An egg bearing snow from an experiment looking at how ocean acidification affected the embryo development.

What We Know

A general pattern has emerged from the body of research conducted on these crab species in Alaska thus far: crab survival decreases at most life stages when they are exposed to more acidic water.

Red king crab and Tanner crab are most sensitive to more acidic (higher CO2) water. Studies consistently show decreased growth and increased mortality at multiple life-history stages in both species. The most likely reason for this is that the crabs have to spend a lot of energy on basic life-sustaining functions in more acidic water. This leaves less energy for other critical activities like growing and fighting disease. While blue and golden king crabs are still sensitive, they seem to be less sensitive than red king and Tanner crabs. Snow crabs are the only species that appear resilient to more acidic water. Since Tanner and snow crabs are closely related, NOAA’s Kodiak Laboratory is planning to conduct more experiments to understand why Tanner crabs are sensitive and snow crabs are resistant. This work will include looking at how their blood chemistry changes in acidic waters and measuring gene expression.

There are currently no Alaska-based Dungeness studies underway. To learn more about Dungeness crab OA response in the lower 48, visit NOAA’s Dungeness Crab Case Study.

Meet the Crabs

Crabs are crustaceans with soft bodies protected by a calcified exoskeleton. A variety of marine crab species are harvested for consumption across the globe, with approximately one-third of annual US crab catches coming from the Bering Sea, Aleutian Islands, and the Gulf of Alaska. Of the ten species of crab that are commercially important in Alaska, ocean acidification researchers have focused their efforts on these six species: red king crab, blue king crab, golden king crab, Tanner crab, snow crab, and Dungeness crab.

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Recent Studies

NOAA’s Kodiak Laboratory has been conducting research on the effects of ocean acidification on commercial crab species for 15 years. Over that time, we’ve learned a lot about how more acidic waters may affect these species and the fisheries that depend on them.

Tanner Crab: Dickinson et al. 2021

Tanner Crab: Dickinson et al. 2021

  • 48 multiparous female adult southern Tanner crabs were exposed to pH 8.1 (ambient), 7.8, or 7.5 for 2 years
  • Examined effects of OA on the exoskeleton (thickness, structural integrity, elemental content, phase of calcium carbonate) of mature (post-molt) female southern Tanner crabs
  • Determine if the hydration state of the cuticle affects micro-mechanical responses to OA

Results

  • 10 survived in pH 8.1, 6 in 7.8, and 7 in 7.5.
  • Average reduction of 60% in hardness of the carapace when hydrated.
  • Hardness of the claw was not affected by hydration.
  • Hardness of the claw was 38% lower at pH 7.5 than pH 8.1 and 27% lower than pH 7.8.
  • Claw was four times harder than carapace when dry and ten times harder when wet.
  • Carapace thickness at pH 7.5 was on average 15% thinner than at pH 8.1.
  • Erosion of inner carapace visible on 57% of crabs at pH 7.5, but never at pH 8.1.
  • Carapace thickness at pH 7.8 was intermediate.
  • Claw cuticle 31% thinner at pH 7.5 than pH 8.1.
  • Greater pollex damage at pH 7.5 than pH 8.1.
  • Carapace Ca content was reduced by 11% on average in pH 7.5 than pH 8.1
  • Mg content increased by 17% on average in pH 7.5 than pH 8.1
  • Suggested transformation of ACC to calcite in crabs at pH 7.5
  • 2yr exposure at pH 7.5 caused reduction in microhardness of claw, alterations in mineral content of the carapace, thinning of the claw and carapace, internal dissolution of carapace, loss of denticles on claw, and a shift to calcite in the carapace.

Golden King Crab: Long, Swiney, and Foy 2021

Golden King Crab: Long, Swiney, and Foy 2021

  • Ovigerous females collected from Aleutian Islands
  • Larvae were reared in a seawater facility and 90 subsequent young-of-the-year were randomly assigned one of three acidification treatments: pH ~ 8.2 (ambient), 7.8, and 7.5 for 127 days.
  • Examined how OA altered juvenile golden king crab growth, morphology, and survival

Results

  • 17 crabs successfully molted in the pH 8.2, 20 in pH 7.8, and 14 in pH 7.5
  • At the end of the experiment, there were 12 crabs surviving in the ambient treatment, 8 in the pH 7.8, and 8 in the pH 7.5.
  • Crabs in pH 7.5 significantly smaller than pH 8.1
  • Crabs in pH 8.1 grew faster than at 7.8 or 7.5
  • Time to molt was greater at pH 7.5
  • OA had no significant impacts to morphology
  • Crabs at pH 8.1 had significantly lower mortality
  • Juvenile golden king crabs exposed to pH levels below surface ambient had significantly lower growth and survival than those exposed to surface ambient water.
  • GKC may be able to adapt to the environment they are reared in, so the results may favor higher pH since they were reared at ambient pH (8.1)

Snow Crab: Long et al 2018 (poster)

Snow Crab: Long et al 2018 (poster)

  • Ovigerous females captured in Bering sea
  • Exposed reared larvae to pH 8.1, 7.8, and 7.5 for one year
  • Mated in lab, exposed to treatments for second year
  • Examined how OA altered larval survival, condition, and calcification
  • Determine if the response is carried over in oogenesis and embryogenesis

Results

  • Low pH exposure as embryos decreased survival at year one
  • Larval pH had little effect on survival at year one
  • Low pH exposure as embryos did not affect survival in year two
  • Larval pH did not affect survival in year two
  • Embryo exposure to low pH carried over positive effects to larvae reared in low pH in both years
  • Less Ca uptake in embryos at pH 7.5 in year one, but not year two
  • Exposure to low pH as embryos has negative effects, but exposure during oogenesis and embryogenesis mitigates or eliminated the effects

Blue King Crab: Long et al 2017

Blue King Crab: Long et al 2017

  • Larvae were reared in a seawater facility and 90 subsequent juveniles at the first crab stage were randomly assigned one of three acidification treatments: pH ~ 8.1 (ambient), 7.8, and 7.5 for 1 year.
  • Examined how OA altered juvenile blue king crab growth, morphology, and survival

Results

  • Exposure to seawater at pH 7.8 had no effect on morphology or mortality and had only a minor effect on growth compared with the ambient treatment.
  • Exposure to seawater at pH 7.5 substantially increased mortality and decreased growth compared with the ambient treatment.
  • Phenotypic variability or plasticity in juveniles and may indicate acclimation by blue king crab to ocean acidification.
  • Findings suggest blue king crab may have scope for evolutionary adaptation in response to gradually changing pH levels.

Tanner Crab: Swiney et al and Long et al 2016, Part I and Part II

Tanner Crab: Swiney et al and Long et al 2016 Part 1 and Part 2

  • Mean embryo stage was not different among pH treatments in either year 1 or 2.
  • Larvae from embryos of oocytes developed in control water were not affected by low pH during embryogenesis.
  • Oocytes sensitive to low pH, hatching success decreased: ~70% fewer viable larvae in pH 7.5, but no difference between control and 7.8 pH hatching success. Reduced calcium content in females exposed to 7.5 pH.
  • Embryos developed in acidified water produced larvae with morphometric differences between 7.8 and 7.5 treatments. Differences in rostrum-dorsal length likely not relevant to larval survival or performance: trivial effects.
  • For Oocytes developed in acidified waters, larvae were bigger in the control and pH 7.8 treatments than the 7.5 treatment: 7.5 had 10% smaller carapace width.
  • Acidified embryo larvae had higher C and N content than larvae only exposed to low pH.
  • Ca and Mg reduced when exposed to acidified conditions for acidified larval embryos
  • Acidified embryos produced larvae with increased starvation survival time (indicative of lower metabolic rate), decreased Ca content and mass, and changed morphology: effects mostly apparent at pH 7.5. All of these effects increased in severity when oocytes and not just embryos were exposed to acidified waters.
  • Increased mortality in 1st larval stage at pH 7.5 for acidified oocyte larvae.

Results

  • Exposure to seawater at pH 7.8 had no effect on morphology or mortality and had only a minor effect on growth compared with the ambient treatment.
  • Exposure to seawater at pH 7.5 substantially increased mortality and decreased growth compared with the ambient treatment.
  • Phenotypic variability or plasticity in juveniles and may indicate acclimation by blue king crab to ocean acidification.
  • Findings suggest blue king crab may have scope for evolutionary adaptation in response to gradually changing pH levels.

Implications

When we look at how ocean acidification is likely to affect fisheries, the more sensitive species are predicted to have declining populations over time which means fewer crabs will be available for fisheries, resulting in declining harvest over the next 50 years in Alaskan waters.

It is important to note though, that these crab species may be able to evolve over time to be better adapted to lower pH (more acidic) water. However, it is very difficult to estimate how much that will help in the face of rapidly-changing ocean conditions. NOAA’s Kodiak Laboratory is planning research to shine light on these important questions.

To learn more about the computational models that are being developed to explore the economic impacts on the Alaska crab fishery from ocean acidification, start with these important publications:

Researchers

Chris Long

W. Christopher Long

Chris Long

Katherine M. Swiney

Bob Foy

Robert Foy

Chris Long

Leah Zacher

Chris Long

Jennifer Gardner

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