How might climate change affect baby sharks?
SHOW NOTES
Before diving into Noémie’s work, we first get to know her a little better and her journey to studying for a PhD [5.02]. Noémie was actually afraid of the water at first, and felt scared when swimming especially when the visibility was bad and she couldn’t see what lay beneath. Then, one year, she injured her ankle badly. During a holiday with family, Noémie’s parents hired a small boat and Noémie dangled her feet over the side, allowing the cool ocean to splash over her injured ankle. It was one of the few things that provided some relief from the pain, and allowed Noémie to appreciate the healing nature of the ocean for the very first time. That appreciation slowly turned into curiosity, and she began overcoming her fear in favour of exploring. Learning to scuba dive really cinched the deal; a whole new world had opened up to her, filled with fascinating creatures to learn about.
Noémie had always held a particular fascination for sharks, but it was a strange twist of fate that led her to be working with them [8.10]. There wasn’t much opportunity to study sharks in France, so Noémie started out researching other fish species and began to specialise in oceanography. She first looked at the impact of the El Niño events on the larval recruitment of commercially important fish species on Reunion Island, before learning to use acoustics to study the abundance of pelagic fishes in the Bay of Biscay. Noémie was about to head into the field for the latter project when the pandemic hit – derailing her plans and keeping her locked indoors, like the rest of the world! Although this seemed detrimental at first, it also afforded Noémie the freedom to look into online courses. She took one on sharks and this led to a research project on the functional rarity of shark and ray species in the northeast Atlantic. Noémie was able to demonstrate that species were not only scarce in abundance, but also possessed unique morphological and behavioural traits that set them apart. It was this experience – along with the skills she had gained in her previous research projects – that led Noémie to be accepted onto her PhD programme, the first project to assess the impact of warming and ocean acidification of sharks and skates in the northeast Atlantic.
Today, we are mainly focussing on one part of Noémie’s PhD – the impacts of warming and ocean acidification on shark development – but a PhD is a lot of work, and Noémie has done important work on other areas [11.16]. In a paper published in the journal Global Change Ecology earlier this year (2024), Noémie examined the effects of climate change on different habitats for sharks and skates in this region. She found that as a result of warming temperatures and increased acidification, particularly in coastal regions, six out of nine species have experienced significant change in their suitable habitat between 1997 and 2020. As a result, these species have shifted their distribution, moving into other areas mainly in the north. Additionally, warming and acidification are affecting critical habitats where egg-laying species – such as the small-spotted catshark – would lay their eggs. This prompted Noémie to spend the rest of her PhD investigating what this could mean for these sharks in their earliest stages of life.
Before hearing about what Noémie has been up to and what she has been able to find out, we take some time to meet the star of her PhD: the small-spotted catshark [14.17]. They are a small species of shark that are very common to the northeast Atlantic ocean, and can be found hanging out near the seabed or weaving their way through kelp forests. With their large, almond shaped eyes, spotted pattern, graceful, elegant movements and propensity for napping, it’s not hard to see where the comparison to cats came from. Small-spotted catsharks are an egg-laying species, and come to shallow, coastal habitats to lay their eggs. The eggs themselves are rectangular in shape, with a tough, outer casing that is designed to look like a small piece of kelp. This helps to camouflage the vulnerable embryo and hide it from predators. At either end of the egg are long, curly tendrils that attach onto the substrate and prevent it from being swept away with the tide and current. And each egg is as unique as a human baby – Noémie, who has raised hundreds in the lab, says that she has seen all kinds of different colours of eggs from the same female. Within this protective casing and over many months, the blob-like embryo slowly develops into something more shark like, getting nutrients from its yolk-sac [21.33].
So, how is Noémie able to observe the effects of warming temperatures and acidification on shark embryos [24.00]? Well, it’s all simulated in a lab. Luckily, Noémie’s research takes place in a marine lab, where sea water comes out of the taps! So, she can provide everything a baby small-spotted catshark might need. In separate tanks, Noémie creates different conditions by altering temperature and acidity, that aim to replicate those in the real world so that different future scenarios can be compared. The first scenario is what the sea may look like if we achieve carbon neutrality by 2050 (which is assumed to be an annual temperature increase of 2 degrees Celsius). The second (and slightly more worrying) scenario where we continue on our current trajectory, seeing greenhouse gases triple by 2075 which would result in an annual temperature increase of 4 degrees Celsius. She compares these two future possibilities with what sea conditions are like now. In all scenarios, Noémie and her research team included monthly variations in the conditions to mimic seasonal fluctuations. This is important as egg-laying peaks in June, as temperatures in the north Atlantic are already warm – meaning that embryos are already subjected to temperatures above the annual average. To give you some idea of what that might look like in the second scenario – where no effort has been made to slow climate change – temperatures in the English channel would reach 23 degrees Celsius. In present day, it averages around 15-18 degrees. This may not sound like much (and to those of you who prefer warm water, maybe quite nice!) but it is a big increase and a potential stressor for our cold-water species, like the catshark.
Noémie took pictures of the embryos inside the eggs by shining a torch through one side, so that the silhouette of the embryo was clearly visible [26.11]. In doing this, she could monitor and assess their growth right until the point of hatching. This is where she got the nickname “mother of sharks”! Noémie says although it was fascinating, it was also very stressful, especially when you consider that she was raising not just a few catsharks, but hundreds. And, apparently, they all decided to hatch at once!
Something that Noémie was trying to monitor specifically was how much oxygen an embryo was consuming during a behaviour called ‘freezing’, and whether this increased or decreased in the different climate scenarios [28.20]. ‘Freezing’ is a very important behaviour that occurs when an embryo senses a predator. To hide itself, the embryo will stop moving and breathing for a short time until the coast is clear. However, this is a very costly behaviour in terms of energy. If an embryo is already stressed – due to climate change for example – it might not have much energy left to enact freezing, which could limit the effectiveness of this avoidance strategy, or even stop it altogether. This could result in higher levels of predation, which isn’t good news for the survival of the population.
Another potential problem of acidification specifically is that it could affect the shark’s blood and skeleton [30.40]. Sharks take in water as they breathe; water flows over the gills, where oxygen is absorbed into the shark’s bloodstream. If the surrounding water is more acidic, it can lead to acidification of the blood. The shark has some ways to counteract this, including by producing more bicarbonate. This is a substance called a ‘base’, which counterbalances the acid, helping the body keep a normal pH (pH being a measurement of how acidic something is). But producing more bicarbonate can also change the structure of the mineralised parts of a shark’s body – its skeleton, teeth and even dermal denticles (it’s skin!). It’s still very early days on this part of the project and Noémie is still working on getting answers to these questions. It does seem as though there are discrepancies between the different conditions, particularly in the vertebrae and teeth, and that while some of these continue to the juvenile stage, some do not. But why this is, Noémie and her team are not sure – you’ll have to follow along with her work to see the results when they’re ready!
Noémie’s work so far has highlighted three interesting findings [33.51]. First is that mortality greatly increases with increasing temperatures. Under current conditions, mortality (or, the number of embryos that do not make it to the juvenile stage) is around 17%. However, that percentage increases to 81% under the second scenario (temperature increase of 4 degrees Celcius). This is almost a four fold increase in mortality, which would be absolutely devastating. Embryonic mortality also peaked in August, one of the warmest months. The second interesting finding is that a critical stage in embryonic development – the transition of the gills from outside to inside the body – occurred at this same time, which could be an explanation as to why mortality was so high. And finally, Noémie found that there was variation in individuals exposed to the same treatment. While some did not show “normal” growth in the more extreme climate scenarios, they were still able to hatch. This could suggest that some have a phenotypic adaptation – a change in its body due to changes in the surrounding environment. Whether this means that the species could evolve to adapt to warming seas is yet to be understood – it would require genetic studies, and Noémie has only observed it in four individuals so far. But, it is still very interesting.
But what happens once the embryo has hatched [38.04]? Well, Noémie has found that there is little difference in juvenile sharks in the control group (conditions close to current day) and those in the 2 degrees warmer scenario. This is promising, as it suggests that, if we are able to limit our emissions and keep warming within 2 degrees, the impact on small-spotted catsharks may be fairly minimal.
Noémie is now working on something called a ‘bioenergetic model’ [38.50]. She has already determined that climate change can affect the number of hatchlings, a very important parameter when it comes to population dynamics. But, she is convinced the effects don’t stop there. Temperature governs all the physiological processes taking place in the body – whether an individual grows, whether they will produce offspring, and whether they will survive. In the model, Noémie is linking population dynamics to the physiological constraints of the species, and testing the effects of increased temperatures on these parameters. As the small-spotted catshark is a ‘model’ species – small, easy to breed in the lab, and found just about everywhere in the northeast Atlantic. Therefore, any effects of climate change on this species are the least to be expected from other, less adaptable or more endangered species.
ABOUT OUR GUEST
NOÉMIE COULON
Since she was a child, Noémie has been attracted to all kinds of animals and she always knew she wanted to work with them. However, she was afraid of the ocean and the creatures in it. It wasn’t until her first dive, at the age of 19, that fear turned into passion. After that, Noémie dedicated her studies to oceanography and marine ecology.
She first studied larval fish recruitment during El Niño events as an undergraduate, determined the abundance of small pelagic fish using acoustics in the Bay of Biscay (in France) and, as a Master’s student, studied the functional rarity of endangered fish communities in the north-eastern Atlantic. Today she is a doctoral student at the mixed research unit Biologie des organismes et des écosystèmes aquatiques (BOREA) where she studies the effects of climate change on sharks and rays in the north-eastern Atlantic.
You can follow Noémie’s work on Instagram (@noemie.coulon.9); Twitter (@Noemie_Coulon); the SOSF website and her own website.
You can also read Noémie’s latest research article here.
