A palette of cool, colorful hues light up the waters of Tatakoto Atoll, part of the Tuamotu island network, in French Polynesia. Until December 2008, the only giant clam to inhabit French Polynesia—Tridacna maxima—was abundant in all of its blue, purple, and green glory. In early 2009, white and empty clam shells occupied the once vibrant ocean floor. In 2013, researcher Serge Anréfouët of Institut de Recherche pour le Développement and colleagues authored a paper in the journal Biological Conservation, which stated that the T. maxima population density decreased by 83% during that span of several months (Andréfouët et al., 2013). What could possibly be responsible for such a large loss, within mere months? Andréfouët and colleagues set out to determine the cause of this mysterious population loss, as well as formulate conservation strategies to prevent future giant clam die-offs in French Polynesia.
The population loss, though large (roughly 20 million clams in 2004 decreased to 1.9 million clams in 2012), surprisingly went unnoticed by the local population. The appearance of field clues (e.g. smaller size structures and recruitment loss) pointed to evidence of mass mortality. Andréfouët and colleagues proposed that the mortality occurred roughly three years before their 2012 survey, due to the shortage of smaller individuals (i.e. less than 6cm). This deficit indicates a loss of recruitment for the past three years.
Since 2004, there has been a no-take area, the first of its kind in all of French Polynesia. The population loss affected all of the atoll, no-take area and all. Therefore, overfishing was eliminated as a cause of the die-off, since the giant clams were dying both in and out of the no-take area, as well as the nature of non-intense local hunting of clams. Additionally, pollution was eliminated as a cause because Tatakoto has no large source of land-based pollution.
A second explanation for the mass mortality is environmental factors. For example, El Niño Southern Oscillation (ESNO) causes increased sea surface temperature and lowered sea levels, both of which have major consequences for giant clams. Lowered sea levels expose the clams for an unusually long time, resulting in the clams drying out and dying. The increased sea surface temperatures also affect giant clams negatively because giant clams, like coral, contain zooxanthellae, a symbiotic micro-algae. Without this, corals are considered to be “bleached,” due to the white color left when the organism is driven out by physical or thermal stress. Perhaps extreme sea temperature led to the loss of symbiosis with zooxanthellae, leading the clams to have an eerie, white appearance.
A third explanation suggested in Andréfouët and colleagues is an unconfirmed report of bivalve mortalities from blooms of cyanobacteria Lyngbia majuscule. It is unclear whether this mortality occurred because of the cyanobacteria’s toxicity or a decrease in light from massive blooms.
Another threat to giant clams that can lead to their death is ocean acidification. Ocean acidification occurs when CO2 reacts to form carbonic acid, which in turn lowers the ocean’s pH. A 2015 PloS ONE study by researcher Sue-Ann Watson of James Cook University showed th that elevated levels of CO2 reduced survival and growth in giant clam T. squamosal juveniles (Watson, 2015). Just how bad are the effects of CO2 on our oceans? According to ocean acidification expert Dr. Sue-Ann Watson, “by the end of the century, if we carry on with business as usual [CO2 production], [oceans] will be 150 percent more acidic than they were 250 years ago” (DelViscio, 2014).
With all of these possible explanations, Andréfouët and his colleagues have yet to discover the exact causal mechanism for the massive die-off. Despite this, the study uses the potential causes of mortality to formulate a possible solution for management.
Overall, Andréfouët and colleagues suggest a new management design in which a “common set of conservation objectives” is established for the network of islands, rather than for an individual-island approach (Andréfouët et al., 2013). They suggest this new approach because working together as an island network can result in measures such as nationwide stock maintenance (which would promote long-term sustainability of T. maxima stocks in the case of mortality), and systematic co-monitoring by communities and scientists to monitor local physical and clam population parameters, as well as standing stock.
Critics of this nationwide support network may argue that there is not enough local attention paid towards the individual islands, and too much focus on a national scale goal, which can result in compliance issues. However, if populations continue to decline after environmental effects as they did in 2009, we are facing additional future mass mortalities. A nationwide support network, as suggested by Andréfouët and colleagues, could help prevent these die-offs by having stock maintenance and a nationwide network of giant clam spat-collecting structures (spat is the stage in which when a free-swimming oyster larva attaches to a hard substrate).
Though more research will be needed to determine the exact mechanism that led to the die-off, the benefits and importance of Tridacna maxima to both French Polynesia and marine ecosystems as a whole (e.g. coral reefs), such as being a food source and shelter for fish (Neo et al. (2014), prove that we cannot allow further mass mortalities to occur. As Andréfouët and colleagues suggest, new management design for the network of islands in the Tuamotu network, rather than atoll-specific design is needed to prepare for environmental events and restore clam stocks if needed.
Andréfouët, Serge, Simon Van Wynsberge, Nabila Gaertner-Mazouni, Christophe Menkes, Antoine Gilbert, and Georges Remoissenet. “Climate Variability and Massive Mortalities Challenge Giant Clam Conservation and Management Efforts in French Polynesia Atolls.” Biological Conservation 160 (2013): 190-99. Web. 15 Sept. 2015.
Delviscio, Produced Jeffery, Jessie Dewitt, Claire Maldarelli, and Larry Buchanan. “On the Cusp of Climate Change.” The New York Times. The New York Times, 21 Sept. 2014. Web. 22 Oct. 2015. <http://www.nytimes.com/interactive/2014/09/22/science/on-the-cusp-of-climate-change.html>.
Neo, Mei Lin, William Eckman, Kareen Vicentuan, Serena L.-M. Teo, and Peter A. Todd. “The Ecological Significance of Giant Clams in Coral Reef Ecosystems.” Biological Conservation 181 (2015): 111-23. Web. 21 Oct. 2015. <http://ac.els-cdn.com/S0006320714004212/1-s2.0-S0006320714004212-main.pdf?_tid=13cd9e1a-787a-11e5-922d-00000aacb360&acdnat=1445490321_760cc46339078a257569578bebb4cc16>.
Pollock, Amanda. Both Palmyra Atoll and Kingman Reef National Wildlife Refuge Are Home to Rare Giant Clams by Amanda Pollock / USFWS. 2010. Flikr, n.p.
Watson S-A (2015) Giant Clams and Rising CO2: Light May Ameliorate Effects of Ocean Acidification on a Solar-Powered Animal. PLoS ONE 10(6): e0128405. doi:10.1371/journal.pone.0128405