Ocean Acidification makes corals quicker to sting each other with their guts

Written by: Melissa Naugle and Edited by Lynn Bonomo

When researchers study ocean acidification and corals, they usually study how well corals survive or how fast they grow. But, it’s also important to consider interactions between corals, such as competition. Corals have an interesting way of competing for space where they sting nearby neighbors. In this article, the researchers studied how corals will compete for space in the future under ocean acidification.

Why study this?

As we already know, global climate change increases the amount of dissolved carbon dioxide in the ocean, leading to ocean acidification. Increased acidity in ocean water can slow coral growth¹ since increased carbon dioxide concentrations throw off their ability to form aragonite- the main component of their skeletons. In addition to physiological effects, ocean acidification can also affect species interactions, such as competition. Past research has shown that ocean acidification tends to benefit algae over coral, since algae can grow faster than corals in more acidic ocean water². However, fewer scientists have asked how ocean acidification will affect coral-coral competition. It is important to understand how corals compete because these competitive interactions contribute to the species distributions that we see on coral reefs. This is especially true in the context of ocean acidification so that we can understand how corals will compete in the future and can identify potential ‘winners’ and ‘losers’. These results will help provide insight into future reef structure and coral diversity.

Rude Neighbors

Before we continue, lets go over coral competition. Since corals don’t move much, it’s easy to forget that they are actually active competitors. In addition to outgrowing or shading over their competitors, corals can also use their mesenterial filaments (Fig 1), little strings of internal tissue that contain stinging cells called nematocysts. If another coral or alga gets too close, a coral can extend its mesenterial filaments to sting and literally digest part of its neighbor. This sting damages the neighbor’s tissue to free up nearby space so the coral can continue growing. While some coral species are better at this than others, it is an effective strategy they can use to compete for valuable space.

So how does ocean acidification affect this?

That’s what this study aimed to find out! The researchers used two species of coral: Galaxea fascicularis and Acropora hyacinthus. Both species are stony corals found in shallow waters of the Red Sea, but G. fascicularis is more aggressive than A. hyacinthus and has been shown to win in competitive interactions³. Researchers found one colony of each species in the Red Sea and broke off eight single polyps of each. Next, they randomly assigned each to an ambient (600μatm) treatment or an elevated pCO₂ (1200μatm) treatment. pCO₂ is the partial pressure of carbon dioxide, which reflects the amount of carbon dissolved in the seawater. So, a higher pCo₂ value means that the water is more acidic. After being given one week to adjust to their respective treatments, the polyps were glued onto plates and assigned into pairs, creating a miniature boxing ring for the corals to compete in (Fig 2). In this structure, corals were glued to glass slides perpendicular to each other with approximately one millimeter of space between them at their tips. To study competition, the researchers looked for evidence of their mesenterial filaments over the course of seven days.

Effects of increased CO₂

 The researchers found that G. fascicularis exposed to high CO2 extended more mesenterial filaments on day four of the study, while the ambient (normal CO₂) treatment extended more mesenterial filaments on day six (Fig 3). Also, the high CO2 corals extended more mesenterial filaments (~10) than the controls (~6), but this finding was not statistically significant (Fig 1). In their analysis of dead coral tissue, the researchers did not find any difference in the amount of damage that G. fascicularis inflicted on A. hyacinthus between the two treatments.

What does this all mean?

This study found no effect of elevated CO2 on the amount of competition between corals or the amount of damage done by mesenterial filaments, but there was an effect on the timing. The timing advantage did not affect the eventual damage, which was the same between the two treatments. Therefore, during ocean acidification corals may be able to continue competing in the same way they do normally. This provides hope in the context of coral-algae interactions that have been affected by ocean acidification. In increased CO2, algae tend to outcompete corals since coral cannot grow as fast as skeleton-less algae. Yet, this study provides evidence that corals will be able to continue competing using their mesenterial filaments, even in more acidic oceans. Further studies will have to continue studying competitive coral interactions under climate change in order for us to get a better idea of the future of coral communities.

Source: Evensen, NR and Edmunds, PJ. (2018) Effect of elevated pCO₂ on competition between the scleractinian corals Galaxea fascicularis and Acropora hyacinthus. Journal of Experimental Marine Biology and Ecology, 500: 12-17. https://doi.org/10.1016/j.jembe.2017.12.002

References:

1. Hoegh-Guldberg, O., Mumby, P.J., Hooten, A.J., Steneck, R.S., Greenfield, P., Gomez, E., Harvell, C.D., Sale, P.F., Edwards, A.J., Caldeira, K. and Knowlton, N., 2007. Coral reefs under rapid climate change and ocean acidification. Science, 318(5857), pp.1737-1742.
2. G. Diaz-Pulido, M. Gouezo, B. Tilbrook, S. Dove, K.R.N. Anthony High CO2 enhances the competitive strength of seaweeds over corals. Ecology Letters, 14 (2011), pp. 156-162
3. Dai, CF. Interspecific competition in Taiwanese corals with special reference to interactions between alcyonaceans and scleractinians . Marine Ecology Progress Series, 60 (1990) pp 291-297.