Watch What Happens When This Giant Clam Opens Up

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In the sand surrounding coral reefs, the giant clam reigns supreme; they are massive, multi-colored, and statuesque. As the largest living bivalve mollusk, the giant clam acts as a lynchpin for its environment. They are filter feeders, which means they acquire nutrients by eating organic matter or tiny organisms that happen to pass. Simultaneously, giant clams provide a habitat for algae and other marine life in areas of the ocean often deprived of food.

This special filter-feeding system allows giant clams (Tridacna gigas) to grow to incredible proportions. While they are often several feet wide, some specimens have been found with a four-and-a-half foot span and a weight of 500 pounds. They hardly seem to move, but when they do, the results are remarkable. What compels them to open up their shells? Watch the video above, and we will explore the topic further.

More About Giant Clams

Filter feeding is an important part of a giant clam’s life. They get their nutrients from plankton and organisms passing by. While their digestive process helps them grow, they reach epic sizes thanks to the process of biomineralization. This is when living organisms produce minerals as tissue. Giant clams seem to have mastered this, as their shells are thick, wavy, and vibrantly colored. That said, this makes them very sensitive to seasonal temperatures.

They enjoyed widespread populations in ancient times but the growth of human civilization has infringed upon the giant clam. Global warming has reduced their numbers but so has interest in their shells. Widespread fishing of the creature for its meat, as well as its prized, vibrant armor, has significantly reduced giant clam populations. Several East Asian locations have even seen giant clams go all but extinct. It’s hard to blame them for not opening their shells, but when they do, what’s happening?

The Giant Clam Shell

They may seem like statues at first glance, immobile, and striking, but giant clams move a fair amount. Remarkably, that movement is almost entirely dependent on perception. The border of the clam’s mantle features thousands of 0.5 mm-wide eyespots. They may not look like much but each eyespot contains a pupil-like opening with a base of at least 100 light-sensitive photoreceptors.

As the sun moves its position throughout the day, giant clams close or open their shells using their large abductor muscles (the clam meat that people eat). This allows the algae that live inside to undergo photosynthesis, thus feeding the clam and algae simultaneously. Remarkably, giant clams may be the only mollusk that’s sensitive to ultraviolet rays. Come nightfall, the giant clam closes its shell. The mantle on its shell also contains two holes, the inhalant and exhalant siphon. These orifices work together to bring in seawater to filter for food before expelling it outward.

They are one of the most majestic creatures in the ocean, but overfishing has put giant clams at serious risk. The more they are harvested for their large, colored shells, the less they add to the vibrancy of coral reefs. Worse, their reduced populations mean fewer habitats for important marine organisms. Ironically, much like their biggest predator, humans, giant clams only open up when they find the light.