Giant clams are the largest bivalve mollusk upon the planet with the Tridacna gigas being the largest species measuring in length up to around 1.2m and with a weight estimated at up to 200 kgs when full size, they are a fascinating marine creature, growing on the sea floor or reef structures.
Not all giant clams are giants though as the different species vary in sizes with the smallest being the crocea commonly sighted at around 13 cms.
What is a bi-valve ? The main feature is that their shell is made of two halves that look similar to each other, these are called valves, there is an elastic ligament which joins them at the bottom side underneath allowing it to open and close.
Underneath the clam where it sits, there is a byssal opening, which vary in size between species, this is where the foot containing the byssal organ which produces attachment threads to a surface exit the shell so they can secure to the substrate which assist in keeping them in place, byssal threads may be used at varying ages in different species, but the little crocea is also aided by the use of an acid secretion to bore into reef substrate until it is mostly submerged safely from predators and often only its mantle may be seen on the substrate surface while other clam species are often free standing or the shell partially obscure.
Not all giant clams are giants though as the different species vary in sizes with the smallest being the crocea commonly sighted at around 13 cms.
What is a bi-valve ? The main feature is that their shell is made of two halves that look similar to each other, these are called valves, there is an elastic ligament which joins them at the bottom side underneath allowing it to open and close.
Underneath the clam where it sits, there is a byssal opening, which vary in size between species, this is where the foot containing the byssal organ which produces attachment threads to a surface exit the shell so they can secure to the substrate which assist in keeping them in place, byssal threads may be used at varying ages in different species, but the little crocea is also aided by the use of an acid secretion to bore into reef substrate until it is mostly submerged safely from predators and often only its mantle may be seen on the substrate surface while other clam species are often free standing or the shell partially obscure.
Tridacna gigas, the largest of the species.
Clam species have two siphons for water flow, an inlet siphon which is a length of divided tissue allowing water to travel in and an outlet siphon which is a narrow tube shape extending outward and upward with a roundish opening.
The fleshy mantle which varies in color differs within and between species and no two clams are ever exactly the same color, this is due to the symbiotic algae zooxanthellae, which are photosynthetic algae and behave like tiny plant-like organisms that live in the exposed tissue mantle providing nutrient to the clam.
The fleshy mantle which varies in color differs within and between species and no two clams are ever exactly the same color, this is due to the symbiotic algae zooxanthellae, which are photosynthetic algae and behave like tiny plant-like organisms that live in the exposed tissue mantle providing nutrient to the clam.
|
During thermal stress from warm ocean temperatures the zooxanthellae may not be present or as consistent, this results in giant clam species with bleached areas on the upper mantle as evidenced in T.gigas photos inset.
Threats to Giant Clam species can include climate change, habitat degradation, sedimentation, pollution, poaching and disease. |
Tridacna derasa
Tridacna maxima
Tridacna noae or tear drop clam due to the mantle pattern and other factors, formally in the T. maxima species but since 2014 it was found to have different genetic traits and has been re-named.
T. squamosa or fluted clam.
T. crocea is the tiniest clam around 13cm which can use an acid to assist in boring into the reef substrate. They use this weak acid secretion from the pedal mantle which extends from the underneath byssal opening, this is possibly why the byssal opening is larger than other species by shell ratio to the proportion of the opening, this byssal opening at a large opening to shell ratio would allow for the highly active layer of mobile flat tissue to exit and retract when required without limitation to allow for the pedal mantle to be extended or retracted outward and up along the outside shell secreting acid, it assists in the clam burrowing or enlarging the cavity within the reef structure. They can also extend their upper mantles well beyond the upper margin of their shell which is usually even with the reef structure so only the mantle is showing and enough to completely hide the shell from sight when viewed from above. The mantle often has numerous protrusions over it and a row of dots around the outside.
While other giant clams belong to the Tridacna genus, two tridacnids are not in the same genus, in pic Hippopus hippopus, one of the two species, the inlet siphon has no fringing tentacles making it obvious which species it is. Usual colours that may be observed are golds, greens and bluish.