Ocean Warming and Acidification will Crash Crabs’ Metabolism

Based on future climate scenarios, researchers believe coastal ecosystems will see increased extremes in low tide temperature fluctuations and drops in pH levels associated with ocean acidification. This particular study looked at what impacts, if any, warmer water and higher acidity levels will have on intertidal zone crustaceans like the test species, porcelain crabs. (The intertidal zone is the area above water at low tide and below water at high tide.)

Based on future climate scenarios, researchers believe coastal ecosystems will see increased extremes in low tide temperature fluctuations and drops in pH levels associated with ocean acidification. This particular study looked at what impacts, if any, warmer water and higher acidity levels will have on intertidal zone crustaceans like the test species, porcelain crabs. (The intertidal zone is the area above water at low tide and below water at high tide.)

“The way that I interpret our data is that in the future, [porcelain crabs] are going to have an overall lower metabolism,” says SF State biology professor Jonathon Stillman. Stillman is the ranking academic on the study. His lab oversaw the project, though his graduate student, Adam Paganini, led the study and is listed as lead author.

Less of their energy will be available for things like growth, behaviour, reproduction or energy storage.

Porcelain crabs “will have less energy overall to do stuff and more of that energy is going to get used for the basics of survival – cellular and physiological maintenance,” Stillman told me over the phone during his morning commute. “Less of their energy will be available for things like growth, behaviour, reproduction or energy storage.”

There are more than 50 species of porcelain crabs in the eastern Pacific Ocean, ranging from Alaska to Chile. The wide range and large variety of related species is one reason Stillman, Paganini and Miller chose porcelain crabs as their test case. The species of Petrolisthes they chose to study – Petrolisthes cinctipes – ranges from Santa Barbara, California to British Columbia.

Stillman, an associate professor at the Romberg Tiburon Center for Environmental Studies, has been with San Francisco State University for the past decade. Through his work on genomics and ocean acidification at the University of California Berkeley, Stillman and others completed previous research on the physiology of invasive clams and how evolution unfolds in an acidifying ocean.

To conduct this multi-year study, Stillman, Paganini and fellow graduate student Nathan Miller collected several hundred adult male crabs near Fort Ross, California on five separate trips between September 2012 and January 2013 and housed them in a specially built aquarium.

The task of designing and constructing the aquarium fell to Paganini who, Stillman told me, took six months to test and modify different variations before finding a suitable model. Even still, data from the first several tests were later discarded as the group tinkered with how to perfect an artificial habitat for dozens of crabs in a laboratory. It was no easy task, as the habitat had to accurately mimic the real world, allowing Stillman and his co-authors to test for temperature and acidity stressors at both high- and low-tide simulations.  

In order to simulate what many biologists expect will happen as a result of climate change, temperatures in the aquarium were raised in separate studies from 11ºC to 30ºC while the pH level dropped from 8.1 to 7.15 – still alkaline, but nearing closer to acidic water. While the porcelain crabs were found able to withstand the heat, a drop in metabolism was identified when the rise in temperature mixed with a drop in pH levels. The crabs’ respiration rate plummeted a dramatic 25 per cent overall and they focused a larger portion of their remaining aerobic energy into basic survival.

Having less energy available for things other than basic cellular maintenance could pose numerous challenges. They might reproduce less, or put less energy into the embryos of what offspring they do have. There will be less energy available to defend themselves against the shallow water fish, shore birds and other crabs that prey on them. They could shift behaviour and move less to conserve energy, impacting their interactions with other porcelain crabs, predators and the intertidal zone ecology in general.

“We think this is going to cause an energetic imbalance in these animals where one of these factors will be compromised,” Stillman told me.

This report is one of the first of its kind to study both temperature stresses alongside complex drops in pH level. 

Or maybe the decreased metabolism could impact all of these factors to various degrees. Stillman doesn’t know; they simply don’t have the data yet. This report is one of the first of its kind to study both temperature stresses alongside complex drops in pH level. Further studies are at various stages of completion to answer the questions this study leaves behind.

While the double-whammy of rising temperatures and lower pH levels had a significantly greater impact on the tested crabs than either of these changes had on their own, individually, rising temperatures exerted greater stresses on the crabs than pH level changes.

The paper, funded by the National Science Foundation and published today in the Journal of Experimental Biology, builds on previous research Stillman co-authored in 2013 at Berkeley on the impacts of pH levels on porcelain crabs.

While the study was conducted on porcelain crabs alone, Stillman and his co-authors believe the effects witnessed in their research can be applied to other crustaceans and species living below water at high-tide and above water, exposed to the elements, at low-tide.

“I would like to think our results are generalizable to other crustaceans in the intertidal zone, as well as mussels or barnacles,” Stillman told me. But it’s very difficult to say. These organisms evolved very differently and have learned to adapt to temperature and pH level fluctuations in their own ways, he said.

“One of the big challenges in climate change and ocean acidification research is that we would like to generalize broadly – we learn how one intertidal zone animal responds and we would like to be able to say this applies broadly to all intertidal zone animals,” Stillman said. “It might be that way, but we have to actually make the measurements before we can say for sure.”