Marine Creature of the Month FEB '19

Giant Clam

Tridacna Gigas

Photo from Maritime Review

Kingdom: Animalia

Class: Bivalvia

Conservation Status: Vulnerable (Vu)

WHERE ARE THEY FOUND?

Giant clams are found across the South Pacific and Indian Oceans. They live off the coast of Australia, the Philippines, India, and East Africa. These are mostly tropical waters.

WHAT IS THEIR HABITAT?

They live in reefs like the Great Barrier Reef off the coast of Australia. They can be found in depths up to 20m (66ft) upon coral sands or broken coral. Being relatively shallow, these waters are light and warm, and support quite a complex ecosystem. Giant clams share their home with microscopic plankton and phytoplankton as well as bigger creatures such as dolphins, whales, sharks, and sea turtles. Not to mention the thousands of fish species which reefs support.

Photo from 1770 Reef

WHAT DO THEY EAT?

Giant clams are filter feeders. They eat the microscopic life forms floating in the water column. However, reefs are generally nutrient-poor waters, so living off plankton alone would make these clams a much smaller species. They achieve their size thanks to a symbiotic relationship with algae. The tissues in their mantle can support algae growth. In return for the home, the clams can use the energy the algae gain from photosynthesis. Fully grown clams get up to 2/3 of the carbon needed for food and growth this way! But until they are big enough to support the algae, clams rely on filter feeding alone.

HOW BIG DO THEY GET?

Thanks to their combined diet of filter feeding and photosynthesis, giant clams can grow to sizes of 120cm (47"). This puts them at over an metre long. I am 1.6m (5'6) tall which puts the giant clam's length at only 40cm (1ft) shorter than me! They do, however, weight considerably more than I do. They can weigh over 200kg (440lbs)! A lot of this weight in carried in their shell.

Photo from National Aquarium

WHAT IS THEIR LIFE CYCLE?

In the wild, they are estimated to live up to 100 years. They start out life planktonic as larval clams. These microscopic life forms hatch only twelve hours after fertilisation and are born with a locomotive muscle, or "foot", which helps them to search for an ideal habitat. They usually settle after a week or so. Over time, they amass carbon and grow - scientists are unsure of the rate but have observed them growing 12cm a year in captivity. 

It is considered a juvenile at around 20cm. As it grows, the giant clam becomes sessile. This means it has a lack of self-locomotion; it cannot move itself about. This is because it loses its foot muscle as it settles. This process can actually kill the clam and does claim many clams' lives. Giant clams grow rapidly due to their symbiotic relationship with algae.

When they reach maturity, giant clams reproduce through broadcast spawning. They are hermaphrodites; they produce both male and female sex cells. They cannot self fertilise, however, but by producing both sex cells they increase the amount of offspring produced as well as making finding a mate much easier. When a giant clam broadcast spawns, it triggers signals in other mature and breeding clams to spawn as well. Effective chat-up lines, considering how they cannot move!

WHAT MAKES THEM VULNERABLE (VU)?

Unfortunately, the answer is us. We are the major threat to giant clams. They are intensely fished for the usual reasons: food, decorations, and medicine. And the biggest ones are hunted because they gain the most profit. Giant clams are considered a delicacy in Japan, France, and South East Asia; and their muscles are used in many dishes. Their shells fetch a high price on the black market and the foot muscle is used as an aphrodisiac.

Our penchant for fearing the huge has not helped matters. Throughout history these clams are referred to as "man-eating clams" or "killer clams". While they physically could grip a person in their shell, it would be a defensive move not an offensive move. It would not be an act of aggression. Mostly because they close very slowly. Well done, guys, well done.

Photo from Thala Beach Nature Reserve

FUN FACTS

• They release 500 millions eggs each time they broadcast spawn.
• Sadly, it is the most endangered species of clam.
• They were discovered, it is believed, as early as 1521.
• It is the only species of clam which cannot fully close its shell. 

Pinterest Board: #MCotM Feb '19

Listening to: Blue Planet II - Hans Zimmer