Warmer temperatures and more acidic oceans put crabs into survival mode
The combined effect of rising temperatures and a more acidic ocean will make it harder for seashore crabs to grow and reproduce, a new study finds.
The results suggests that other marine species could be affected too, the researchers say.
Basic functions
The oceans absorbs around 30 per cent of the carbon dioxide we emit into the atmosphere. When carbon dioxide dissolves in water, carbonic acid is produced, which makes the oceans more acidic. The oceans have become around 26 per cent more acidic since the industrial revolution and this is projected to increase further under all scenarios of the Intergovernmental Panel on Climate Change.
New research, published in The Journal of Experimental Biology, studies the impact of ocean acidification and higher air temperatures on the porcelain crab, which lives in rocky shorelines on the coasts of the Indian and Pacific Oceans.
The results show that warmer, more acidic conditions mean the crabs have to put a greater proportion of energy into the basic functions of living and breathing, leaving less for anything else.
This could be an indicator for other species in the ‘intertidal zone’, or seashore, too. As co-author, Professor Jonathon Stillman, explains: “future intertidal zone animals may experience reduced rates of growth, behavior, or reproduction.”
High- and low-tides
While previous studies have tested the impact of constant high temperatures and acidity on sea creatures, this isn’t typical of the conditions on the seashore.
Air temperatures in the intertidal zone can change by 20°C within six hours, while acidity levels can vary between day and night and from one season to the next. This study tests temperature and acidity that peak and fall during the day.
In order to test the crabs under these conditions, researchers constructed a specially-designed aquarium, which could simulate high- and low-tide as well as different temperatures and acidity.
They simulated ‘low-tide’ in the aquarium for seven hours each day, reducing the water and increasing the air temperature. Then for five hours of ‘high-tide’ they submerged the crabs with water and brought the air temperature back down. They tested three scenarios of higher temperature and higher acidity (no change, moderate change and extreme change).
Combined effects
After two and a half weeks in those conditions, the researchers tested the metabolic rate and thermal tolerance of the crabs.
The findings show the combination of higher temperatures and more acidic water cause the crabs’ metabolic rate to fall by as much as 25 per cent. You can see this by the green line in graph A below. The metabolism of the crabs slowed down, leaving them less energy to use in finding food, reproducing, or growth.
The effects of increasing acidity (pH) and temperature (°C) stress severity on the porcelain crab in terms of metabolic rate (A) and thermal tolerance (B). Shows results for only the impact of acidity (blue line), only temperature (red) and the combined effect (green). Paganini et al. (2014).
By monitoring the crabs’ heart rate response to temperature, the researchers also find that the crabs’ thermal tolerance went up as the acidity and temperature increased (see graph B). This suggests that the crabs were using more energy to cope with the more stressful conditions, the researchers say.
The results also show the impact on the crabs was greatest when they were exposed to the combined effects of increased acidity and higher temperatures, rather than one or the other.
Less energy
Overall, with higher temperatures and acidity the crabs have less energy and are having to use more of it just to survive. This means there is less energy available to put towards growth or reproduction. As the paper concludes:
Intertidal zone organisms may be able to withstand increased warming and acidification, but with performance effects that could lead to reduced ecological resilience.
Prof Stillman also warns that the impacts may not be limited to just the porcelain crab:
Though our research focused on one intertidal zone animal, the results may hold true across species.
The porcelain crab is a food source for coastal fish, birds and other crabs, which means that were their numbers to fall, it could disrupt food networks, particularly if other species are affected too.