Ocean Acidification Has Multiple Modes of Action on Bivalve Larvae

Abstract

Ocean acidification (OA) is altering the chemistry of the world's oceans at rates unparalleled in the past roughly 1 million years. Understanding the impacts of this rapid change in baseline carbonate chemistry on marine organisms needs a precise, mechanistic understanding of physiological responses to carbonate chemistry. Recent experimental work has shown shell development and growth in some bivalve larvae, have direct sensitivities to calcium carbonate saturation state that is not modulated through organismal acid-base chemistry. To understand different modes of action of OA on bivalve larvae, we experimentally tested how pH, PCO2, and saturation state independently affect shell growth and development, respiration rate, and initiation of feeding in Mytilus californianus embryos and larvae. We found, as documented in other bivalve larvae, that shell development and growth were affected by aragonite saturation state, and not by pH or PCO2. Respiration rate was elevated under very low pH (~7.4) with no change between pH of ~ 8.3 to ~7.8. Initiation of feeding appeared to be most sensitive to PCO2, and possibly minor response to pH under elevated PCO2. Although different components of physiology responded to different carbonate system variables, the inability to normally develop a shell due to lower saturation state precludes pH or PCO2 effects later in the life history. However, saturation state effects during early shell development will carry-over to later stages, where pH or PCO2 effects can compound OA effects on bivalve larvae. Our findings suggest OA may be a multi-stressor unto itself. Shell development and growth of the native mussel, M. californianus, was indistinguishable from the Mediterranean mussel, Mytilus galloprovincialis, collected from the southern U.S. Pacific coast, an area not subjected to seasonal upwelling. The concordance in responses suggests a fundamental OA bottleneck during development of the first shell material affected only by saturation state.

Fig 1. Experimental Carbonate Chemistry Treatments.

Carbonate chemistry treatments of Ω_ar and P_CO2 (μatm), with pH (total) isopleths to illustrate the relationship among the three variables in the experiments. Grey circles are the actual treatment values, and star indicates the treatment chemistry of the control (freshly collected seawater bubbled with CO₂-reduced air for 24 hours).

Fig 2. Target and Measured DIC and Total Alkalinity.

The target and measured values of total alkalinity and total dissolved inorganic carbon (DIC) in μmol kg^-1. The 1:1 line represents if our measured measured values were exactly aligned with the target values.

Fig 3. Proportion Normal Shell Development.

The proportion of normally developed shells in relation to (A) P_CO2, (B) pH, and (C) Ω_aragonite. Greyscale symbols denote Ω_ar treatments in panels A and B, and the star is the control treatment. Error bars are standard deviations of the three replicate BOD bottles per treatment. See text for fit of model noted on panel C.

Fig 4. Normal Shell Length.

Shell length of normally developed larvae (48 hours post fertilization and exposure) in relation to (A) P_CO2, (B) pH, and (C) Ω_aragonite. Greyscale symbols denote Ω_ar treatments in panels A and B, and the star indicates control data. Error bars are standard deviations of the three replicate BOD bottles per treatment. See text for fit of model noted on panel C.

Fig 5. Larval Respiration.

Respiration rate of larvae immediately following the 48 hour post-fertilization and experimental period in response to (A) P_CO2, (B) pH, and (C) Ω_aragonite. Error bars are standard deviations of the five replicate experimental chambers per treatment. The grayscale in this graph represents the three pH categories for the treatments used in the reduced experimental matrix (see Table 1).

Fig 6. Comparisons Among Respiration Treatments.

Respiration rates presented on a bar graph showing pairwise comparisons across all treatments, with the common line over values representing no significant difference. Error bars are standard errors. Note that values are arranged in increasing pH along the x-axis, and P_CO2 and Ω_ar are included for each treatment. All values are in units previously noted.

Fig 7. Initiation of Larval Feeding.

The proportion of larvae feeding 44 hours post-fertilization and exposure in relation to (A) P_CO2, (B) pH, and (C) Ω_aragonite. Greyscale symbols denote P_CO2 treatments in panels B and C. Error bars are standard deviations of the three replicate experimental containers per treatment. Bold value on panel B indicates the significant linear relationship between pH and initiation of feeding for the high P_CO2 treatment. As noted in the text, the removal of this high P_CO2 treatment results in extremely poor fit of the response to pH.

Fig 8. Reponses of Two Mussel Species to OA.

Comparison of normal shell development and shell length (insert) of normal larvae between M. californianus (black circles, this study) and M. galloprovincialis (open squares, from previous work by our group [20]). Error bars for both studies are standard deviations. Proportion normal data for M. californianus is been standardized to the control values make the data more comparable with the M. galloprovincialis study.