The elegant shapes and colorful patterns of shells often define the identity of the molluscs we encounter and gleefully collect. But what we hold in our hands is a largely inert structure of calcium carbonate, devoid of its occupant, its essence incomplete. What is missing are the squishy bits — the animal itself. The soft tissues of clams and snails can be breathtaking in their colors and patterns. One of the better known examples of this are the giant clams, Tridacna spp.
Residing the shallow sunlit seas of the tropical Indo-Pacific, tridacnids are the largest clams in the world, with one individual measuring 1.4 m (4.6 ft) in length and 263 kg (863 lbs) in weight (Beckvar 1981). While the shells have been collected as curios for their child-bathtub-sized novelty and undulating, fluted edges, there is little else that stands out about these shells. What is mind-blowing about tridacnids are the vivid colors and patterns of iridescent blue, green, black, yellow, brown and violet of the exposed living tissue, the mantle.
What makes these fleshy frills so psychedelic is a complex interplay between the tridacnid clam and its unicellular dinoflagellate symbionts called zooxanthellae. The zooxanthellae primarily reside in the upper layers of tissue of the enlarged mantle and siphon, where they are exposed to sunlight and photosynthesize. The simple sugars that are the result of photosynthesis — glucose and glycerol — are released into the bloodstream, thus providing a source of energy for the tridacnid (Goreau, Goreau, & Yonge 1973; Morton 1978; Fitt & Trench 1981; Wilkins 1986; Buck, Rosenthal, & Saint-Paul 2002). Generally, clams are filter feeders, drawing in water and sorting out all the plankton and organic particulates for digestion. While giant clams retain a fully functional and efficient filter feeding apparatus, the clear, nutrient-sparse waters of the tropics has likely driven the evolution of symbiotic relationships that provide alternate energy sources. Adult tridacnids can probably sustain themselves on photosynthates alone and it is hypothesized that this influx is behind the spectacular growth rate of approximately 100 mm (4 in) per year observed in the species Tridacna gigas (Bonham 1965; Klumpp, Bayne, & Hawkins 1991).
So here is our first color component: zooxanthellae use green chlorophyll and a red carotenoid called peridinin as photosynthetic pigments, rendering the single-celled organisms a brown or reddish-brown color (Fatherree 2007).
The host clam itself generates accessory pigments — greens, browns, and yellows — that extend the range of usable wavelengths of light available for the symbiont’s photosynthesis (Fatherree 2007). In addition, specialized cells (iridocytes) form clusters in the mantle called iridophores that act both as reflectors to redirect light into deeper tissue and as a sunscreen to prevent the zooxanthellae from getting fried from intense sun exposure. Each iridocyte contains a stack of crystalline reflective platelets that scatter light, which makes pigments appear more vivid, creates those eye-popping electric blue-green colors, and makes the mantle shimmer with iridescence (Goreau, Goreau, & Yonge 1973; Griffths, Winsor, & Luong-Van 1992; Fatherree 2007; Todd, Lee, & Chou 2009).
Photosynthetic pigments of zooxanthellae, accessory pigments and iridophores of the host tridacnid clam completes the palette of colors. But how are these colors arranged into striped, spotted, and mottled patterns? While variation in color is caused by localization of zooxanthellae (Todd, Lee, & Chou 2009), the pattern-generating mechanism remains unknown. Despite some broad parameters of patterning seen between species, the role of genes remain ambiguous. Offspring are observed to have a tremendous variety of colors and patterns different from their parents, suggesting a weak genetic link (Fatherree 2007).
Much of the tridacnid’s anatomy and behavior is focused on fostering the intimate relationship with their zooxanthellae symbionts. The super-sized mantle and siphon blossom from the shell during the day and orient themselves to the light like flowers tracking the sun (Morton 1978; Wilkins 1986). This makes the exposed fleshy parts vulnerable to predation, especially from roaming fish looking for a quick bite. Tridacnids do have several hundred well-developed pinhole eyes along the mantle and siphon that can detect shadows and moving objects, triggering immediate retraction and closing of the shell (Wilkins 1986; Land 2003; Todd, Lee, & Chou 2009).
However, responding to each threat by clamming up is hardly beneficial to the zooxanthellae that need to be exposed for hours a day for photosynthesis. Todd, Lee, and Chou (2009) hypothesized that the colors and patterns of the mantle are actually cryptic, breaking up the outline of the animal and camouflaging it against the cluttered background of the reef. High levels of morphological variation, i.e., polymorphism, is common in organisms inhabiting complex and unpredictable environments. Analysis of RGB color values of mantles and background reef substrate indicate some color matching camouflage against fish predators with trichromatic color vision. That a given set of parents spawns wildly polymorphic offspring might be a bet-hedging strategy that potentially some of the dispersing larvae will match their given substrate. This also allows for colonization of a large range instead of being limited to one type of microhabitat for which they are a perfect background match.
Though the flamboyant mantles of tridacnids may serve to hide the clams from predators, they have captured the attention of human collectors. Giant clams have long been an easily-accessible and prized food source for native Pacific Islanders. With the opening up of global markets, tridacnid meat and shells have come into high demand, quickly exhausting local stocks (Jameson 1976; Benzie & Williams 1995; Foyle et al. 1997). Overfishing has stimulated successful efforts at industrial-scale aqua- and mariculture (Klumpp, Bayne, & Hawkins 1991). However, the heavy investment necessary to build large nursery facilities and the seven year growing time to bring species like Tridacna gigas to meat harvesting size was not within reach of many communities throughout the Indo-Pacific. Instead, villagers have turned their attention to smaller species like T. crocea, T. maxima, and T. squamosa for the saltwater aquarium trade. It only takes five to seven months for those species to reach a size suitable for shipping, making this an economical feasible option (Foyle et al. 1997). The ability to breed tridacnids in captivity also creates opportunities to reseed depleted reefs (Beckvar 1981).
The success of raising tridacnids in captivity may have saved them from overfishing, but there is a new threat, one that impacts all marine creatures with photosynthetic zooxanthellae. The symbiont in giant clams is a dinoflagellate named Symbiodinium microadriaticum, and this same species is supposedly found in all reef-building corals, many sea anemones, gorgonians (sea fans), and the Upside-Down Jellyfish, Cassiopeia (Fitt & Trench 1981). With warming of ocean surface waters, symbiotic zooxanthellae are expelled, leaving their hosts bleached white. One experiment recreated the conditions for an observed bleaching event on the Great Barrier Reef of Australia and determined that increased light intensity and water temperature were the main factors resulting in loss of zooxanthellae. It is unclear whether the zooxanthellae are leaving or if they are being ejected by their hosts (Buck, Rosenthal, & Saint-Paul 2002). In some cases, bleaching is a temporary loss of color that can last well over a year. But in more severe cases, the damage is permanent — the spaces that once held zooxanthellae degenerate and the giant clam is left to struggle on its own, undoubtedly with poor outcomes (Norton et al. 1995).
Incidence of coral reef bleaching is becoming more frequent and widespread. This is not necessarily always fatal, but it does require immediate response to global climate change.
A clam is more than just its empty shell, to be discarded or set upon a shelf. By delving into something as superficial as color and pattern, we have uncovered something about the essence of the tridacnid giant clam — the symbiotic relationship with zooxanthellae that is a vital focus of its form and function.
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