Imagine for a minute that the air in which you live is a thicker, fluid medium growing gradually more and more acidic as another species much larger than yourself pumps gas into it. Now imagine it dissolving away your skeleton leaving you defenceless to predators.
Apart from the fact that you wouldn’t be able to breathe, it may seem a little extreme, but this is what is happening to the exo-skeletons of the ocean’s tiny butterfly snails; pteropods.
Pteropods are free-swimming marine gastropods, most less than 1cm long, which have developed wing-like flaps instead of a muscular foot which they beat to stay afloat. They are a type of zooplankton and make up a hugely important part of the base of the ocean’s food-web. They are prey to krill, small fish and even shellfish.
With the rise in atmospheric CO2,the oceans have absorbed an estimated one third of human carbon emissions. The oceans act as a carbon sink, slowing the effect of global warming, which is great for us terrestrial beings, but poses multiple threats to marine life.
CO2 gas from the atmosphere reacts with water molecules to make carbonic acid (CO2 + H2O = H2CO3) this is why the ocean is ‘souring’ and becoming more acidic.
This poses the first problem to the pteropods as it causes the dissolution of their shells. The second problem is that the carbonic acid partly dissociates to make bicarbonate (H2CO3 = H+ + HCO3–). This dissociation decreases the availability of carbonate ions that the pteropods use with calcium to lay down new shell.
The declining pH of the water not only affects the sea butterflies, but all the shelled marine organisms and has costly knock-on effects for marine food-webs and the rest of the ocean.
In this “Sea” Butterfly Effect, changes in the abundance of these small and seemingly inconsequential critters could result in large-scale trophic shifts or ‘algal blooms’. There will be less food for their predators and they will no longer be controlling the phytoplankton populations.
A group of scientists recently discovered that the shells of Limacina helcina, a species of pteropod, are dissolving and have proposed that they be used as an indicator of declining habitat suitability due to ocean acidification. As Kintisch (2014) put it, ‘Sea Butterflies are a canary for ocean acidification’. They could be our early warning system, like canaries in a coal-mine.
The study measured aragonite, a relatively soluble form of calcium carbonate that organisms use to form their shells. Rising levels of CO2, combined with the increase in sea-ice melt, decrease the saturation of aragonite meaning that less is available to lay down new shell material. They looked at the proportion of shells that were damaged and related it to the saturation of aragonite in the water. They found that the less aragonite was available the more individuals’ shells were damaged.
This next picture shows how a shell (A) is damaged after being in acidic conditions for six days (B).
The study focused on the pteropod-rich California Current ecosystem, an area known to be an acidification hotspot. Winds drive coastal upwelling which brings colder, naturally more acidic water to the surface. The organisms which live there are well adapted to the conditions and that was why the researchers did not expect to see a high degree of damage.
Previously, it has been shown that this damage makes pteropods more vulnerable to infection and predators and can make it harder for them to maintain buoyancy and metabolic activity. Some snails have been able to patch themselves up from the inside, but the decrease in carbonate ion availability will make this harder to do.
Further research into the resilience of these pteropods is imperative for the economy of local fisheries. The Californian fish stocks as well as stocks all over the world rely on stable pteropod populations and the things that eat them as a food source. If pteropods can’t adapt then these fisheries are at the risk of collapse.
Using pteropods as ‘canaries’ may be more important in the polar regions as CO2 is more readily absorbed in colder waters. It is also important in coastal areas as these appear to be breeding grounds due to the high numbers of juveniles found.
The impacts that they have observed will form a baseline with which to compare the effects of further ocean acidification. There is much uncertainty around what impacts changes in sea butterfly populations will have on their ecosystems. Much will depend on the resilience of the pteropods themselves; whether their predators can switch prey and whether other zooplankton which don’t have shells can fill the niche. Whatever the ramifications, these sea butterflies and their response to the rising CO2 will put the effect in motion.
For a very short-and-sweet video on the importance of pteropods take a look at this video.
This post was inspired by an article in Science (paywall).
Kintisch, E. (2014) ‘Sea Butterflies’ are a Canary for Ocean Acidification. Commenting on:
Bednarsek, N. et al. (2014) Limacina helicina shell dissolution as an indicator of declining habitat suitability owing to ocean acidification in the California Current Ecosystem. Proceedings of the Royal Society B. 281:2-8.