The effects of reduced pH on decorator crab morphology, physiology and behavior
Crabs in the family Majoidae camouflage by decorating their exoskeletons with organisms and debris from their environment. This form of camouflage, involving both the act of decorating and carrying of these decorations, is thought to be energetically costly, and may present a trade-off under stressful environmental conditions. The energetic cost of decoration behavior has been evinced by reduced organic content due to elevated metabolism. In the context of previous research demonstrating that many marine calcifiers experience metabolic costs under experimental ocean acidification conditions, we hypothesized that decorator crabs exposed to reduced pH will have insufficient energy to support regulatory processes along with decoration behavior. Thus, we predicted that energy will be allocated towards growth and calcification at the expense of decoration behavior. Dwarf teardrop crabs, Pelia tumida, were exposed to ambient (pH=8.0, pCO2=613 µatm) and reduced (pH=7.75, pCO2= 894 µatm) pH conditions for five weeks. Half of the animals in each treatment were given two sponge species, Halichondria panacea and Haliclona permollis, to decorate with, whereas the remaining animals were not allowed to decorate. At the end of the experiment, all animals were analyzed for exoskeleton mineral content (Ca and Mg) using EDX and ICP-MS, organic content (a proxy for metabolism) using TGA, and decoration behavior by quantifying sponge mass and percent cover. Overall, decorator crabs showed no signs of energy limitation under reduced pH conditions. Neither growth, exoskeleton mineral content, nor organic content of crabs differed among pH or decoration treatments. In addition, both sponge mass and percent cover remained the same across pH treatments, indicating no effect of reduced pH on decoration behavior, and thus the ability to camouflage. The maintenance of physiological processes without metabolic costs in P. tumida exposed to reduced pH radiates from the emerging trends on the susceptibility of crustaceans to changes in ocean chemistry associated with ocean acidification.