Color Beneath the Waves: Giant Clams of Bunaken

travelertina

07:07:00 July 7, 2025

Asia,Indonesia

When you think about coral reefs, the first things that spring to mind might be turtles, sharks, or clownfish (who doesn’t want to find Nemo?). I’m guessing “clams” aren’t on the list. But once you spot your first giant clam — anyone hanging out in the Tridacna genus — you’ll probably want to find more. Bright, colorful, and sometimes enormous, they stand out in a way you don’t expect. This post is a photo celebration of the giant clams I’ve seen while diving in Bunaken National Park, North Sulawesi, Indonesia — specifically around the reefs of Bunaken, Siladen, and Bangka Islands.

Close-up of a vibrant blue and electric turquoise giant clam nestled in coral, with intricate iridescent patterns on its mantle, photographed while snorkeling at Bunaken National Park, North Sulawesi, Indonesia.

What’s a Giant Clam?

Giant clams are the largest bivalve shellfish in the world! They generally live in crevices in coral or rock, though some are right out there on the sandy ocean floor. And while they will retreat into their shells if you get close, they are the only bivalves out there that are unable to close their shells completely. So even if they retract their bodies, you’ll always be able to see a sliver of color.

Sliver of a blue giant clam mantle barely visible within a mostly closed shell, nestled inside a large piece of white coral at Bunaken National Park, North Sulawesi, Indonesia.

Giant Clam Fun Facts

They can weigh over 500 pounds and live for over 100 years
They are hermaphrodites that reproduce by releasing millions of eggs into the water — timed with the phases of the moon.
People used to think that giant clams could attack humans (but they can’t — refer back to the fact that they can’t close their shells all the way)
Some of the largest giant clams have been found in Japan, the Great Barrier Reef, and — yes — Bunaken National Park!

And a not-so-fun fact: giant clams have been so overharvested that they are now classified as Vulnerable under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). So please don’t try to move or touch them!

What Gives Giant Clams Those Rainbow Hues?

Close-up of a giant clam with a speckled brown and beige mantle and a pale yellow shell, surrounded by coral and marine life, photographed while snorkeling in Bunaken National Park, North Sulawesi, Indonesia.

Get ready for some science! Giant clams bring the reef to life with color for two main reasons: (a) they have light-reflecting cells called iridocytes in their mantles, and (b) symbiotic algae (zooxanthellae) usually live within their tissues. The thickness of the iridocyte platelets determines the wavelength of light they reflect: thicker stacks reflect longer wavelengths like red and yellow, while thinner stacks reflect blue and green hues. Meanwhile, the symbiotic algae often gives these clams a green or blue tint — but as you’ll see, they come in color variations you might never imagine.

Clams by Color

Shades of Green

Pale green giant clam with black splotches and dotted edge markings, resting among densely packed coral structures in Bunaken National Park, Indonesia.

Giant clam with deep green, brown, and yellow markings in rocky coral with algae nearby in the reef at Bunaken National Park, North Sulawesi, Indonesia

Giant clam with a mottled mantle of cream, green, and dark tones, partially open within coral structure at Bunaken National Park, Indonesia.

Tropical Turquoise

Giant clam with a shimmering turquoise and black-spotted mantle embedded in coral, photographed in Bunaken National Park, North Sulawesi, Indonesia.

Vivid green and turquoise giant clam with glowing blue edges resting on a coral reef in Bunaken National Park, Indonesia.

Bold Blues

Small giant clam with a deep blue mantle and bright blue spots nestled into coral, photographed in Bunaken National Park, Indonesia.

Electric blue giant clam nestled among brain coral and branching coral, with a clownfish peeking from an anemone nearby in Bunaken National Park, Indonesia.

Small giant clam with a vivid electric blue mantle nestled within a lobe of pinkish-brown coral, photographed in bright sunlight while snorkeling at Bunaken National Park, North Sulawesi, Indonesia.

Dark blue giant clam with tiny electric blue spots nestled tightly beneath branching coral on a reef in Bunaken National Park, North Sulawesi, Indonesia

Golden Hues

Giant clam with a golden mantle with brown spotted markings and a thick shell, nestled into various types of coral at Bunaken National Park, Sulawesi, Indonesia

Large clam with a yellow and green mantle with black spots, surrounded by coral and a sea urchin, at Bunaken National Park, North Sulawesi, Indonesia

Earth-Tone Ripples

Partially concealed giant clam with a speckled dark brown mantle and cream edges, partially covered by bright green algae on the reef flat at Bunaken.

Partially closed giant clam with a wavy brown mantle and sandy shell, sitting on the seafloor in shallow water at Bunaken National Park, Indonesia.

Giant clam with cream and black patterned mantle surrounded by tunicates, algae, and encrusting coral, photographed at Bunaken National Park, North Sulawesi, Indonesia.

Giant clam with a dark greenish-black mantle and pale white shell edges, surrounded by a variety of hard and soft coral at Bunaken National Park, Indonesia.

Giant clam with a mantel of motteld cream and brown, situated among branching coral, other hard coral, and tunicates, seen while snorkeling at Bunaken National Park, North Sulawesi, Indonesia

Giant clam with a black mantle patterned in cream and tan speckles, surrounded by branching and encrusting corals on a reef in Bunaken National Park, North Sulawesi, Indonesia.

A Pop of Pink

Giant clam with swirling cream and black markings with a pink and white shell among coral, a red feather star and algae, photographed in Bunaken National Park, North Sulawesi, Indonesia.

Want to see more of what Siladen has to offer? Don’t miss my 2023 post on snorkeling Siladen’s house reef, or my 2025 post highlighting more of the island’s incredible marine creatures. And if you’re hunting for clownfish, you’ll find them — along with their anemones — in this post.

Curious about what comes out under these waters at night? All sorts of critters lurk here.

8 responses to “Color Beneath the Waves: Giant Clams of Bunaken”

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