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Coastal Management

Economically vital Blue Crabs having a hard time dealing with marine hypoxia and acidification

Tomasetti SJ, Morrell BK, Merlo LR, Gobler CJ (2018) Individual and combined effects of low dissolved oxygen and low pH on survival of early stage larval blue crabs, Callinectes sapidus. PLoS ONE 13(12): e0208629. 

 

The Blue Crab, Callinectes sapidus, found along the Atlantic coast from Nova Scotia to South America (NOAA)

Blue Crab is the 5th most valuable fishery in the US.

Known as Callinectes sapidus throughout its range, the Blue Crab is one of several native crab species found along the Western Atlantic coast, from Nova Scotia all the way to Argentina. It forms a key fishery in the Gulf and North East United States, where it produces more than 60 metric tons and $175 million in landings annually and is increasingly seen in Canadian waters, where it may be a boon to northern fisheries struggling with climate change.

Like many other coastal species, the Blue Crab inhabits both marine and estuary environments, depending on its life stage. Mating and juvenile development take place in estuaries, where there’s lots of food and protection from the harsh environment of the open ocean. Here, they play a major role in coastal food chains and the ecology of both freshwater and marine environments. In their juvenile stage, crabs are tiny and pelagic (drifting through the water column), and together with algae, juvenile jellyfish, and other young arthropods, they make up a plankton-soup that feeds and sustains a vast number of marine animals. As they age and settle down into a benthic lifestage (on the ocean floor), they become both prey to other larger invertebrates, mammals, and birds, and key predator of smaller invertebrates and fishes. Blue Crabs have also been observed fending off invasive Green Crabs, which are blamed for damaging vital eelgrass habitat along the temperate American coast. These tasty little guys are important for natural ecosystems and the human communities that border them!

Pelicans, godwits, geese, gulls and sanderlings share this coastal wetland in San Diego (Circe Denyer)

Humans can be bad neighbours, though.

Living in coastal estuaries puts species at risk of many anthropogenic hazards. Coastal wetlands, rivers, and the estuaries they feed are exposed to increasingly high nutrient levels, a result of unfettered urban and agricultural runoff. High nitrogen and phosphorus levels produce algae blooms that suffocate the coast of necessary dissolved oxygen. Shallow coastal environments are also prone to periods of acidification, where high nutrient levels and warming waters make for overproduction of carbon dioxide. Crabs have gills and, obviously, need adequate dissolved oxygen to “breathe”. The pH, or acidity, of their aquatic environment is also important for maintaining normal physiological functioning. Low pH (or high acidity) can make shell production difficult and, at extremes, can interfere with immune system function. Adult Blue Crabs are pretty tolerant of moderately hypoxic (low oxygen) and acidic conditions, thanks to a handy ability to regulate their blood chemistry, but there’s been little research on how young crabs fair under these conditions.

Delaware and Chesapeake Bays, both heavily developed estuaries and threatened habitat of the Blue Crab and many other important marine species (NASA)

To examine the effects of these stressors on young Blue Crabs, researchers from Stony Brook University in New York manipulated the environments of crab larva raised in laboratory tanks to mimic hypoxic and acidic conditions of a natural estuary. Hypoxia and acidification don’t always happen at the same time in nature, though their combined effects may be especially detrimental to crabs. So the researchers looked at each stressor both independently and together, to see if there were any differences in the crabs’ reactions under one or both conditions. The researchers also looked at two time periods, 4 or 14 days of exposure, to see if the length of exposure might also play a role in determining the crabs’ reactions. Most incidences of intense acidification and hypoxia last a few days to over a week, so this range allowed the researchers to compare realistic events likely to occur under climate change.

It’s tough being a crab.

Larval crabs experience high mortality under normal conditions, which is why individual females produce millions of eggs at a time. There’s a really cool video of some mama crabs getting ready to spawn their millions of babies here! In the lab, the Stony Brook researchers found natural survival to range from 19-80% of the total larva they started with, with pretty big differences between tanks. This natural variation can be explained by differences between the crabs themselves, as each tank contained larval crabs from only one or very few mothers, minimizing genetic diversity. Some young are just more fit than others.

Under short (4 day) exposure to low oxygen waters, survival of larval Blue Crabs decreased to just 1% of the original population, significantly less than even the lowest natural survival rate under control conditions. Meanwhile, acidic conditions lasting 4 days didn’t appear to affect larval survival. When the duration of the exposure increased to 14 days, however, low oxygen and acidic conditions both reduced survival (6-17% under low oxygen and 9-23% under acidic conditions). Combined, the two stressors brought crab survival down to just 3-5%.

While adult Blue Crabs seem to be moderately tolerant of environmental stress, larval crabs appear to have significantly reduced defense mechanisms for dealing with hypoxic and acidic conditions.

Low oxygen was especially detrimental to larval survival in the Stony Brook study, significantly lowering crab numbers even under short duration. While acidification had less dramatic effects on larval survival, when combined with hypoxia or lasting 14 days, both stressors resulted in a nearly total decimation of the juvenile population. Considering that hypoxia and acidification are more likely to occur in the highly urbanized estuary environments where young crabs live, and under climate change these events are expected to occur both more frequently and for longer lengths of time, this apparent juvenile vulnerability could have lasting effects on the Blue Crab.

Young stages of the Common Shore Crab, a relative of the Blue Crab (Calman, W. T)

The Stonybrook researchers call for more study on the combined effects of environmental stressors on juvenile crab survival. A myriad of marine organisms depend on larval blue crab as a regular part of their seasonal diet. The American fishing industry depends on them directly for fisher income as well as the indirect benefits they offer tourism in coastal communities. Understanding the complex life histories and adaptation strategies of marine organisms will be increasingly important under climate change as we struggle to maintain coastal livelihoods.

You can help

If you’d like to support research on important Atlantic coastal species, you might consider supporting organizations like Sea Grant Maryland in the US or the Fishermen and Scientists Research Society in Canada. These groups work directly with researchers and fishers to develop and distribute findings on the ongoing effects of climate change and other anthropogenic stressors on the Atlantic ocean, with a focus on key fishery species. You can also support groups working on wetland and estuary restoration, like the members of Restore America’s Estuaries, to protect vital coastal habitats. The time to act is now!

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