{"id":28486,"date":"2019-08-20T00:05:58","date_gmt":"2019-08-20T07:05:58","guid":{"rendered":"https:\/\/hakaimagazine.com\/?post_type=custom_visuals&p=28486"},"modified":"2019-11-15T15:18:30","modified_gmt":"2019-11-15T23:18:30","slug":"raising-giants","status":"publish","type":"custom_visuals","link":"https:\/\/hakaimagazine.com\/videos-visuals\/raising-giants\/","title":{"rendered":"Raising Giants"},"content":{"rendered":"

One of my fondest childhood memories is of day trips with my Austrian grandmother. We\u2019d cross the nearby Italian border and order spaghetti alle vongole<\/em>\u2014spaghetti with clams\u2014in a favorite restaurant for lunch. I was fascinated by the little Venus clams on my plate of pasta\u2014caught from the ocean, a world completely alien to me then\u2014and greatly enjoyed their taste. Now, about 40 years later, I often scuba dive between rows of giant clams off the coast of the island of Luzon in the northern Philippines, about 10 hours by road north of the capital of Manila. Each clam is more than 10,000 times the size of the vongole I enjoyed decades ago. Four species of giant clams are bred here and raised in an ocean nursery by my colleagues at the University of the Philippines in an effort to restock the bivalves, whose wild populations are perilously low. Though it is currently illegal to harvest giant clams, places such as the Philippines and other countries in Southeast Asia, where human populations are large and people get a significant amount of their protein from the ocean, are especially dangerous for such immobile giants.<\/p>\n

A reef assessment done in the 1970s showed that giant clams were close to becoming locally extinct in the Philippines due to poaching, overharvesting, and habitat destruction. As a response, in the mid-80s University of the Philippines Marine Science Institute founding director Edgardo Gomez imported clam larvae from the Solomon Islands and Palau. Gomez and his colleagues then reared the clams at the university\u2019s marine lab in the coastal municipality of Bolinao on the west coast of Luzon. From there, the scientists relocated the juvenile clams to different parts of the archipelago. These efforts, spanning more than 30 years, have been successful due to the enthusiastic work of young Filipino marine biologists. Where clam populations had been completely depleted in the wild, they now have been replanted. In an age of constant bad news about the state of the environment, the re-establishment of giant clams in the Philippines is a welcome environmental success story.<\/p>\n

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\"researcher<\/a>

University of the Philippines marine lab research assistant Renato Adolfo is a grown man, but this giant clam looms large in comparison. Of the 13 known species of giant clams, the largest is Tridacna gigas<\/em> (shown here), which can weigh up to 500 kilograms\u2014about five times as much as me, an amateur heavyweight strongman competitor\u2014and have a shell over a meter long. This species is the second-heaviest invertebrate on the planet; only the deep sea\u2019s giant squid is larger. Although giant clams filter-feed plankton, they also derive energy from photosynthesis, so must live in shallow, light-flooded habitats, such as seagrass meadows, as in this image, or in coral reefs.<\/p><\/div>\n

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\"mantle<\/a>

When a clam\u2019s shell is open, you can see a frill of flesh along its edges. This is the clam\u2019s mantle, the part that encloses its internal organs. Its colors and patterns explain some of the clam\u2019s biology. The mantle of this T. gigas<\/em>, for instance, is dark yellow because of a symbiotic algae, Symbiodinium<\/em>. Through photosynthesis, the algae provide energy to the clam. The blue dots are iridocytes, specialized cells that funnel light onto the algae deep in the mantle, akin to biological fiber-optic cables. Different clam species show different patterns of iridocytes and the color of the mantle also varies. It can be yellow, blue, purple, or orange depending on the particular species of Symbiodinium<\/em> the clam hosts. The partnership between giant clams and the algae occurs early in a clam\u2019s life. About four days after fertilization, when the eventual giants are still tiny larvae, the clams absorb the symbiotic algae from the surrounding seawater. The algae are initially ingested like food, but instead of moving through the digestive system, they\u2019re translocated into the tissues of the growing animal.<\/p><\/div>\n

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\"bleaching<\/a><\/p>\n

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Like tropical coral, giant clams are susceptible to bleaching. When water temperatures are several degrees Celsius above normal for weeks at a time, the clams expel their symbiotic algae, and, as a result, areas of their mantles turn eerily white. In some cases, the tubular system in the mantle that normally houses the algae\u2014sort of an algae farm for the clam\u2014atrophies.<\/p>\n

In contrast to corals, which often die after bleaching due to a lack of algae-derived nutrition, most clams usually recover when the temperatures normalize and the population of symbiotic algae is re-established from the seawater. The clam\u2019s color returns, as does its energy budget as the algae begin to photosynthesize.<\/p>\n<\/div>\n

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\"siphon<\/a><\/p>\n

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Without a frame of reference, this exhalant siphon of a giant clam could be mistaken for Jupiter\u2019s Great Red Spot. In reality, it\u2019s the opening through which a giant clam expels water after it has filtered out the plankton. Egg and sperm cells, as well as metabolic waste products, are also released through the siphon. Clams bring water, full of fresh oxygen and food, into the mantle cavity through a slit-like inhalant siphon.<\/p>\n

Divers can sometimes feel the power of the exhalant siphon if they swim close. Clams have rudimentary vision and can only distinguish light and dark. If a diver looms overhead, the clam contracts its mantle and closes its shell, forcing the water out of its siphon and blasting the diver with a powerful jet of water.<\/p>\n

This is the smooth giant clam, Tridacna derasa<\/em>, and it shows a different pattern of iridocytes\u2014made of lines rather than dots\u2014than the T. gigas<\/em> shown in the last two images.<\/p>\n<\/div>\n

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