Crab larvae already showing effects of coastal acidification

Dungeness crab support the most valuable fishery on the US West Coast (Credit Theo Stein, NOAA)

Ocean acidification is having a profound impact on a prized crustacean that supports the most valuable fishery on the west coast of the United States.

According to a new study involving the University of Plymouth, the effects of climate change are impacting the shells and sensory organs of some young Dungeness crabs.

An analysis of samples collected during a 2016 research cruise identified examples of damage to the carapace, or upper shell, of numerous larval Dungeness crabs as well as the loss of hair-like sensory structures the crabs use to orientate themselves to their surroundings. 

The study, funded by the National Oceanic and Atmospheric Administration (NOAA), is published in the journal Science of the Total Environment.

Prior to this study, scientists had believed Dungeness crabs were not vulnerable to current levels of ocean acidification, although a laboratory study conducted on crab larvae in 2016 had found that their development and survival suffered under pH levels expected in the future.

Dr Nina Bednarsek, senior scientist with the Southern California Coastal Water Research Project and the study’s lead author, said:

“This is the first study that demonstrates that larval crabs are already affected by ocean acidification in the natural environment, and builds on previous understanding of ocean acidification impacts on pteropods. If the crabs are affected already, we really need to make sure we start to pay much more attention to various components of the food chain before it is too late.”

John Spicer, Professor of Marine Zoology at the University of Plymouth and one of the study’s co-authors, added:

“Projecting the future effects of ocean acidification is one thing, but here is evidence that the young stages of a commercially important crab species are affected here and now by upwelling acidic water off the west coast of the United States. Climate change, and its effect on biodiversity (the variety of living things in all their different forms) is not just the future, it is our now.”

Read more about Professor John Spicer and our work on ocean acidification

As part of the study, which involved scientists from across the USA, Canada and Slovenia, the crabs were examined under a high-magnification scanning electron microscope.

It revealed that the corrosive conditions of coastal waters had affected portions of the fragile, still-developing external shell and legs of the post-larval crabs, leaving tell-tale features such as abnormal ridging structures and scarred surfaces. This could, the researchers say, impair larval survival by altering swimming behaviours and competence, including the ability to regulate buoyancy, maintain vertical position and avoid predators.

The study showed that crabs with signs of carapace dissolution were smaller than other larvae. This was disconcerting, scientists said, because the damage during the crab’s larval stages could cause potential developmental delays that could increase energy demands and interfere with maturation.

The team also found that the low pH water in some coastal areas damaged the canals where hair-like bristles called mechanoreceptors stick out from the shell. These receptors transmit important chemical and mechanical sensations to the crab, and may help crabs navigate their environment.

Examination showed that carapace dissolution destabilises the attachment of the mechanoreceptor anchor, resulting in them falling out in some individuals. This is a new aspect of crustacean sensitivity to ocean acidification that has not been previously reported.

The team hypothesise that the absence or damage of mechanoreceptors within their neuritic canals may in part explain potential aberrant behavioural patterns, such as slower movement, less tactile recognition, and prolonged searching time, as well as impaired swimming, that have been observed in various crustacean species exposed to low pH conditions in laboratory settings.

The full study - Exoskeleton dissolution with mechanoreceptor damage in larval Dungeness crab related to severity of present-day ocean acidification vertical gradients by Bednaršek et al - is published in Science of The Total Environment, doi: 10.1016/j.scitotenv.2020.136610.

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