An essential relationship in the Red Sea
(Tridacna maxima clams extend their mantle tissue outside of their shells to allow their symbiotic micro algae to photosynthesize and produce a food source for both the algae and the clam. These clams live on shallow reef tops and exhibit various colors including blue, green, purple and brown. Image sourced from NOAA via Unsplash)
Giant clams, just like corals, have a symbiotic relationship with microscopic algae where the algae provide the host animal the majority of its food source and the host provides the algae with a safe place to live and nutrients for their growth and survival. This relationship in giant clams has completely altered the bivalve's anatomy. The algae living in their tissues require sunlight and clear water to photosynthesize. Thus, the giant clam has evolved to keep its shells open, exposing beautiful blue, purple and green mantle tissue to the sky, whereas non-photosymbiotic clams are frequently closed, facing downward toward the substrate or buried in the sand.
This upward and open position is very risky for soft-bodied invertebrates. To detect predators, the giant clam has eye spots that sense when light is blocked by shadows of fish swimming above them. The clam can quickly shut its shells when it detects a a shadow by contracting its strong adductor muscles. Once shut, a giant clam is very hard to open.
The symbiotic relationship with microscopic algae has also contributed to the clam's giant size. While Tridacna maxima, the species in the Red Sea, are not human-sized, they are extremely large (about the size of a dinner plate) compared to bivalves that filter feed. It is the excess carbon that photosynthetic algae produce that allows for giant clams to get so giant. This relationship has allowed the largest of the giant clams, Tridacna gigas, to grow larger than six feet long and weigh up to 200 Kg or 440 lbs! This is true for corals as well. While the coral polyp is relatively small, coral colonies can be massive, and it’s all thanks to their microscopic partners and their ability to source large amounts of carbon from the power of the sun.
The giant clam, however, stores their algae very differently than coral. Corals have a layer of symbiotic algae in their tissue while clams have a system of tubules that stack algae throughout their mantle tissue. This stacking design allows the clam to pack a tremendous number of algae into its tissue, however, algae on top of each other poses the problem of lack of sunlight penetration to all of these symbionts. Nature came up with a design to solve that issue as well.
Lining these tubules are specialized cells called iridocytes that act as mirrors and reflect sunlight coming from the ocean’s surface to each stacked algal cell. These iridocytes also give the giant clams their beautiful translucent appearance. This intricate system of tubules, not unlike our capillary system, allows for a more embedded relationship with the algal symbionts that may be part of their ability to withstand bleaching.
Researching the giant clams of the Red Sea
As the Red Sea is relatively understudied, not much is known about the diversity of giant clams in this region. I set out to better understand that diversity in my first research project at KAUST which required clam tissue samples for genetic sequencing.
Right, easy enough. I got my SCUBA gear, my dive buddy and some scissors to descend down onto the reef to cut a small piece of the mantle tissue from the clam.
We get down there. I get my scissors out. I go to cut a piece of the mantle and the clam closes its heavy bivalve shells. I look at my dive buddy and we both start laughing. Of course that would happen! My hand holding the scissors cast a shadow over the clam, awakening its defense mechanism, and then one touch to the delicate tissue, and the clam shut tight. I thought, ‘what are we going to do now?’
And then an idea under pressure
My dive buddy unclips a free carabiner from her vest and gestures to me to follow her to another clam. She carefully avoids swimming above the clam to prevent casting shadows and gently places the carabiner in between the shells as the clam begins to close. Now there is a gap between the shells to snip out a piece of the mantle tissue. Snip, snip, we got it! But quickly, the tissue starts oozing its dinoflagellates and the surrounding water turns blue-green. This chemical cue tips off nearby fish and they start to swarm, hungry for a free lunch.
My dive buddy grabs the piece of tissue out of the feeding frenzy and we secure it into one of the sample tubes. Whew! That was harder than we thought!
Almost a hundred tissue samples later and I am the expert on sampling giant clams.
So, what did I find?
After I collected the tissue, sequenced the DNA, and matched each sequence to known clam sequences through GenBank, I found that the giant clam population was heavily dominated by Tridacna maxima harboring symbionts in the genus Symbiodinium or Clade A Symbiodiniaceae. I had sampled at three different reefs, two different depths and both the protected and exposed sides and realized the lack of clam species diversity suggests an environmental boundary occurring north of my study site where a smaller giant clam, Tridacna squamosa, contributes more to the giant clam population.
T. squamosa clams are slightly smaller and live in shallower waters than T. maxima, making them easy targets for fishermen. While most species of giant clams are protected, their numbers are declining. Giant clams have been and continue to be harvested for their shells, meat and trade in the aquarium market. It is possible that fishing pressures in the central Red Sea wiped out the population of T. squamosa. But now, giant clams might be facing another threat – bleaching.
Can giant clams bleach?
Because giant clams have the same relationship as corals with microscopic algae that give them both food and color, it is possible for them to bleach. Interestingly, bleached clams are rare to witness in the wild, and the reasons for this can be many.
Understanding how these symbiotic clams will respond to warming seas starts by understanding the partners involved in the relationship. My samples showed that giant clams in the central Red Sea harbor Symbiodiniaceae in Clade A, now known as the genus Symbiodinium.
Characterizing these different species of Symbiodiniaceae is still an ongoing process. Some have been found to be extremophiles, or organisms that thrive in extreme environments. For example, symbionts in Clade D, or genus Durusdinium, have been found to resist bleaching and quickly repopulate corals that have undergone bleaching.
Clade A, Symbiodinium, has been labeled as an opportunistic genus with algal cells thriving in coral or clam cells after bleaching has wiped out other algal species. In the Red Sea, an unusually warm body of water, hosting symbionts in the genus Symbiodinium may be beneficial. A result that supports this theory is that the clams from the inshore reefs, reefs that experience the warmest temperatures, all predominantly host a single species within Symbiodinium.
What does this mean for clams in the face of climate change?
Hundreds of Symbiodiniaceae species are now being discovered and their heat tolerance, symbiosis compatibility, and stress resistance are being investigated. While some species may be heat tolerant in some hosts, those benefits are not necessarily consistent across host species. Although the theory of symbiont switching or shuffling offers a solution for corals to acclimatize and adapt to warming oceans, scientists are still unsure if this is a viable option. Because both coral and clams have coevolved with their symbionts for millions of years, it may not be possible for them to form new relationships with other species of algae in a matter of months to years.
Bleaching is possible in both of these photosymbiotic invertebrates, however, my heat experiment with giant clams show that they are more resistant to bleaching than corals. This may be due to the species of symbiont they host, the way they stack their algae in tubules, or acclimation to warmer temperatures in their shallow water habitats. Giant clams are also more dependent on their heterotrophic acquisition of carbon through filter feeding than coral and may be able to avoid starvation even when bleached.
Unfortunately, as global ocean temperatures increase, bleaching is becoming an annual event for corals and potentially more frequent in clams. Although organisms in the Red Sea are already acclimated to warm temperatures, they may be living at their maximum. With summertime water reaching up to 34°C on reef flats, a slight increase in this region may be enough to induce bleaching. This has already been a phenomenon witnessed during the 2016 bleaching event, something previously unusual for the reefs of the Red Sea. This indicates that both coral and clam bleaching is a present threat that will add to the list of reasons why these animals are declining.
Is recovery possible?
The decline of reefs and the frequency and intensity of bleaching events threatens not only the corals and clams, but the other animals that rely on coral reef ecosystems. Clams provide structure to the reef, a food source for other animals, and potentially a symbiont source for photosymbiotic invertebrates. Their decline is linked to negative effects of many other animals on the reef.
However, bleaching does not always lead to a host animal’s death. Both clams and coral can recover from bleaching if temperatures return to normal soon after these animals start to lose their algal symbionts. Recovery experiments in both corals and clams show that some species are able to take up different symbiont species, but it is still unclear if those symbionts will provide long-term benefits. Continued research on the characteristics of different species of Symbiodiniaceae will help scientists to understand the potential of giant clams and corals to adapt to warming oceans through their relationship with their microscopic algae.
To learn more about actions you can take to decrease your carbon footprint, a goal that ultimately helps mitigate bleaching, read our blog post on coral bleaching.