Edible crabs won't cope with the effects of climate change on seawater – new study

October 23, 2018 by Nia Whiteley, The Conversation
Cancer pagurus, the edible crab. Credit: davemhuntphotography/Shutterstock

We are only just beginning to learn how aquatic organisms will respond to climate change, and the effect that this will have on their communities and ecosystems. One way to find out more is to look at whether species will be able to compensate for changes in their environment. Particularly if they can survive any immediate fluctuations in temperature, and reductions in ocean pH brought about by increasing levels of atmospheric CO₂.

Coastlines and estuaries are already challenging places for marine organisms to live. The physical properties of seawater – salinity, temperature, pH and oxygen levels – vary frequently. And with further environmental fluctuations due to climate change, they are becoming even more demanding. Patterns of sea surface salinity are changing, as fresh water input increases, due to exceptional storm events and runoff from flooding.

Scientists have started to examine the combined effects of global warming and a reduction in seawater pH – otherwise known as ocean acidification – on marine communities. To date, it has appeared that multiple factors have more of an effect on these creatures than each factor in isolation. Together they influence the ability of species to compensate and survive the changes.

However, not much is known about the combined effects of and seawater dilution on these organisms. This is important as changes in salinity tolerance are known to influence distribution patterns of marine species and their community structures.

Comparing the plight of crabs

For our newly published study we decided to look at this combination of factors by focusing on two species of marine crabs: the edible crab (Cancer pagurus) and the shore crab (Carcinus maenas). Both are common to UK waters, but experience different degrees of environmental variation in their natural habitats. For edible crabs, home is typically the low intertidal shallow shelf waters for juveniles, and down to 100 metres for adults away from the influence of freshwater. While shore crabs typically live in estuaries and experience dilute seawater on a regular basis.

We studied how the crabs reacted to what are predicted to be the business as usual levels of CO₂ in 2100 (1,000 micro-atmospheres) and a biologically relevant reduction in . We were interested to see whether the edible crab will be less capable than the shore crab which regularly experiences salinity variations. We were also keen to find out why one species is likely to be more vulnerable than the other by investigating the ways they naturally compensate for environmental changes.

We exposed juveniles of both species to the different CO₂ and salinity conditions for up to one year. The crabs were fed regularly and they continued to grow by moulting throughout the exposure time. We found that the shore crab was fully capable of surviving the conditions for up to a year, but the edible crab struggled.

The – which is a widely invasive species in countries outside Europe – increased its response to a stimulus (upregulated) its capacity to exchange bicarbonate ions across the gills. This mechanism helps buffer changes in body fluid pH associated with increased CO₂ in seawater. The edible crab, meanwhile, showed no such upregulation, and had limited ion transporting capacities. Instead, this species accumulated CO₂ within its haemolymph (crustacean blood) supply.

There was some attempt at compensating for the conditions, but remarkably the edible crabs were better off in dilute seawater. This was a surprise as the edible crab typically spends all of its adult life living in marine environments separated from the influence of freshwater. The reason behind it is difficult to explain, but it may come down to passive changes associated with exposure to dilute seawater making the haemolymph more alkaline.

Our work demonstrates that the juvenile edible could survive elevated CO₂ conditions by moving into freshening seawater – but only for limited periods. This species also proved to be vulnerable to longer term exposures to dilute .

Our study helps us appreciate that there are fundamental differences in the biological capacities of marine species to compensate for climate change. Even within a taxa of crustaceans that is generally regarded as being relatively tolerant to change.

Fully , such as the edible crab, with its preference for stability, are poorly equipped for survival in a variable natural environment. They are likely be to more vulnerable to climate change and further studies on this and similar are urgently needed.

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5 comments

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Shootist
1 / 5 (2) Oct 23, 2018
no . . . 350 million years and crabs have never experienced warm and cold.

where do you people come up with this drivel?
barakn
5 / 5 (1) Oct 23, 2018
I'm not sure who "you people" are. The article never mentioned that crabs have never experienced warm and cold, so I'd have to assume that you pulled it out of your rectum, a place where you seem to store a lot of your ideas.

For my part, I can tell you that the same species has not lived for 350 million years and even amongst those species that did survive temperature transitions, their population levels may have gotten uncomfortably close to extinction levels.

Nor do you appear to have yet clued in to the idea that it's not so much whether temperatures change, but how quickly they change that matters. All in all, the drivel has been sourced... to you.
aksdad
1 / 5 (2) Oct 24, 2018
Yet another study doing what every salt water aquarium owner knows not to do: rapidly alter the chemistry of the water. It's bad. But that's not what's happening in the oceans.

Most prople don't know that the oceans experience fairly large seasonal fluctuations in pH; much more than the predicted long-term decrease in pH. Organisms are well adapted to these seasonal fluctuations. It beggars belief that they wouldn't be able to adapt to fractionally lower pH over decades that is significantly less than the seasonal fluctuations they experience annually.

See the NOAA PMEL Carbon Program observations page for data on measured ocean pH here. Click on a site to see the graphs:

https://www.pmel....map/text

Check, for example, the measurements at Grays Reef and notice how pH fluctuates by 0.3 per year but the long-term trend is so small it can't be seen:

https://www.pmel....ays+Reef
barakn
not rated yet Oct 25, 2018
The pH measured by a surface buoy is not representative of the entire water volume. We can see from https://www.pmel....nding+OA that the day-to-day variability of pH at the surface is large, reflective of the fact that the buoy is at the surface, the interface where a lot of the CO2 enters the ocean and that relatively small blobs of water with varying pH pass by the buoy. Mobile animals have the option of moving to avoid water with non-optimal pH because they're not stuck in a fucking aquarium, and one option is to move lower down in the water column where conditions are far less variable - the article specifically mentions edible crab adults living as far down as 100 meters. There is a strong seasonal component to pH as well, and animals also have the option of timing pH-sensitive activities - spawning, molting, etc.- to times of the year with optimal pH.
barakn
not rated yet Oct 25, 2018
But additional CO2 from the atmosphere will shift both the lows and highs lower. Animals will find it harder to use movement or timing to find water of optimal pH, because the whole baseline will shift.
notice how pH fluctuates by 0.3 per year but the long-term trend is so small it can't be seen:

You were expecting to see a pH trend in 6 years of extremely spotty data from just one site in a buffered system? Or maybe you were hoping people weren't going to notice you cherry picking data again.

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