We examined the response of multiple structures used for predator defense in the California spiny lobster, Panulirus interruptus, to a series of ocean acidification-like conditions. Lobsters were collected by modified commercial traps offshore La Jolla, CA (in the area around 32.8534193, -117.2687516) in October 2016 and held at ambient conditions (pH 7.97, 16.5℃) before exposure to stable or diurnally fluctuating reduced pH conditions established by bubbling CO2 and as measured using best practices (ambient pH/stable, 7.97, 16.5℃; reduced pH/stable 7.67, 16.6℃; reduced pH with low fluctuations, 7.67 ± 0.05, 16.4℃; reduced pH with high fluctuations, 7.67 ± 0.10, 16.4℃). After three months, the carapace spine and rostral horn tip were tested for hardness and stiffness using a nanoindentation materials testing machine (Nano Hardness Tester, Nanovea, Irvine, CA, USA) equipped with a Berkovich tip. Fresh samples (<12 hours, except for two samples tested within 24 hours) were kept hydrated in seawater until testing. Samples were secured to an aluminum block with cyanoacrylate glue such that the outer surface was facing up. Indentations were performed by applying a load of 40 mN to the outer surface of the sample at loading and unloading rates of 80 mN/min with a 30 sec hold for creep. At least three indents were taken per sample
