Ocean acidification alters properties of the exoskeleton in adult tanner crabs, Chionoecetes bairdi

Ocean acidification can affect the ability of calcifying organisms to
build and maintain mineralized tissue. In decapod crustaceans, the
exoskeleton is a multilayered structure composed of chitin, protein, and
mineral, predominately magnesian calcite or amorphous calcium carbonate
(ACC). We investigated the effects of acidification on the exoskeleton of
mature (post-terminal-molt) female southern Tanner crabs, Chionoecetes
bairdi. Crabs were exposed to one of three pH levels—8.1, 7.8, or 7.5—for
two years. Reduced pH led to a suite of body-region-specific effects on
the exoskeleton. Microhardness of the claw was 38% lower in crabs at pH
7.5 compared with those at pH 8.1, but carapace microhardness was
unaffected by pH. In contrast, reduced pH altered elemental content in the
carapace (reduced calcium, increased magnesium), but not the claw.
Diminished structural integrity and thinning of the exoskeleton was
observed at reduced pH in both body regions; internal erosion of the
carapace was present in most crabs at pH 7.5, and the claws of these crabs
showed substantial external erosion, with tooth-like denticles nearly or
completely worn away. Using infrared spectroscopy, we observed a shift in
the phase of calcium carbonate present in the carapace of pH-7.5 crabs: a
mix of ACC and calcite was found in the carapace of crabs at pH 8.1,
whereas the bulk of calcium carbonate had transformed to calcite in pH-7.5
crabs. With limited capacity for repair, the exoskeleton of long-lived
crabs that undergo a terminal molt, such as C. bairdi, may be especially
susceptible to ocean acidification.