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Written By

Mykala Wright


College of Science and Engineering

Publish Date

14 March 2022

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Considering the chemistry

The level of carbon dioxide in the Earth’s atmosphere has officially reached an all-time high, and it’s showing no signs of slowing. JCU PhD Student Jodi Thomas says this could spell trouble for some of our smallest cephalopods in the seas.

The oceans absorb about one quarter of all excess carbon dioxide (CO2) emissions released into the atmosphere. In the process, a series of chemical reactions occur and the water becomes more acidic. This is problematic for some of our most sensitive sea species, as ocean acidification can impact their ability to build shells and skeletons or even their capacity to detect and avoid predators.

Jodi is combining her two passions — zoology and neuroscience — to find out how changes in ocean chemistry can affect the nervous system of the two-toned pygmy squid (Idiosepius pygmaeus). With a maximum mantle length of two centimetres, the two-toned pygmy squid is a cephalopod characterised by its small size and short lifespan.

“Previous research has shown that high levels of CO2 can lead to behavioural changes in squid. My research is looking at the mechanisms in the nervous system that are leading to those behavioural changes at increased levels of CO2,” Jodi says.

“For me, this research is the perfect intersection of my two disciplines. It allows me to use a whole range of techniques, from hands-on experiments and molecular laboratory work to the computer-based bioinformatics and statistical analysis that follows.”

A headshot image of JCU PhD Student Jodi Thomas.
A close-up image of the two-toned pygmy squid in a tank. The small squid has translucent skin covered in black speckles.
Left: PhD Student Jodi Thomas. Right: A two-toned pygmy squid. Supplied by Jodi Thomas

The highway to hyperactivity

Despite new and ongoing reduction efforts, climate models predict CO2 emissions will continue to rise throughout the rest of the century, eventually reaching levels that will have extensive effects on marine life.

For her research, Jodi placed squid in seawater with current-day levels of carbon dioxide as well as those forecast for the future.

“I kept the squid in the different CO2 levels for seven days and I monitored any behavioural changes that occurred at the higher CO2. Seven days is a relatively short period of time, but previous research has shown behavioural changes occur within at least five days following CO2 exposure,” she says.

“Also, the two-toned pygmy squid has a lifespan of about 90 days, so seven days is quite a large portion of their life.”

Jodi found that exposure to high carbon dioxide levels led to behavioural changes in the squid, namely they became increasingly active.

“I measured three types of activity: the amount of time the squid spent moving, the total distance they moved and the average speed at which they moved. All of these showed increases at higher levels of CO2,” she says.

Jodi also investigated if and how an increase in carbon dioxide would impact the way the squid interacted with one another.

“I placed the squid in a tank that has a mirror along the entire length of one wall so they’re looking at a mirror image of themselves. But they don’t really recognise themselves; they see the mirror image as though it’s another squid,” she says. “I analysed the interactions the squid had with the mirror and found that at higher levels of CO2 the squid were more interested in their mirror image.”

Once the squid had been exposed to the higher levels of carbon dioxide they behaved more aggressively toward their reflection, spending longer periods of time up close to and touching the mirror.

“After discovering the increased activity and aggression I hypothesised that a specific receptor in the nervous system was involved in creating these behavioural changes,” Jodi says.

To explain the receptors’ role in behavioural outcomes, Jodi uses an analogy that compares the nervous system to a highway.

“The nervous system is made up of brain cells called neurons, and electrical signals travel along those neurons. The connections between neurons are like intersections, and they have receptors. The receptors act like traffic lights at a busy intersection, except instead of controlling the movement of cars through the intersection, receptors control the movement of the electrical signals between the neurons — or brain cells,” Jodi says.

“The receptor I was focusing on is a type of inhibitory receptor, which is similar to a red light at an intersection. As you can imagine, having red lights at a busy intersection is very important to control traffic flow. So, those inhibitory receptors are really important for controlling the flow of electrical signals.”

To discover if the receptors were underlying the behavioural changes, Jodi took her research a step further and commenced a pharmacological study where she administered pharmacological agents that targeted those specific inhibitory receptors in the squid’s nervous system.

“I placed the squid in the same conditions —the current-day levels of CO2 and the predicted future levels — and I treated them with the drugs before measuring their behaviour once again,” she says.

“The drugs acted specifically on those ‘red light’ receptors and I was able to determine that these receptors were changing function when the squid were exposed to higher levels of CO2.”

Jodi describes how elevated levels of carbon dioxide had caused the squid’s ‘red light’ receptors to become dim, stop working completely or even switch function entirely.

“Some have gone from a red stop light to a green go light, changing from inhibitory to excitatory. As you can imagine, this would send cars at an intersection into chaos. And that’s what I’m hypothesising is happening with the squid: the changes in function of this receptor are leading to those behavioural changes at higher CO2 levels,” Jodi says.

Will squid survive?

So, what does this mean for the future of the two-toned pygmy squid?

Before any definitive conclusions can be drawn there is a need to take the experiments out of the lab and into the wild.

“We need to see exactly how those behavioural changes will translate into the natural environment,” Jodi says. “I do have a number of hypotheses and ideas about what could happen, though. For example, an increase in activity could disrupt reproduction because if the squid are more active, they’re likely to be using more energy, leaving less energy available to do other important things, like reproduce.”

The two-toned pygmy squid also relies on its ability to be cryptic and change colour to escape predators. So, an increase in activity could potentially put their survival at risk.

“This specific species of squid likes blending in and camouflaging when predators are near. If they’re bouncing around more, they’re going to be easier for predators to see,” Jodi says.

Cephalopods are critical components of many marine ecosystems, and squid are often considered a keystone species because of their role as both predator and prey. Changes to the population of the two-toned pygmy squid could have widespread consequences, impacting major marine food webs and entire ecosystems.

“Ecosystems are extremely interconnected, so disturbing even one small element can have pervasive effects,” Jodi says. “The two-toned pygmy squid sits in the middle of the food web. They feed on lots of different animals, such as small fish and crustaceans, and big animals like fish and sharks feed on the squid. If these behavioural changes occur at elevated CO2 levels, the squid are sitting in a spot where their actions will effect a lot of different animals and those effects will cascade throughout the ecosystem.”

But it’s not all bad news. As we have seen in the past, many species can adapt to changes in their physical environment. Because the two-toned pygmy squid has a short lifespan, they may be able to adapt and survive in increasingly acidic ocean conditions.

“Identifying the mechanisms that lead to this increased activity will help us to understand the cause-and-effect relationship between higher levels of carbon dioxide and behavioural changes. This will lead to a better understanding of why there is so much variability between animals that are effected by elevated CO2 and animals that aren’t,” Jodi says.

“Ultimately, this research will help us make predictions about how animals will fare in the future, as well as develop conservation strategies for the long-term sustainability of our oceans.”

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