As more reefs degrade, however, scientists continue to struggle to understand why. Top-down theories suggest overfishing of predator fish leaves fewer herbivores to eat the seaweed. Bottom-up viewpoints contend that rising temperatures and runoff from fertilizers on land cause the seaweed to over grow, throwing the ecosystem out of whack.
Realistically, both things are happening, Dixson said.
She believes other things are happening, too. Her research teams study findings suggest that seaweeds alter the corals biochemistry, and that increased seaweed cover may cause further coral declines by changing the ways organisms forage or interact there.
The paper grew out of Dixsons post-doctoral work with Mark Hay, a professor of biology at the Georgia Institute of Technology who studies seaweed-coral competition.
We know from Marks work that seaweed placed near coral improves the seaweeds taste because the algae spends its energy fighting coral instead of producing nasty chemicals to deter herbivorous fishes. My study looked at the other side of the coin–how the presence of seaweed affects coral palatability in fish, Dixson said.
In the experiment, Dixson videotaped and counted butterflyfish interactions with reefs that had chemically active (Galaxaura filamentosa) and chemically inactive (Sargassum polycystum) seaweeds and control reefs without seaweed. Because butterflyfish are territorial, she moved the experiment to multiple locations to ensure new butterflyfish visited the test site each time.
Parents are always trying to find ways to add vegetables into their childrens diet, by hiding extra carrots in tomato sauce, for example. But what if the child could smell the carrots even without seeing them? Dixson said. Its not about memory, there is something happening with the coral-seaweed interaction that makes the coral unattractive to butterflyfish.
Whether the corals are chemically changing internally in a way that makes them less nutritious or they actually taste bad due to the energy expended to defend themselves against the seaweed is unknown.
The ripple effect
Then there is the ripple effect–how does this negative coral-seaweed interaction affect other reef organisms and what is the larger implication for the ocean if this imbalance continues to grow?
Drastic differences already can be seen in places like Fiji, where a beautiful coral reef can be found right next to a seaweed-choked wasteland. Seaweed overgrowth is a consequence of environmental change, but it is not as simple as more seaweed hurts coral and less corals hurts fish – there are animal behaviors that could be driving this, as well.
Butterflyfish clearly have evolved the ability to detect changes or differences in coral reefs and are choosing habitat and food supplies based on these cues. If this pattern is present in other fish, it could have ramifications including putting undue pressure on healthy corals by overeating them while avoiding those they dont like Dixson said.
To begin answering these larger questions, Dixson said, requires adding behavioral effects into the mix of how scientists think about habitat degradation.
We need to start understanding these interactive effects, especially the behavioral choices that could be exacerbating issues that were not even thinking to give fish credit for, she said.
So, where does Dixson go from here?
She and Brooker plan to dive deeper into the chemistry of the coral-seaweed interaction to explore whether seaweed affects the corals nutrition, taste or something else they havent considered. They are also curious about whether coral-seaweed interactions affect a fishs ability to chemically camouflage itself against predators.
We know animals hunt with chemistry, so it makes sense that they would hide themselves with chemistry, too, Dixson said. Chemical camouflage is an emerging field with potential for helping us better understand how organisms interact and predator-prey dynamics. I think UD can be a leader in this field.
Chemical warfare on the reef: How certain seaweed species harm corals
Rohan M. Brooker et al. Cryptic effects of habitat declines: coral-associated fishes avoid coral-seaweed interactions due to visual and chemical cues, Scientific Reports (2016). DOI: 10.1038/srep18842