Tag Archives: giant clams

Underwater ‘banquets’ are a feast for the eyes

Published October 21, 2017 in the “Ocean Watch” column, Honolulu Star-Advertiser ©2017 Susan Scott

A giant clam sits on a bed of coral in Britomart Reef in Queensland, Australia. ©2017 Susan Scott

ORPHEUS ISLAND, AUSTRALIA >> After nearly a week of snorkeling on the outer reefs of the Great Barrier Reef Marine Park, stormy weather forced Craig and I to sail our 37-foot boat, Honu, back to the shelter of this island. Although Orpheus is also a national park with its own charming beaches, birds and reefs, we missed our giant clams.

While anchored on the reef, we had discovered a several-acre area that looked like the site of a mermaid’s banquet. Table after table was set with pink, green and lavender plate corals sitting between runners of gold and tan leather corals. In the midst of these place settings sat dazzling centerpieces: blue, green and gold giant clams.

A major downfall of these beautiful bivalves is that they’re sitting ducks, big pieces of meat that can’t run away and have only minimal defense. (They can close their shells somewhat but don’t slam shut like they do in cartoons.) Because the colorful algae in their soft tissues need sunlight to thrive, the clams grow mostly in clear, shallow water, making them easy to find, kill and eat.

Like stony corals, giant clams host algae in their tissues that grow sugars for the clams. If conditions are prime and the algae get too dense, the clam simply eats them, too.

A giant clam gathers fertilizer for its algae directly from the seawater that bathes it. Covering the soft, gaping lips of the creature is a type of tissue that can absorb ammonia, nitrate and phosphates, nutrients crucial for plant growth.

The clam breathes, and gets nutritional supplements, by inhaling water through a round siphon on one end of its soft body and sifting out tiny plants, animals and oxygen. The filtered water exits out another siphon on the clam’s opposite end.

Because giant clams grow well in tanks, aqua-farmers in Palau, the Philippines, the Cook Islands and other areas grow them for food, for the aquarium trade and to replenish reefs where people have overharvested.

Giant Clams being grown in a tank in the Cook Islands. ©2006 Scott R. Davis

Just when Craig and I were reminiscing about our amazing experience in what we named the Super Duper Royal Clam Garden, we made a joyful discovery. Near the James Cook University’s marine science center in Orpheus Island’s Pioneer Bay, researchers had planted a giant clam garden in the 1970s.

We found the man-made plot by accident while going ashore in our dinghy. From the surface we could see the 3-foot-long creatures smiling up at the sun, their oval bodies resembling iridescent place mats, and their siphons projecting from the flesh like white porcelain cups. Today 40 to 50 (my guess) mature clams thrive there.

Swank dinner parties aren’t Craig’s and my usual thing, but last week we attended two and we didn’t get a bite to eat. In giant-clam gardens the feast is for the eyes.

Colorful Giant Clams-Bora Bora, French Polynesia. ©2006 Scott R. Davis

Acid-making tissue helps giant clams thrive on reefs

Published December 1, 2014 in the “Ocean Watch” column, Honolulu Star-Advertiser ©2014 Susan Scott

A giant clam lives on its coral home in Palau. ©2014 Susan Scott

Why don’t giant clams growing deep inside living coral heads get squished by growing coral?

My friend asked me that in Palau as we snorkeled over brilliantly colored clams sunbathing in all their glory from their coral head homes.

Good question. I did not know.

Then last week Mary sent me a link to a marine aquarist magazine article that explained how giant clams bore. It was a well-written piece, with references, but I wondered: Why do people who keep home aquariums want to know about giant clams?

Giant clams, I learned, are suitable for the home aquarium trade because despite their family name, some of the eight giant clam species are small. Six of the little ones, as gorgeous as their colossal cousins, are sold as saltwater pets. Besides enjoying the clams’ unique colors and patterns (no two clams are alike), aquarists like giant clams because they’re hardy, grow fast and need little care.

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This is good because people also like to eat giant clams of all sizes, and as a result, they’re scarce or absent in most unprotected areas of the tropical Indian and Pacific oceans. Today most species are on the International Union for Conservation of Nature red list of threatened species.

But there’s hope. Because the clams do well in tanks, aquaculture farmers grow giant clams throughout the Pacific. Some of the young clams go to the aquarium industry, but the clams grown in state-sponsored farms get transplanted in their native habitat.

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Giant clams being farmed in Aitutaki, Cook Islands. Courtesy Scott R. Davis

Tank-grown clams never bore into coral heads after release, but remain free-standing. If born on the reef, however, baby clams often dig in. After drifting in currents for about 10 days, a larval clam drops hinge-side down onto a coral head, anchoring itself with threads protruding from a hole where the clam’s two shells meet.

Also protruding from the hole is a flap of tissue that excretes a weak acid, killing the coral bodies beneath it and weakening the limestone below. (Only the top layer of a coral head is alive. The bulk below consists of the living coral’s long-dead ancestors.) Meanwhile, topside, the clam’s lovely “lips” release a chemical that keeps budding coral polys at bay.

The exposed lips also contain algae that feed the clam and iridescent color cells that act as sunscreen to protect the clam’s soft parts from burning. The resulting color combinations make giant clams of all sizes look like brilliant smiles lighting up the face of the reef.

Thanks, Mary, for the question and link. The expression “happy as a clam” finally makes sense.


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Marine biologist Susan Scott writes the newspaper column, “Ocean
Watch”, for the Honolulu Star-Advertiser, www.staradvertiser.com

©2014 Susan Scott