Giant Clam Largest Hard-shelled Mollusc in the World
Giant clams are huge molluscs that can grow up to 1.5 meters and weigh as much as 230 kilos. They have zigzag shaped opening with large protruding brilliant blue iridescent lips. Many are found in the Great Barrier Reef off the north-eastern coast of Australia.
The Giant clam is the world’s largest sessile (attached to a fixed location) mollusc in the world. It has a rather uninteresting hinged double-sided shell with a serrated zigzag pattern on it. The two halves of the shell hinge on one side and are held together by strong muscles. This shell may have blotches of colour. The most notable characteristic of this clam, besides its huge size, is its beautiful blue iridescent lips. Because of these large lips, the giant clam cannot, unlike other clams, close its shell completely. At the centre of its blue lips it has two siphons (tubes) which it uses to suck in water and also eject waste.
Giant Clam's Big Blue Lips Why the Giant Clam's has Blue Lips
The giants clam has special iridescent cells, called iridocytes, in its lips that give them their beautiful blue and aqua colour. These brilliant coloured lips actually serve a very useful purpose — they direct sunlight into the interior of the clam providing the sunlight necessary for algae it hosts in its lips.
These clams live in a symbiotic relationship with algae. This symbiotic relationship is referred to as mutualism, because both clam and algae mutually benefit from their partnership. Typically the shallow coral reefs the clams live on is exposed to levels of sunlight strong enough to kill the algae. But by hosting the algae within its body, clam provides the algae with a safe and sunny environment. The algae in return produce sugars and proteins which they share with the clam.
The clam can open and close its shell thereby expanding or contracting the surface and angle of its mantel to manage the amount of light penetrating to its microscopic algal tenants below. It can also lighten or darken its colour pigmentation to regulate light penetration.
Why are its lips blue?
The reason the giant clam's lips are blue is because the clam’s iridocytes transmit mainly red and blue light, the wavelengths that the algae photosynthesise most efficiently, and deflect much of the green and yellow wavelengths because it’s of no use to the algae. This is what gives the clam’s lips their colour. Interestingly the algae themselves are a dull grey-brown colour.
Giant clams are filter feeders that suck in large quantities of water through an inhalant siphon which is usual a flat hole in its mantel. The water is circulated over its gills where a sticky mucus causes microscopic plankton and other particulate organic matter from the water to adhere to it. Tiny hair-like structure called cilia continuously sweep these food particles towards the clam’s mouth and into its digestive system. Digestive waste is excreted through an anus. And filtered water is ejected through an exhalent siphon with a tubed spout.
Giant clams also supplement diet this with sugars (glycerol) and proteins produced by the microscopic algae that they host within their mantel. The algae feed on carbon dioxide and other products supplied by the giant clam and using photosynthesis with the light directed to them by the clam's iridocytes produce these nutrients for the clam.
Giant clams are sedentary animals that are found in the relatively shallow water not more than 20 meters deep. Once a young clam larvae finds a suitable location on the reef it attaches itself to the reef and stay there for the rest of its life. They inhibit inhabits reefs and lagoons with good sunlight penetration.
Giant clams are hermaphrodites. They have both male and female reproductive organs but they don't self-fertilise.
They release millions of eggs and sperm into the water through their siphon where they intermix with eggs and sperm from other giant clams and are fertilised. This spawning, known as broadcast spawning, is synchronise by chemical signalling using a substance referred to as the spawning induced substance that individual clams release into the water signalling their readiness to spawn. Many individuals then time the release of their eggs and sperm to have the best opportunity of being cross-fertilised by other clams. Spawning is most common during the incoming tide and lasts for 30 to 180 minutes. .
Once fertilised the eggs float in the water for about 12 hours before the larvae hatch out and immediately start to develop a shell. Within a few days the little clams descend to the bottom the ocean and for the first few weeks will move from place to place in search of a suitable location. It will then attach itself to the ocean floor and remain there for the rest of its life.
Even though million upon millions of eggs and sperm are produced very few clams survive into adulthood. A giant clam reaches adulthood in 4 to 9 years and can live for up to 100 years.
Legend - Giant Clam Attack Killer Clams?
There are many unfounded legends, myths and tales about attacks by giant clams. These tales usually claimed that these giant clams gobbled up unwitting divers. A U.S. Navy diving manual even gave detailed instructions on how to free oneself form the grip of a giant clam.
Today the giant clam is known to be non-aggressive. It will only close is shell in self-defence. It is possible that a human could get a limb caught during such a closure and drown as a consequence. But this is very unlikely because the clam closes its shell very slowly, giving a person plenty of time to extricate themselves.
Threats What Treats Do Giant Clams Face?
Giant clams are preyed upon by many animals much smaller than themselves. They include snails, eels and starfish that nip off pieces of the giant clam's lips.
One of the major treats facing giant clams are humans who harvest them for their shells, as displays in their aquariums and for their flesh which is considered a delicacy in some countries. This has led to a drastic decline in the number of these beautiful clams world-wide. The giant clam is listed as vulnerable to extinction.
The giant clam is considered to be vulnerable because of humans. As with all plants and animals the giant clam is protected in the Great Barrier Reef sanctuary.