Abstract
The anticipated effects of CO2-induced ocean acidification on marine calcifiers are generally negative, and include dissolution of calcified elements and reduced calcification rates. Such negative effects are not typical of crustaceans for which comparatively little ocean acidification research has been conducted. Crustaceans, however, depend on their calcified exoskeleton for many critical functions. Here, we conducted a short-term study on a common caridean shrimp, Lysmata californica, to determine the effect of CO2-driven reduction in seawater pH on exoskeleton growth, structure, and mineralization and animal cryptic coloration. Shrimp exposed to ambient (7.99 ± 0.04) and reduced pH (7.53 ± 0.06) for 21 days showed no differences in exoskeleton growth (percent increase in carapace length), but the calcium weight percent of their cuticle increased significantly in reduced pH conditions, resulting in a greater Ca:Mg ratio. Cuticle thickness did not change, indicating an increase in the mineral to matrix ratio, which may have mechanical consequences for exoskeleton function. Furthermore, there was a 5-fold decrease in animal transparency, but no change in overall shrimp coloration (red). These results suggest that even short-term exposure to CO2-induced pH reduction can significantly affect exoskeleton mineralization and shrimp biophotonics, with potential impacts on crypsis, physical defense, and predator avoidance.
Highlights
Which could make them vulnerable to changes in ocean pH
Percent growth in carapace length decreased with increased body size for shrimp in both ambient [least-square regression (LSR): slope = –0.75, d.f. = 10, R2 = 0.68, F = 21.6, N = 12, P < 0 .001] and reduced pH (LSR: slope = –0.93, d.f. = 10, R2 = 0.86, F = 60.9, N = 12, P < 0 .0001)
Crustaceans are generally thought to be tolerant of fairly broad changes in ocean carbon chemistry, such as those associated with ocean acidification or upwelling events, potentially because of their pH regulation abilities, less soluble form of CaCO3, and biological control of the calcification process[14,15,16,17]
Summary
Which could make them vulnerable to changes in ocean pH. crustaceans may respond to increased acidity relatively quickly because they repeatedly shed their exoskeleton (cuticle) and secrete a new one in the process of moulting. The impact of OA on net calcification appears neutral, there is evidence that multiple crustacean species increase calcification in reduced pH conditions, which perhaps appears counterintuitive and an exception compared to what has been learned from most other marine calcifiers. The epicuticle is relatively thin and contains lipoproteins impregnated with calcium salts, but lacks chitin and lamellar organization[27,28] Both the exocuticle and endocuticle layers are composed of chitin-protein fibers with mineral in between. Changes in cuticle structure and calcium carbonate content due to ocean acidification could cause discontinuities in the refractive index of the shrimp body, reducing transparency and having negative consequences for their ability to avoid detection by predators
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