Is seaweed stank preventing coral reefs from recovering?

By: Dana Sackett

Chemical cues are ubiquitously used by animals and plants in every environment; often with the intended purpose to attract a nearby friend or repel an unwanted enemy.  A recent article by Dixson and others examined these chemical cues from corals and seaweeds, enemies that fight for habitat in our shallow ocean environments.  Indeed, seaweeds have been winning the battle over the last several decades turning many vibrant species-rich coral reefs into flattened species-poor beds of seaweeds and rubble (to better understand the struggle between corals and seaweeds see Coral Calls for Help and Fish Respond).

coral-reefs-algae-pnas

Top panel: a thriving coral reef. Bottom panel: a reef that has been degraded and taken over by seaweed. Source: http://web.ornl.gov/info/library/ornlnews/archive101811.shtml

In fact, corals have declined by 80% in the Caribbean and by 50% throughout the tropical Pacific in the last several decades.  Some primary reasons for these declines are overfishing (for an example of coral loss from overfishing see our article Trophic Downgrading), climate change (to better understand how this occurs see our article on ocean acidification), and pollution.  As coral reefs are an essential part of our ocean ecosystems, these rapid declines are very concerning.

Some of the stats listed in the Dixson article make the importance of coral reefs very clear.  For instance, despite making-up less than 0.1% of ocean area, coral reefs support 32 of 34 total animal groups (or phyla).  Comparatively, rainforests can only boast supporting 9 of these groups (phyla).  The importance of corals can also be seen in their economic value; creating $29 billion per year in revenue from fisheries, tourism, and other activities.  In addition, coral reefs support the animals that provide protein to hundreds of millions of humans (for more information on corals see our article What Do Las Vegas and Coral Reefs Have in Common).

fig1a1

Acropora coral planula prefer water-borne cues from MPAs to those of non-MPAs (graph). MPA cues can make non-MPA water more attractive, just as non-MPA cues can make MPA waters repulsive (boxes). Source: Science; http://oceanbites.org/chemical-healing-how-coral-larvae-and-juvenile-reef-fish-are-using-good-chemistry-to-choose-a-neighborhood/

This Dixson study not only highlights these important facts but then goes on to examine how the chemical signals from a healthy coral reef within a no-take marine protected area (MPA) and from an adjacent seaweed dominated non-MPA (or fished area) affects the recruitment of coral larvae and juvenile fish.  First, using water from the MPA and non-MPA in a controlled lab setting the authors were able to determine that both coral larvae and fish showed an overwhelming preference for the water from the coral-dominated MPA.

F2.large

Juvenile fish overwhelmingly chose to spent their time in water from the MPA over water from the non-MPA. Source: Science,http://oceanbites.org/chemical-healing-how-coral-larvae-and-juvenile-reef-fish-are-using-good-chemistry-to-choose-a-neighborhood/

The authors went on to identify the specific sources of water-bourne chemical cues that caused this preference by holding seaweed, coralline algae, and corals in MPA and non-MPA water, allowing their chemical cues to leach into the water.  The addition of chemical cues from coral and coralline algae to MPA water caused the water to be even more attractive to the coral larvae and fish, and even made the non-MPA water more attractive.  Conversely, seaweed soaked water caused a decrease in the preference for that water by coral larvae and fish.

F3.large

Adding chemical cues from corals made MPA and non-MPA water more attractive to juvenile fish, while adding seaweed chemical cues to MPA and non-MPA water made those waters less attractive to the fish. Source: Science, http://oceanbites.org/chemical-healing-how-coral-larvae-and-juvenile-reef-fish-are-using-good-chemistry-to-choose-a-neighborhood/

Even more interesting, juvenile fish responded differently to different seaweeds and different corals.  Juvenile fish showed a preference for chemical cues from seaweeds that occur in relatively low abundance on both healthy and degraded reefs over the chemical cues from seaweeds that are abundant on degraded reefs.  Fish also preferred the chemical cues of more sensitive coral species (that generally exist in the healthiest coral reefs) than the more resilient coral species (that exist even in the presence of some degradation).  This finding demonstrates that juvenile fish actively avoid those chemical cues that indicate a degraded reef.

Figure41

Reef fish display nuanced decision-making, preferring cues from sensitive “indicator” coral to more resilient coral species. Similarly, fish avoid seaweeds that are indicative of a full-reef take over, as opposed to seaweeds that occur in both healthy and unhealthy reefs. Source: Science, http://oceanbites.org/chemical-healing-how-coral-larvae-and-juvenile-reef-fish-are-using-good-chemistry-to-choose-a-neighborhood/

While fish and coral larvae likely use a number of senses to determine where to make their home, chemical cues, as the authors point out, are important because they can extend over long distances, letting a fish and coral larvae know that swimming in the direction of the stinky seaweed smell may be a waste of time because there is likely bad seaweed-dominated habitat in that direction, while the delicious smell of corals coming from another direction is probably worth investigating.

The authors of this study were able to demonstrate that coral larvae and juvenile fish are able to respond to chemical cues of reef quality, avoiding the degraded seaweed-dominated reefs and preventing coral recovery in those areas.  The question is then, how do you get rid of the seaweed stank to allow corals to recover in these degraded areas?  One option would be to allow seaweed-eating fish to recover and remove the seaweed.  In addition, adding some of those delicious coral chemical cues may help to expedite the process.

References:

Dixson DL, Abrego D, Hay ME. 2014. Chemically mediated behavior of recruiting corals and fishes: A tipping point that may limit reef recovery. Science 345:892-897.

http://web.ornl.gov/info/library/ornlnews/archive101811.shtml

http://www.wired.com/2011/10/toxic-algae-coral-reefs/

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