Bleached Sea Anemone
It’s a scene that left us sobbing: little, adventurous clownfish Nemo plucked away from his loving father. It’s a story that left us with tears of joy: good-hearted Dory finally reunited with her devoted parents. Now, all are safe and sound in the Great Barrier Reef, right? Wrong. Turns out that the Great Barrier Reef is in much danger due to intense coral bleaching. So, perhaps Disney needs to consider a much needed third installment: Saving Nemo.
Coral reefs are among the most productive and diverse environments in the world. Even though they constitute less than 1 percent of Earth’s aquatic ecosystems, these “tropical rainforests of the ocean” support nearly 25 percent of marine species, including coral and anemone species.
Yet, coral and anemone species are sensitive; they can survive only in narrow temperature, UV radiation, and nutrient quantity ranges. Unfortunately, human-induced climate change, ozone layer depletion, and water pollution are disrupting these temperature, UV radiation, and nutrient quantity balances. As a result of these changes, the Great Barrier Reef is undergoing the worst bleaching event the world has ever seen, endangering many animal species that depend on reefs, including clownfish.
But what exactly is coral bleaching? Picture a vibrant reef packed with corals and anemones of every color imaginable. Now imagine the same scene but with only one color: white. This depressing image, resembling a lifeless forest in the dead of winter, is a bleached coral reef.
Coral reefs get their vivid colors from tiny algae called zooxanthellae that live within their tissues. The algae provide corals and anemones with food and structural reinforcement and, in return, receive protection and carbon dioxide for photosynthesis. When corals and anemones are stressed due to changes in water temperature, light, or nutrients, they expel the algae, leaving the corals and anemones white and susceptible to disease. Coral reefs can recover, but if the stress continues for long periods of time, they eventually die.
Clownfish, in particular, have an intimate relationship with anemones: anemones protect clownfish, while clownfish provide anemones food. Due to the concerns over the Great Barrier Reef’s extreme fragility, a recent study in the Proceedings of the Royal Society B highlights the effects of habitat degradation on clownfish survivability. Anna Scott of Southern Cross University and Danielle Dixson from University of Delaware tested five clownfish species and three anemone species to assess whether clownfish can identify if their potential home is bleached or healthy and how this affects where they choose to live. The fish were released into a device called a choice flume containing two water streams flowing at the same speed but leading to two different water sources with different chemical cues.
What the researchers found was concerning. While the clownfish can identify their host’s health status through smell, they are inflexible in selecting their home. Essentially, clownfish associate with certain anemone species only, regardless of health status. Even when unbleached non-hosts are available, once they’re settled, clownfish cannot adapt.
This inflexibility puts clownfish at risk. Habitat destruction decreases egg production. Moreover, clownfish in bleached anemones have an increased predation risk because they stand out against the white background. Thus, declining habitat quality has harmful cascade effects: bleaching leads to anemone mortality and subsequently fish mortality due to interdependent relationships. Unfortunately, this could mean no more Nemo.
However, some might argue that bleaching events are temporary, and coral reefs will adapt to changing conditions. Protective mechanisms such as repopulation with algal types with greater stress resistance or alteration of current algae’s photochemistry and heat stress response could reestablish reef dynamics. This was certainly true the past three decades. However, Tracy Ainsworth of James Cook University emphasizes that “[these] protective mechanisms are likely to be lost under near-future climate change scenarios.” Thus, while bleaching could be overcome in the past, predicted drastic increases in water temperature will disable these mechanisms and leave bleaching permanent.
Dishearteningly, if nothing is done, coral death could exceed 95% regionally, leaving many species at risk for extinction. World Wildlife Fund’s Rick Leck stresses that “coral bleaching is the most visible example of climate change.” The severity of current human-caused conditions demand change. Actions such as converting to renewable energy, reducing agricultural run-off, and even planting trees can help reefs and the species they support flourish.
Bottom line? Coral reefs and clownfish need our help. While Disney can wait 13 years to release Finding Nemo’s sequel, the Great Barrier Reef cannot wait this long for action.
Ainsworth, Tracy D., Scott F. Heron, Juan Carlos Ortiz, Peter J. Mumby, Alana Grech, Daisie Ogawa, C. Mark Eakin, and William Leggat. “Climate Change Disables Coral Bleaching Protection on the Great Barrier Reef.” Science 354.6283 (2016): 338-42. Web. 5 Sept. 2016. <http://science.sciencemag.org/content/352/6283/338.full?ijkey=n33yVo7R6IEKc&keytype=ref&siteid=sci>.
Scott, Anna, and Danielle Dixson. “Reef Fishes Can Recognize Bleached Habitat during Settlement: Sea Anemone Bleaching Alters Anemonefish Host Selection.” Proceedings of the Royal Society B 283.1831 (2016): n. pag. Web. 5 Sept. 2016. <http://rspb.royalsocietypublishing.org/content/283/1831/20152694>.