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Of all the beautiful and varied natural ecosystems on earth, few have inspired awe like coral reefs. And rightly so – these colorful structures found on shallow ocean bottoms in tropical and subtropical regions of the world are not only pretty to look at (Figure 1), but also support an incredible diversity of life and are thus often called “rainforests of the sea” [1]. Some estimates suggest that a quarter of all ocean species depend on reefs for their survival, even though reefs only cover 0.2% of the ocean floor [2]. Additionally, coral reefs are incredibly important for humankind because they provide food for millions of people through fishing, generate revenue through tourism, and protect coastlines from storms. Indeed, their economic value has been estimated to be almost $30 billion per year [3]. It is no surprise, therefore, that the rapid decline of reefs observed since the 1970s has caused alarm among environmentalists worldwide.


Figure 1 ~ Coral reefs are ocean habitats formed by thousands of tiny coral polyps and found in tropical and sub-tropical regions of the world. (A) ‘Coral reefs in Mbonege beach, Honiara, Solomon Islands’ by Sharon Suri available at https://www.flickr.com/photos/theworldfishcenter/9243632999. Licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic license. (B) Representative drawing of a coral polyp, the individual organism that, as part of a whole community of corals, builds a coral reef. See http://www.barrierreefaustralia.com/the-great-barrier-reef/coralfacts.htm for more details. (C) Map showing distribution of corals around the world. Image courtesy of the National Oceanic Atmospheric Administration ( http://oceanservice.noaa.gov/education/kits/corals/media/supp_coral05a.html).

What are coral reefs, and why are they at risk?

Coral reefs are started by tiny organisms known as coral polyps (see Figure 1B), which are invertebrates in the phylum Cnidaria. Their closest relatives are sea anemones and jellyfish. The polyps have soft, cup-shaped bodies but generate a hard external skeleton made of calcium carbonate or limestone using ions absorbed from seawater. These skeletons form the structure of coral reefs, with different coral species forming different shapes [4]. The vibrant colors of many reefs come from species of algae known as zooxanthellae that live among the corals – the corals provide shelter for the algae while the algae generate energy through photosynthesis and provide food for the corals [4]. The relationship between these species is therefore mutually beneficial, or symbiotic. Once these organisms die, their skeletons remain for new corals to build upon. A living reef is therefore a colony of corals, algae, and the multitude of marine species they support. It is estimated that some of the reefs we see today started growing more than 50 million years ago [5].

These ancient ecosystems have been struggling to cope with the many changes that are occurring in the world today, such as fluctuations in temperature, sea level, and ocean salinity. When corals are stressed by these and other environmental changes, they start to evict the zooxanthellae. This leads to “bleaching” (the corals turn white following the loss of the algae) and can eventually cause the death of the reef [6]. Corals are also sensitive to the balance between the various life forms living among them. For example, if overfishing removes natural predator species in the food chain, prey species such as seaweed, algae, or starfish can start growing out of control and take over the reef, which jeopardizes the health of the corals. Therefore, better management of fishing practices, along with tactics to limit water pollution and other environmental fluctuations experienced by the corals, are being applied worldwide to protect these important ecosystems.

Corals can “smell”?

While the conservation efforts described above might help the survival of adult corals, reef recovery is still not fully understood, with some reefs showing signs of recovery and others not. Therefore, it is also important to understand how young corals are recruited to new sites. With this in mind, a group of scientists led by Danielle Dixson at the Georgia Institute of Technology set out to determine if young corals might be able to distinguish between different reef sites based on the “chemical cues” they emit. Such an ability could be considered analogous to our sense of smell, which allows us to detect chemicals in the air. The findings, published last month in the journal Science, show that young corals as well as young fish are attracted to water from healthy reefs over that from dying reefs [7].

To test for these preferences, the scientists took samples of ocean water from a number of different reef sites off the coast of Viti Levu in Fiji. They chose pairs of sites that were right next to each other and should therefore experience similar environmental conditions, but in each case, one site fell in a marine protected area (MPA) where fishing is prohibited, while its paired site did not. The protected sites had healthier reefs with a higher percentage of coral cover while the unprotected sites had degraded reefs dominated by seaweed instead. The scientists placed juveniles of three different coral species and fifteen different fish species in flow tanks containing water from both sites (Figure 2). In each case, the test organism was placed downstream of where the two different kinds of water were flowing into the tank, and was free to swim towards either water source. Both the corals and the fish showed a strong preference for the water from healthy reefs – each test species spent more than 80% of its time swimming towards water from MPAs instead of water from non-MPAs. These results suggest that the young corals and fish are not only able to detect some difference in the water coming from different environments, but also to respond to these cues with movement, swimming towards the water they prefer.

Figure 2 ~ Schematic showing how the preference of individual fish or coral polyps for different kinds of water was measured. (A) The authors used tanks in which water from two different sources flowed in from one end and the fish/corals were placed at the other end (the view shown here is from above the tank). A partition (shown on the right hand side) minimizes mixing of water near the source. At the end of the tank where the fish or coral is placed, the organism is free to swim between the two kinds of water and towards either water source. After a 2 minute adjustment period, the organism was observed for 3 minutes and its position recorded every 5 seconds. Then the organism was removed from the tank, allowed to rest for 1 minute, and then placed back in the tank with the two water sources being swapped — for example, if in the first test MPA water came in from the top right and non-MPA from the bottom right, in the second test MPA water would come in from the bottom right and non-MPA from the top right [7]. (B) The amount of time spent on each side of the tank was quantified to measure the organism’s preference for one type of water over the other. 

As the authors of this study told the BBC in an interview, this was particularly surprising for the corals because they are such simple organisms and don’t have noses or tails like the fish do, and their movement is generally thought to be driven by ocean currents [8]. Interestingly, the idea that corals can detect different types of environments is not completely unprecedented [7]. For example, it was known previously that corals do not attach to surfaces close to colonies of seaweed. However, if the corals can respond to signals in the water rather than the seaweed itself, it suggests that these signals can act over much greater distances than previously appreciated.

The complexities of conservation

The results of this study suggest a few ways in which coral reef conservation could be made more effective. For example, current tactics include the demarcation of areas where fishing is not allowed, such as the MPAs described above. While it is crucial that local people have access to areas where fishing is allowed, the results described here suggest that some fish species are particularly important to protect, even outside MPAs. One of the studies’ authors, Mark Hay, is working in Fiji to encourage locals to specifically avoid capturing certain species of herbivorous fish, those that eat the seaweed that discourages corals from settling nearby, as well as to physically remove seaweed during the time of year that young corals settle down [8]. These measures should help keep seaweed levels under control, and hopefully increase the attractiveness of the reefs as sites for young corals and fish to settle.

While these findings mark an exciting advance in the field, there are still open questions remaining. For example, it remains to be clarified what exactly the chemical signal is that attracts the corals, or how the corals detect these chemicals. The authors addressed the former question by adding different species of coral, algae or seaweed to the water they were testing, and saw that adding healthy corals or algae increased the attractiveness of the water, while adding seaweed made the water less attractive. Interestingly, while corals were generally attracted to their own species compared to other single species, they were even more attracted to diversity, with water that had been exposed to a mixture of different corals trumping the water coming from the same species [7]. This suggests that the signaling taking place is complex, and will likely vary between reefs with different amounts of diversity. This potential complexity also raises the question of whether the results of this study will be broadly applicable to reefs worldwide. As pointed out by marine ecologist John Bruno, the sites used in the study either had very high seaweed coverage (49 to 91%) or very low (1 to 2%), but in nature most reefs fall somewhere in between these two extremes [9]. In this range, the signals coming from healthy versus unhealthy reefs may not be as easy to discriminate.

Despite these unanswered questions, the study led by Dixson et al. demonstrates the remarkable ability of the “simple” coral polyps to distinguish between different environments, and show that other organisms such as fish care about the health of a reef too. Restoring a reef environment will therefore depend not just on the reintroduction of corals, but also on controling of seaweed and perhaps other species that invade dying reefs — or at least, getting rid of the “smell” of these species. You wouldn’t want to live in a place that smelled funny, would you?

Niroshi Senaratne is a graduate student in the Biological and Biomedical Sciences program at Harvard Medical School. Special thanks to Kaitlyn Choi, a preceptor in the Department of Stem Cell and Regenerative Biology at Harvard University, for figure design.

REFERENCES

1) “Corals and Coral Reefs” by the Ocean Portal Team; reviewed by Nancy Knowlton, Smithsonian NMNH. http://ocean.si.edu/corals-and-coral-reefs

2) Spalding MD, Ravilious C & Green EP (2001). World Atlas of Coral Reefs. Prepared by the UNEP-World Conservation Monitoring Centre. University of California Press, Berkeley, USA.

3) Cesar H, Burke L & Pet-Soede L (2003). The Economics of Worldwide Coral Reef Degradation. Cesar Environmental Economics Consulting (CEEC), The Netherlands. https://www.wwf.or.jp/activities/lib/pdf_marine/coral-reef/cesardegradationreport100203.pdf

4) “Coral Polyps – Tiny Builders” from the Coral Reef Alliance. http://coral.org/coral-reefs-101/coral-reef-ecology/coral-polyps/

5) “Coral (Anthozoa)” from the National Geographic Society. http://animals.nationalgeographic.com/animals/invertebrates/coral/

6) “What is Coral Bleaching?” from the National Oceanic and Atmospheric Administration. http://oceanservice.noaa.gov/facts/coral_bleach.html

7) Dixson DL, Abrego D & Hay ME (2014). Chemically mediated behavior of recruiting corals and fishes: A tipping point that may limit reef recovery. Science 354(1699): 892-7.

8) Webb J (2014, August 21). Coral and fish can “smell” bad reefs. BBC World Service. http://www.bbc.com/news/science-environment-28880216

9) Bruno JF (2014). How do coral reefs recover? Science 354(1699): 879-80.

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