Effects of seawater temperature and pH on the Japanese stone crab

There has been an increase in atmospheric carbon dioxide (CO2) levels due to deforestation and burning of fossil fuels. This has impacted the ocean by increasing its water temperatures and decreasing its pH. This decrease in pH is referred to as ocean acidification. There have been predictions that ocean temperature will increase 1.8-4°C and ocean pH will decrease 0.2-0.3 units by the end of the century. Temperature and pH are important for driving biological processes. Several studies have shown that ocean acidification affects reproduction, growth, physiology, and survival of a variety of marine species. Ocean acidification has also been found to affect feeding, swimming, predator-prey interactions, predator escape, and assessment and choice behavior in crustaceans. However, there is little known on how decreased pH affects feeding and foraging behaviors in crustaceans. Ocean warming has been found to affect habitat use, development, reproduction, and survival in crabs, but there is also little known on how it affects foraging behaviors. This study by Wu et al. (2017) examines how decreased pH and increased temperature affects the foraging behaviors of the Japanese stone crab Charybdis japonica.

The survival temperature of C. japonica ranges from 3.0-32.0°C, with their optimal growth temperature falling from 20.0-27.0°C. It is known that decreased pH can negatively impact predation efficiency and increased temperature can increase hunting activities. These researchers hypothesize that high temperature and high pH will produce optimal foraging behavior. Several factors are examined including breaking time, eating time, searching time, handling time, prey preference, and predation rate. Foraging behaviors were examined at two different temperatures (18 and 25°C) and two different pH levels (7.3 and 8.1). Four bivalve prey items, Potamocorbula laevis, Ruditapes philippinarum, Tegillarca granosa and Mactra veneriformis, were introduced to the crabs in their aquariums. Twelve of each prey item was offered to each crab simultaneously. The prey eaten were recorded over 48 hour periods. 

Prey preference consistently followed the same pattern, P. laevis being the most preferred and M. veneriformis the least (Figure 1). Several studies have found that crabs prefer smaller prey according to energy maximization. This statement is consistent with the results with P. laevis being the smallest and most preferred prey item. The prey preference could also be explained by prey escape abilities. T. granosa and M. veneriformis have better burrowing and escape abilities than P. laevis and R. philippinarum. 

Figure 1. Prey preference of C. japonica

Breaking time, searching time, eating time, and handling time were all prolonged with decreased pH but diminished with increased temperature. Prey profitability displayed the opposite pattern. Figure 2 displays differences in eating times in each scenario. 

Figure 2. The effects of temperature and pH on eating time of C. japonica

The results of this study suggest that decreased pH and increased temperature significantly impact the foraging behavior of C. japonica. Decreased pH resulted in reduced feeding rates. This could be due to various physiological processes. Several studies have shown that escape behaviors, foraging, and response to chemical cues have been negatively impacted by decreased pH. This study also found that increased temperature positively impacted foraging behaviors by increasing predation efficiency and increasing activity. While temperature and pH individually produced significant effects in this short term study, the interaction of the two did not. Future long term studies will need to be done to get a better understanding of ocean acidification and ocean warming. More studies will also need to be done to determine if there are other variables affecting foraging behaviors. 

Wu et al., (2017). Effects of seawater pH and temperature on foraging behavior of the Japanese stone crab Charybdis japonica. Marine Pollution Bulletin, 120(1-2), 99-108. https://doi.org/10.1016/j.marpolbul.2017.04.053