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°C) 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°C; reduced pH/stable 7.67, 16.6°C; reduced pH with low fluctuations, 7.67 ± 0.05, 16.4°C; reduced pH with high fluctuations, 7.67 ± 0.10, 16.4°C). After three months, inductively-coupled x-ray spectrometry (ICP-MS) was performed on cuticle samples at the Scripps Isotope Geochemistry Laboratory (SIGL) for a precise quantification of elements (µmol/mg sample). The carapace spine was air-dried and then trimmed so only the spine remained with no setae, and the abdominal segment was cut as a 1 x 1 cm² from the center of the second abdominal segment and air-dried. Samples were weighed and placed in Teflon vials for digestion with 0.5 ml of concentrated Teflon-distilled (TD) nitric acid (HNO3) on a hotplate at 100°C for >24 h. Samples were dried down and diluted by a factor of 4000 with 2% TD HNO3 before being transferred to pre-cleaned centrifuge tubes for analysis. Samples were doped with an indium solution to monitor instrumental drift. Measurements were done using a ThermoScientific iCAPq c ICP-MS (Thermo Fisher Scientific GmbH, Bremen, Germany) in standard mode. Masses of Mg and Ca were sequentially measured for 30 ratios, resulting in internal precision of <2% (2 s.d.). Elements were corrected for total mole fraction. Total procedural blanks represented <0.3% of the measurement for Mg and Ca. Raw data were corrected offline for instrument background and drift. Samples were bracketed by internal standards of crab carapace (n=3), which allowed for calculation of absolute values as well as weighted averages of isotopes from natural abundance. The standards yielded external precision of 2% and 3% for Mg and Ca, respectively.