Haemolymph, or circulatory fluid, was extracted by needle and tested for total CO2 levels and pH. From these values, pCO2 and carbonate ion concentrations were determined by calculation. The immune response to acidification was determined by examining lipid peroxide concentrations in haemolymph samples. Calcium and magnesium haemolymph concentrations were found by atomic absorption spectrometry. Levels of shell mineralization were found by dissolving a portion of the shell, or chelae, in acid and measuring calcium and magnesium ion concentrations.
The oxygen uptake decreased under acidified conditions, and total CO2 levels in haemolymph decreased as a function of decreasing pH. Haemolymph calcium and magnesium concentrations also decreased in acidified conditions, but magnesium concentration in the shell increased slightly at lower pH. Thermal tolerance, lipid peroxidation, and calcium ion concentration in the shell were unaffected by acidification.
Small et al. concluded that N. puber is able to compensate for a medium-term exposure to ocean acidification. This compensation comes without affecting thermal tolerance, or immune response. Decreases in metabolic activity corresponded to increases in calcium in the haemolymph and magnesium in the chelae. Since no impact upon mineralization was observed, the authors concluded that shell dissolution is not the most important process in buffering the body against acidified conditions in the medium term. The ability to buffer pH is likely energetically expensive, which could incur extra costs on physiological processes not examined here. The authors conclude that the decrease in oxygen consumption observed corroborates other papers as a proposed mechanism for conservation of energy and regulation of internal pH in harsh external conditions. As a key predator in its ecosystem, tolerance for ocean acidification will likely give this species an ecological advantage over other predators. However, since oxygen uptake limitations were observed, there could be additional restrictions on the ability of these predators to maintain existing population dynamics in the face of ocean acidification.