The Sea Butterfly Effect

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.

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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.

sea butterfly 2

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.

Ocean FoodWeb

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.

Damage related to undersaturation

This next picture shows how a shell (A)  is damaged after being in acidic conditions for six days (B).

pteropod before and after exposure to acidification

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.

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Don’t judge a fruit by its mesocarp!

The old saying goes ‘don’t judge a book by its cover’, but for anyone who’s studied a bit of botany the fruity twist will be appreciated. (Credit to Stephen Vander Wall)

The reason I criticise the judgement of outward appearances is because the two atypically pretty plants below are going to be the subjects of my attention this year and had someone cast them aside as unattractive I might not have such an interesting albeit ‘nerdy’ project to work on.

blog erica flowers

These two species are endemic to small areas in the Western Cape and are likely to have out-of-the-ordinary pollination systems as they are not attractive to the usual pollinators that we associate with Cape Fynbos.

Erica nabea and Erica occulta have not yet been studied and this leaves me with the urge to figure out what is pollinating them or if they are fertilising themselves.

E. nabea is found in the Outeniqua Mountains above George and Knysna and grows to about 1.5m in height. Its flowers are green and white and appear between May and August.

E. occulta is found exclusively on a small patch of limestone on the Southern Agulhas Plain near Pearly Beach. It flowers between August and October and its flowers are hidden in a mass of hairy leaves. It is extremely localised and its <6 km² distribution is threatened by the alien invasive, Acacia cyclops and the potential construction of a nuclear power plant.

Blog eric distribution

Unfortunately, removing the threat of the competing acacia and preventing the construction of a nuclear power plant may, in this case, be an easier feat than rescuing this species as its population is so small that it could be on the verge of extinction. The population consists of about 50 individuals and their genetic diversity and their subsequent ability to adapt to change may be very low.

As part of my Honours project I am going to look at the micro-satellites of these two species to assess the level of heterozygosity in their populations. The heterozygosity of the micro-satellites is a fancy way of saying that I am going to investigate the genetic variation of the plants in the population.

But, some species are adapted to being selfers (they fertilise their ovules with their own pollen). These species don’t need to invest in making colourful flowers or lots of nectar to attract pollinators. E. nabea is an Adelopetalum meaning unseen or secret and E. occulta is named from occultus meaning secret or hidden. Maybe these two species are adapted to selfing and hopefully by the end of my experimenting and observing I will be able to understand what these plants are doing so secretly. I also hope that they still have enough genetic diversity to see them through the imminent climate change as it will be sad to see such unique plants go extinct.